Lyotropic Liquid Crystals - American Chemical Society

4 Liquid Crystalline Phases in Biological

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Model Systems K. LARSSON and I. LUNDSTRÖM

1

University of Göteborg and Chalmer's University of Technology, Fack, S-402 20 Göteborg 5, Sweden The present knowledge about molecular organization in lyotropic liquid crystalline phases is summarized. Particular attention is given to biologicaly relevant structures in

lipid-

-water systems and to lipid—protein interactions. New findings are presented on stable phases (gel type) that have ordered lipid layers and high water content. Furthermore, electrical properties of various lipid structures are discussed. A simple model of 1/f noise in nerve membranes is presented as an example of interaction between structural and electrical properties of lipids and lipid-protein complexes.

' H p h e significance of l i q u i d crystals i n b i o l o g i c a l systems is o b v i o u s f r o m the f a c t that m o s t l i f e processes r e q u i r e m o l e c u l a r d i s o r d e r a n d m o b i l i t y w i t h m a i n t e n a n c e of o r i e n t a t i o n of the f u n c t i o n a l groups i n v o l v e d ; this is just the m o l e c u l a r o r g a n i z a t i o n t h a t is characteristic o f the l i q u i d c r y s t a l l i n e state. I n a f e w cases, f o r e x a m p l e i n muscles a n d i n the

nerve

m y e l i n sheath, there are l i q u i d c r y s t a l l i n e regions w i t h t h r e e - d i m e n s i o n a l extension.

M o r e o f t e n , h o w e v e r , t h e structure is t w o - d i m e n s i o n a l , c o n -

sisting o f a u n i t l a y e r o f a l a m e l l a r l i q u i d c r y s t a l l i n e phase.

The mem-

b r a n e that covers a l l cells a n d c e l l organelles has a m o l e c u l a r

arrange-

m e n t of this t y p e . S i n c e there are v e r y f e w p h y s i c a l m e t h o d s f o r i n v e s t i g a t i n g t w o - d i m e n s i o n a l structures i n a n aqueous e n v i r o n m e n t , the use o f r e l a t e d l i q u i d c r y s t a l l i n e phases as m o d e l s p r o v i d e s a n i m p o r t a n t basis f o r o u r u n d e r s t a n d i n g of m e m b r a n e structure. L y o t r o p i c l i q u i d c r y s t a l l i n e phases h a d b e e n u t i l i z e d t e c h n i c a l l y f o r a l o n g t i m e b e f o r e t h e i r structures w e r e k n o w n . A m i l e s t o n e i n the e l u c i d a t i o n o f t h e i r structure w a s the i n t r o d u c t i o n o f the l i q u i d c h a i n concept. I n 1958, o n the basis o f e v i d e n c e f r o m I R spectroscopy, C h a p m a n p r o 1

Present address: Chemical Center, Box 740, S-22007 Lund, Sweden

43 In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

44

LYOTROPIC LIQUID

CRYSTALS

p o s e d that a h i g h t e m p e r a t u r e phase t r a n s i t i o n i n a n h y d r o u s soaps w a s c a u s e d b y m e l t i n g of the chains ( I ) .

A f e w years later, the

complete

s t r u c t u r e of the m o s t c o m m o n l i q u i d c r y s t a l l i n e phases i n s o a p - w a t e r sys tems w a s r e v e a l e d b y t h e p i o n e e r i n g w o r k of L u z z a t i a n d c o - w o r k e r s ( 2 ) . A c h a r a c t e r i s t i c f e a t u r e of m o l e c u l e s t h a t f o r m l y o t r o p i c l i q u i d crys tals is t h e i r surface a c t i v i t y . B e c a u s e of the a m p h i p h i l i c n a t u r e of t h e m o l e c u l e s , t h e y o r i e n t u p o n contact w i t h solvent m o l e c u l e s , g i v i n g rise to p o l a r a n d n o n p o l a r regions that are s e p a r a t e d b y t h e p o l a r e n d g r o u p s . Downloaded by NORTH CAROLINA STATE UNIV on September 24, 2012 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch004

A l l structures k n o w n fit one of those m a d e p o s s i b l e b y the v a r i o u s c u r v a tures of the i n t e r f a c e b e t w e e n t w o l i q u i d regions, w i t h m o l e c u l a r size t a k e n i n t o c o n s i d e r a t i o n . I t is therefore n o t s u r p r i s i n g that the e a r l i e r t r e a t m e n t of t h e structure of l y o t r o p i c l i q u i d crysals w a s u n s u c c e s s f u l since the m o l e c u l e s w e r e r e g a r d e d as stiff rods. T h e l y o t r o p i c l i q u i d c r y s t a l l i n e phases r e l e v a n t to b i o l o g i c a l systems consist of w a t e r a n d l i p i d s a n d u s u a l l y p r o t e i n s also. T h e l i p i d s l i s t e d i n T a b l e I o c c u r i n c e l l m e m b r a n e s ; a l l f o r m l i q u i d c r y s t a l l i n e phases w i t h water. D i f f e r e n t b i o l o g i c a l tissues are c o m p l e x m i x t u r e s of v a r i o u s l i p i d types, a n d e a c h one is u s u a l l y r e p r e s e n t e d b y n u m e r o u s h o m o l o g s w i t h Table I.

Lipids in Cell Membranes

Typical Example

Lipid Phospho lipids

dipalmitoyllecithin

Formula CH —OCO(CH ) CH | CH —OCO—(CH ) CH 2

2

2

1 4

2

3

1 4

CH —ΟΡ0 Ό (CH ) N 2

2

2

2

Η Sphingolipids

cerebroside

CH

(CH )

3

2

1 2

+

3

(CH ) 3

Η

3

Η

—C=C—C—C—CH —0—CH2

H

O H N H C=0 R

—CH—HCOH—HOCH—HOCH—HC—CH OH 2

I Glycerides

1-monopalmitin

ο

1

CH OH | CHOH 2

CH OCO(CH ) 2

2

1 4

CH

3

In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

4.

LARSSON A N D L U N D S T R O M

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Phases in Biological Model Systems

different f a t t y a c i d c o m p o s i t i o n . A r e m a r k a b l e p r o p e r t y of s u c h m i x t u r e s is t h a t t h e y u s u a l l y b e h a v e as a single c o m p o n e n t w i t h respect t o t h e p h a s e r u l e . A s is d i s c u s s e d b e l o w , l i p i d phase transitions a r e o f f u n c t i o n a l i m p o r t a n c e i n m e m b r a n e s , a n d t h e p o s s i b i l i t y of v a r y i n g t h e f a t t y a c i d p a t t e r n p r o v i d e s a w a y to v a r y t h e phase t r a n s i t i o n t e m p e r a t u r e as w e l l as t h e t r a n s i t i o n range.


Structures in Lipid—Water Systems T h e three f u n d a m e n t a l l y o t r o p i c l i q u i d c r y s t a l structures are d e p i c t e d i n F i g u r e 1. T h e l a m e l l a r structure w i t h b i m o l e c u l a r l i p i d layers separ a t e d b y w a t e r layers ( F i g u r e 1, c e n t e r ) is a r e l e v a n t m o d e l f o r m a n y b i o l o g i c a l interfaces.

Despite the disorder i n the polar region a n d i n the

h y d r o c a r b o n c h a i n layers, w h i c h spectroscopy reveals a r e close to t h e l i q u i d states, there is a p e r f e c t r e p e t i t i o n i n t h e d i r e c t i o n p e r p e n d i c u l a r to the layers. B e c a u s e of this o n e - d i m e n s i o n a l p e r i o d i c i t y , t h e thicknesses of the l i p i d a n d w a t e r layers a n d t h e cross-section area p e r l i p i d m o l e c u l e can be derived directly f r o m x-ray diffraction data. T h e h e x a g o n a l a r r a n g e m e n t of c y l i n d e r s f o r m e d b y l i p i d m o l e c u l e s i n a c o n t i n u o u s w a t e r m e d i u m ( F i g u r e 1, r i g h t ) is o b s e r v e d i n a l l systems w i t h l i q u i d crystals, p r o v i d e d that there is a m i c e l l a r state at h i g h w a t e r content.

O n l y a f e w lipids w h i c h have a large polar head group relative

to t h e h y d r o c a r b o n c h a i n p o r t i o n o f t h e m o l e c u l e s h o w this phase,

e.g.

l y s o l e c i t h i n ( o b t a i n e d f r o m l e c i t h i n w h e n o n e c h a i n is r e m o v e d ) a n d p s y c h o s i n e ( r e l a t e d t o cerebroside i n t h e same w a y ) . T h e x - r a y d i f f r a c t i o n p a t t e r n of this p h a s e c a n b e i n d e x e d as a t w o - d i m e n s i o n a l h e x a g o n a l lattice, a n d o n e m a y a p p l y s i m p l e geometry i n o r d e r t o d e t e r m i n e t h e r a d i u s of t h e l i p i d c y l i n d e r s , t h e distance b e t w e e n adjacent c y l i n d e r s , a n d the area p e r l i p i d m o l e c u l e at t h e w a t e r interface. T h e i n v e r s e d h e x a g o n a l structure, w i t h w a t e r c y l i n d e r s a r r a n g e d i n a m a t r i x f o r m e d b y t h e d i s o r d e r e d h y d r o c a r b o n chains ( F i g u r e 1, l e f t ) , is a c o m m o n structure i n aqueous systems of l i p i d s of b i o l o g i c a l o r i g i n .

There

is u s u a l l y n o p r o b l e m i n d e t e r m i n i n g t h e t r u e a l t e r n a t i v e b e t w e e n t h e t w o h e x a g o n a l structures f r o m t h e x-ray d a t a , a n d t h e m o l e c u l a r d i m e n s i o n s c a n t h e n b e c a l c u l a t e d . T h e o c c u r r e n c e of this structure i n c o m p l e x l i p i d s results f r o m t h e m o l e c u l a r shape; t w o h y d r o c a r b o n chains are u s u a l l y

Figure 1. Schematic of the three most common phases that occur in aqueous lipid systems



46

LYOTROPIC

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Journal of Lipid Research and Nature

Figure 2 .

Egg lecithin. Left: phase diagram (3), and right: details of the region with low water content (4)

l i n k e d t o the p o l a r e n d g r o u p . I f the interface has a c u r v a t u r e so that the chains d i v e r g e , t h e y w i l l h a v e m o r e space t h a n i n t h e l a m e l l a r structure. T h i s structure o f t e n f o r m s f r o m t h e l a m e l l a r o n e u p o n h e a t i n g .

I t is

k n o w n f r o m s p e c t r o s c o p i c studies t h a t c h a i n m o b i l i t y increases t o w a r d the m e t h y l e n d . T h i s h e x a g o n a l structure is also f a v o r a b l e f o r s u c h therm a l m o v e m e n t , a n d the t r a n s i t i o n f r o m the l a m e l l a r phase w h e n t h e r m a l m o v e m e n t increases is therefore n o t s u r p r i s i n g . O n e t y p e o f l i p i d that is d o m i n a n t i n b i o l o g i c a l interfaces is l e c i t h i n , a n d lecithin—water systems h a v e therefore b e e n e x a m i n e d extensively b y different p h y s i c a l t e c h n i q u e s .

Small's b i n a r y system ( 3 ) f o r egg l e c i t h i n -

w a t e r is p r e s e n t e d i n F i g u r e 2. T h e l a m e l l a r phase is f o r m e d over a large c o m p o s i t i o n r a n g e , a n d , a t v e r y l o w w a t e r content, the phase b e h a v i o r i s q u i t e c o m p l e x . T h e i r structures as p r o p o s e d b y L u z z a t i a n d c o - w o r k e r s ( 4 ) are either l a m e l l a r w i t h different h y d r o c a r b o n c h a i n p a c k i n g s or b a s e d o n r o d s ; b o t h types are d i s c u s s e d b e l o w . T h e s w e l l i n g o f l e c i t h i n ( a n d o f a l l other n e u t r a l l i p i d s s t u d i e d ) p r o ceeds u n t i l a w a t e r l a y e r thickness o f a b o u t 20 A is r e a c h e d .

B e y o n d this

l i m i t , the r e g i o n w i t h h i g h e r w a t e r content i s o f t e n d i s c r i b e d as c o n s i s t i n g of t w o phases, w a t e r a n d t h e l a m e l l a r phase.

T h i s i s n o t correct, h o w -

ever, since c o n c e n t r i c structures a r e f o r m e d i n this w h o l e range. T h e s p h e r i c a l s o - c a l l e d l i p o s o m e s , w i t h l i p i d layers a l t e r n a t i n g w i t h

water

layers, a r e not r e s t r i c t e d o n l y to v e r y d i l u t e systems, b u t , together w i t h the c y l i n d r i c a l a r r a n g e m e n t o f c o n c e n t r i c layers, t h e y represent a p a r t i c u lar s t r u c t u r a l state a b o v e the l i m i t o f w a t e r s w e l l i n g . O u r k n o w l e d g e o f the e q u i l i b r i u m state o f these dispersions a n d v a r i a t i o n s i n shape a n d size o f t h e c o l l o i d a l particles is v e r y l i m i t e d e v e n t h o u g h liposomes a r e


4.

LARSSON A N D L U N D S T R O M

47


u s e d f r e q u e n t l y as m e m b r a n e m o d e l s . I t is p o s s i b l e to b r e a k these p a r ticles m e c h a n i c a l l y , f o r e x a m p l e b y u l t r a s o u n d , a n d a l a r g e p r o p o r t i o n o f particles consisting of a single b i l a y e r is t h e n o b t a i n e d . T h e s e s o - c a l l e d vesicles c a n b e separated, a n d t h e y h a v e o b v i o u s advantages o v e r l i p o somes as m e m b r a n e m o d e l s . T h e phase b e h a v i o r o f a synthetic l e c i t h i n , d i p a l m i t o y l l e c i t h i n , as a n a l y z e d b y C h a p m a n a n d c o - w o r k e r s ( 5 ) , is d i a g r a m m e d i n F i g u r e 3 . T h e m a i n features are t h e same as i n t h e phase d i a g r a m o f e g g l e c i t h i n : a Downloaded by NORTH CAROLINA STATE UNIV on September 24, 2012 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch004

m i x t u r e o f n u m e r o u s h o m o l o g s . A s a c o n s e q u e n c e of t h e v a r i a t i o n i n f a t t y a c i d c h a i n l e n g t h , t h e c h a i n m e l t i n g p o i n t is l o w e r e d w h i c h means that t h e c r i t i c a l t e m p e r a t u r e f o r f o r m a t i o n of l i q u i d c r y s t a l l i n e phases is r e d u c e d . T h i s t e m p e r a t u r e is a b o u t 42 ° C f o r d i p a l m i t o y l l e c i t h i n , a n d , i f t h e l a m e l l a r l i q u i d c r y s t a l is c o o l e d b e l o w this t e m p e r a t u r e , phase is f o r m e d .

a so-called g e l

T h e h y d r o c a r b o n chains i n t h e l i p i d b i l a y e r s o f this

phase a r e e x t e n d e d , a n d t h e y c a n b e r e g a r d e d as c r y s t a l l i n e .

The gel

phase a n d t h e transitions b e t w e e n o r d e r e d a n d d i s o r d e r e d chains are c o n s i d e r e d separately. C u b i c phases h a v e also b e e n o b s e r v e d i n l i p i d - w a t e r systems.

Such

a phase is n o t a t r u e l i q u i d c r y s t a l since i t exhibits t h r e e - d i m e n s i o n a l p e r i o d i c i t y , b u t a l l p h y s i c a l p r o p e r t i e s a r e c l o s e l y r e l a t e d t o those o f t h e l a m e l l a r a n d h e x a g o n a l phases.

F o r t h e structure d e r i v e d f o r t h e c u b i c

phase of s t r o n t i u m m y r i s t a t e ( 6 ) , see F i g u r e 4. T h e l i p i d m o l e c u l e s a r e j o i n e d so that t h e p o l a r groups f o r m r o d s , a n d these rods a r e j o i n e d i n t o t w o separate t h r e e - d i m e n s i o n a l n e t w o r k s . T h e l i p i d r e g i o n as w e l l as t h e p o l a r r e g i o n i n this structure f o r m c o n t i n u o u s m e d i a . T h e r e is also e v i d e n c e f o r a c u b i c structure w i t h c l o s e d w a t e r aggregates. It is f o r m e d i n m o n o g l y c e r i d e s of m e d i u m c h a i n l e n g t h w h e n t h e l a m e l l a r phase is h e a t e d

0.8 Concentration

\

Chemistry and Physics of Lipids

Figure 3. Phase diagram of the dipalmitoyllecithin-water system (5)

Nature

Figure 4. The rod systems formed by the polar groups in the cubic phase that forms in strontium myristate (6)

Library American Chemical Society In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.


48

LYOTROPIC

Figure 5.

The binary system of water

LIQUID

CRYSTALS

monopalmitin-

( F i g u r e 5 ) , a n d , w i t h l o n g e r chains ( > C o ) , i t transforms i n t o t h e h e x a 2

gonal structure w i t h closed water cylinders u p o n further heating.

Struc-

tures b a s e d o n s p a c e - f i l l i n g p o l y h e d r a w e r e a n a l y z e d ( 7 ) , a n d t h e x - r a y d a t a are i n g o o d a g r e e m e n t w i t h a structure c o n s i s t i n g of a b o d y - c e n t e r e d a r r a n g e m e n t of p o l y h e d r a w i t h faces f o r m e d b y six squares a n d e i g h t hexagons

(see

F i g u r e 6 ) . T h i s structure w a s later c o n f i r m e d b y freeze-

etching electron microscopy

(8).

A q u e o u s systems of m o l e c u l e s as d i f f e r e n t as l e c i t h i n s a n d m o n o g l y c e r i d e s h a v e v e r y s i m i l a r phase d i a g r a m s (cf. F i g u r e s 3 a n d 5 ) , w h i c h illustrates that l i p i d s w i t h s i m i l a r size relations b e t w e e n h y d r o p h o b i c a n d h y d r o p h i l i c regions (expressed f o r e x a m p l e b y the H L B v a l u e ) g i v e t h e s a m e t y p e of w a t e r i n t e r a c t i o n . I f i o n i c g r o u p s are present, t h e l a m e l l a r

Figure 6. Cubic structures in lipid-water systems based on space-filling polyhedra. The data from the monoglyceride-water cubic phases fit with the body-centered structure to the right.


4.

LARSSON A N D LUNDSTRÔM


49

p h a s e swells t o a h i g h e r m a x i m u m w a t e r content, b u t , besides t h i s , t h e r e are n o m a j o r differences i n t h e b e h a v i o r o f i o n i c l i p i d s . M u l t i c o m p o n e n t systems, w h i c h c o n t a i n d i f f e r e n t types of l i p i d s a n d w a t e r , h a v e t h e same phases as w e r e d e s c r i b e d a b o v e .

T h e phase dia-

grams are of course c o m p l i c a t e d b y t h e coexistence of m a n y phases i n e q u i l i b r i u m , b u t , a p a r t f r o m that, t h e same relations b e t w e e n s t r u c t u r e and

t h e size—shape r e l a t i o n s o f t h e h y d r o p h o b i c a n d h y d r o p h i l i c r e g i o n s


of t h e l i p i d m i x t u r e s as i n t h e b i n a r y systems seem t o b e v a l i d .

Model

systems r e l a t e d t o different diseases i n w h i c h l i q u i d c r y s t a l l i n e phases are involved were studied, particularly b y Small. A s a n example, a ternary s y s t e m i n w h i c h t h e most i m p o r t a n t c o m p o n e n t s of a t h e r i o s c l e r o t i c lesions are i n v o l v e d (10)

is i l l u s t r a t e d i n F i g u r e 7. T h e a d d i t i o n o f p h o s p h o -

l i p i d s a n d w a t e r t o t h e phase d i a g r a m s gives a p h y s i c a l basis f o r u n d e r s t a n d i n g t h e f o r m a t i o n of these p a t h o l o g i c a l l i p i d deposits ( I I ) . T h e f o r m a t i o n of g a l l stones b y c h o l e s t e r o l p r e c i p i t a t i o n i n t h e b i l e w a s examined i n a similar w a y Lipid—Protein

(12).

Interaction

A l t h o u g h t h e association b e t w e e n l i p i d s a n d p r o t e i n s is f u n d a m e n t a l i n u n d e r s t a n d i n g t h e p h y s i o l o g i c a l f u n c t i o n s of m e m b r a n e s , i n f o r m a t i o n on s u c h structures is v e r y l i m i t e d . Studies o f a f e w systems o f l i p i d s a n d g l o b u l a r p r o t e i n s i n d i c a t e that t h e p r o t e i n s t e n d to r e m a i n i n t h e i r n a t i v e form.

T h e structures c a n b e separated i n t o t w o s o m e w h a t

types.

U s u a l l y t h e l i p i d s t r u c t u r e seems t o d o m i n a t e , a n d t h e p r o t e i n

simplified

m o l e c u l e s are i n c o r p o r a t e d i n t o l i q u i d c r y s t a l l i n e structures of l i p i d s . I n o t h e r cases, t h e l i p i d m o l e c u l e s are d i s t r i b u t e d w i t h i n t h e p r o t e i n u n i t s ,

"Surface Chemistry of Biological Systems"

Figure 7. The ternary system cholesteryl oleate-cholesterol-triolein (CO^C-TO) at different temperatures (10). The darkened region corresponds to one isotropic phase whereas the remainder consists of two or three phases.



50

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Biochimica et Biophysica Acta

Figure 8. Molecular arrangements in aqueous precipitates of insulin and lecithincardiolipin (13). Left: the lipid bilayer structure used in the preparations; center: the precipitate formed by a 9/1 molar ratio of lecithin-cardiolipin; and right: the precipitate formed by a 6/4 molar ratio. a n d i t t h e n m i g h t b e p o s s i b l e to c r y s t a l l i z e t h e c o m p l e x a n d to o b t a i n a d e t a i l e d structure d e t e r m i n a t i o n . A n e x p e r i m e n t a l c o m p l i c a t i o n is t h e d i f f i c u l t y i n effecting m o l e c u l a r interaction between the components.

T h e usual technique for preparing

l i p i d - p r o t e i n phases i n a n aqueous e n v i r o n m e n t is to use c o m p o n e n t s o f o p p o s i t e charge. T h i s i n t u r n means that t h e l i p i d s h o u l d b e a d d e d to t h e p r o t e i n i n o r d e r t o o b t a i n a h o m o g e n e o u s c o m p l e x since a c o m p l e x separates w h e n a c e r t a i n c r i t i c a l h y d r o p h o b i c i t y is r e a c h e d .

If the precipitate

is p r e p a r e d i n t h e opposite w a y , t h e c o m p o s i t i o n of t h e c o m p l e x c a n v a r y since i n i t i a l l y t h e p r o t e i n m o l e c u l e c a n take u p as m a n y l i p i d m o l e c u l e s as its n e t charge, a n d this n u m b e r c a n decrease successively w i t h r e d u c t i o n i n a v a i l a b l e l i p i d m o l e c u l e s . I t is thus n o t possible to p r e p a r e l i p i d protein—water m i x t u r e s , as i n t h e case of other t e r n a r y systems, a n d t o w a i t f o r e q u i l i b r i u m . Systems w e r e p r e p a r e d that consisted of l e c i t h i n cardiolipin ( L / C L )

mixtures w i t h ( a ) a h y d r o p h o b i c protein, insulin,

and w i t h ( b ) a protein w i t h h i g h water solubility, bovine serum a l b u m i n (BSA). I n t h e i n s u l i n - L / C L complexes, i t is e v i d e n t f r o m t h e d i m e n s i o n s o f the l a m e l l a r l i q u i d c r y s t a l l i n e phase that i n s u l i n is s i m p l y associated elect r o s t a t i c a l l y w i t h t h e L / C L b i l a y e r a n d that i t replaces w a t e r (13).

The

a m o u n t s d e p e n d o n t h e n u m b e r o f charges (see F i g u r e 8 ) . T h e l i m i t o f p r o t e i n association is r e a c h e d w h e n n o m o r e surface is a v a i l a b l e at t h e b i l a y e r - w a t e r interface. B S A - L / C L complexes are also l a m e l l a r l i q u i d crystals.

There are

t w o alternative models ( F i g u r e 9 ) w h i c h c a n explain the observed lattice


4.

LARSSON A N D LUNDSTROM


51


Biochemistry

Figure 9. Molecular arrangements in aqueous precipitates of bovine serum albumin and lecithin-cardiolipin (14). Center: lipid bilayers; and left and right: two alternative structures of the precipitates based on the x-ray diffraction spacings. d i m e n s i o n s (14).

I n t h e most p r o b a b l e a l t e r n a t i v e , parts o f t h e p r o t e i n

m o l e c u l e s p e n e t r a t e t h e l i p i d l a y e r ; c o n s e q u e n t l y , there is a h y d r o p h o b i c l i p i d - p r o t e i n i n t e r a c t i o n . T h e other a l t e r n a t i v e is that t h e thickness o f t h e l i p i d b i l a y e r is so r e d u c e d that there are d i r e c t contacts b e t w e e n w a t e r and h y d r o c a r b o n chains. A l l available data o n lamellar l i p i d - w a t e r l i q u i d crystals r e v e a l s i m i l a r values f o r t h e cross-section area p e r h y d r o c a r b o n c h a i n at the b i l a y e r - w a t e r interface that are q u i t e different f r o m those f o r this s e c o n d s t r u c t u r a l a l t e r n a t i v e .

T h e probable mechanism for forma-

t i o n o f t h e c o m p l e x is therefore a n i n i t i a l electrostatic effect t h a t b r i n g s the p r o t e i n a n d l i p i d m o l e c u l e s together; s u b s e q u e n t l y , a s t r u c t u r e w i t h o p t i m u m short-range i n t e r a c t i o n b e t w e e n t h e c o m p o n e n t s is a d o p t e d . T h e association o f l i p i d s w i t h proteins i n d i l u t e aqueous solutions w a s s t u d i e d b y T a n f o r d (15).

H e i d e n t i f i e d different types of i n t e r a c t i o n

t h a t d e p e n d o n t h e n u m b e r o f associated l i p i d m o l e c u l e s .

H e also a n a -

l y z e d t h e r e l a t i o n b e t w e e n l i p i d association i n t o m i c e l l e s a n d t h e c o m p e t i n g b i n d i n g of l i p i d s to proteins. O b s e r v a t i o n s w e r e m a d e of l i p i d - p r o t e i n phases i n w h i c h t h e struct u r e is d e t e r m i n e d m a i n l y b y t h e p r o t e i n . R a m a n s p e c t r o s c o p y is a u s e f u l m e t h o d f o r structure analysis of s u c h phases. above were

T h e structures

described

a n a l y z e d successfully b y a n x-ray d i f f r a c t i o n t e c h n i q u e .

L i p i d - p r o t e i n c o m p l e x e s , h o w e v e r , are o f t e n a m o r p h o u s , a n d a l t e r n a t i v e m e t h o d s t o s t u d y t h e i r structures a r e therefore n e e d e d .

It was demon-

strated that R a m a n spectroscopy c a n b e u s e d to o b t a i n s t r u c t u r a l i n f o r m a t i o n a b o u t l i p i d - p r o t e i n i n t e r a c t i o n (16, 17).

I t is thus p o s s i b l e t o

d e t e r m i n e t h e c o n f o r m a t i o n as w e l l as t h e t y p e of e n v i r o n m e n t o f t h e l i p i d m o l e c u l e s . W i t h t h e p r o t e i n , i n t e r p r e t a t i o n is m o r e c o m p l i c a t e d . I t is u s u a l l y p o s s i b l e t o d e t e r m i n e w h e t h e r t h e c o m p l e x has t h e s a m e p r o t e i n c o n f o r m a t i o n as t h e c o m p o n e n t u s e d i n t h e p r e p a r a t i o n , o r , i f a change occurs, i t m a y b e possible to correlate i t w i t h d e n a t u r a t i o n of t h e p u r e p r o t e i n . F o r complexes f o r m e d b y l o n g - c h a i n a l k y l phosphates a n d i n s u -



52

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CRYSTALS

Figure 10. Models of complexes between long-chain phosphate esters (sodium salts of the monoesters) and insulin based on Raman spectroscopy. Shorter chains (e.g. C ) have a protein environment whereas lipid regions with disordered chains are formed when chains are longer (>cj. 10

l i n , t h e s t r u c t u r a l features i n F i g u r e 10 w e r e d e r i v e d f r o m t h e R a m a n spectra. I n s u l i n r e m a i n s i n its n a t i v e f o r m i n c o m p l e x e s f o r m e d b y p h o s phates.

I n the members

w i t h l o n g chains

(>Ci ), 4

the hydrocarbon

chains a r e s u r r o u n d e d b y other chains w h i c h indicates that t h e y f o r m t h e u s u a l t y p e o f h y d r o c a r b o n c h a i n regions. W h e n c h a i n lengths are short, ( < C i o ) , h o w e v e r , t h e p h o s p h a t e ester m o l e c u l e s h a v e a different a n d m o r e p o l a r e n v i r o n m e n t , a n d t h e y are therefore l i k e l y to b e d i s t r i b u t e d i n h y d r o p h o b i c pockets of t h e i n s u l i n m o l e c u l e s . T h e h y d r o c a r b o n chains i n b o t h structure types are d i s o r d e r e d a n d a r e m a i n l y of g a u c h e c o n f o r m a t i o n . O n d r y i n g , t h e c o m p o n e n t s separate a n d t h e chains c r y s t a l l i z e . A n i m p o r t a n t r o l e of w a t e r i n l i p i d - p r o t e i n complexes is p r o b a b l y t o f u n c t i o n as a s p a c e - f i l l i n g p o l a r m e d i u m . T h e p o s s i b i l i t y of o b t a i n i n g i n f o r m a t i o n a b o u t l i p i d - p r o t e i n i n t e r a c t i o n m a k e s R a m a n s p e c t r o s c o p y a u s e f u l t e c h n i q u e f o r s t r u c t u r a l studies of m e m b r a n e s .

A s a n i l l u s t r a t i o n of spectra r e c o r d e d f r o m b i o l o g i c a l

samples, see t h e R a m a n s p e c t r u m of a f r o g s c i a t i c n e r v e i n F i g u r e 11. T h e C-H

s t r e t c h i n g v i b r a t i o n r e g i o n is c h a r a c t e r i s t i c o f l i p i d b i l a y e r s i n a

Figure 11. Raman spectrum of a frog sciatic nerve in Ringer solution



4.

LARSSON AND LUNDSTRÔM

»! 10

20


,

,

,

,

,

30

40

50

60

70

>

Figure 12.

», 80

53

l l

90

%(w/w)H 0 2

The binary system

tetradecylamine-water

m a i n l y d i s o r d e r e d state, w h i c h agrees w i t h findings f r o m s t r u c t u r a l studies of t h i s p a r t i c u l a r m u l t i l a m e l l a r structure b y n u m e r o u s o t h e r t e c h n i q u e s . The Gel State in Lipid-Water

Systems

L i p i d - w a t e r g e l phases w e r e p r e v i o u s l y r e g a r d e d as metastable structures that are f o r m e d b e f o r e separation of w a t e r a n d l i p i d crystals w h e n t h e c o r r e s p o n d i n g l a m e l l a r l i q u i d c r y s t a l is c o o l e d . N e w i n f o r m a t i o n o n g e l phases (see

b e l o w ) reveals t h a t t h e y c a n f o r m t h e r m o d y n a m i c a l l y

stable phases w i t h v e r y s p e c i a l s t r u c t u r a l p r o p e r t i e s . T h i s c h a r a c t e r i s t i c m a k e s t h e m as i n t e r e s t i n g as t h e l a m e l l a r l i q u i d crystals f r o m a b i o l o g i c a l p o i n t of v i e w . T h e b i n a r y system of t e t r a d e c y l a m i n e - w a t e r is d i a g r a m m e d i n F i g u r e 12. T h e r e are t w o g e l phases w i t h d i f f e r e n t w a t e r l a y e r thicknesses b u t w i t h t h e same b i l a y e r structure ( 1 8 ) . T h e m o l e c u l e s are e x t e n d e d a n d v e r t i c a l i n t h e b i l a y e r i n b o t h f o r m s , a n d t h e existence of t w o f o r m s c a n b e e x p l a i n e d as f o l l o w s . T h e gel-I phase swells t o a w a t e r l a y e r t h i c k ness of a b o u t 14 A , w h i c h is a b o u t t h e same as that o b s e r v e d i n a l l k n o w n l a m e l l a r l i q u i d crystals of n e u t r a l m o l e c u l e s ( some of w h i c h c a n b e c o o l e d to g i v e a g e l p h a s e ) .

A t h i g h w a t e r c o n c e n t r a t i o n s , there are e n o u g h

w a t e r m o l e c u l e s to i o n i z e a c e r t a i n p r o p o r t i o n of the a m i n e g r o u p s , a n d , at a c r i t i c a l c o n c e n t r a t i o n , t h e electrostatic r e p u l s i o n of t h e b i l a y e r s w i l l b a l a n c e t h e v a n d e r W a a l s attractive forces. W h e n t h e p r o t o n c o n c e n t r a t i o n of t h e w a t e r present w a s c h a n g e d , this c r i t i c a l c o n c e n t r a t i o n s h i f t e d as e x p e c t e d f r o m t h e c h a n g e d c h a r g e d e n s i t y .



54

LYOTROPIC LIQUID CRYSTALS

Figure 13. The binary system cholesterol dihydrogenphosphate^ivater C h o l e s t e r o l sulfate a n d c h o l e s t e r o l m o n o p h o s p h a t e b o t h f o r m a q u e ous phases of t h e l i q u i d c r y s t a l a n d g e l t y p e . T h i s is r e m a r k a b l e since i t is b e l i e v e d t h a t t h e o c c u r r e n c e of l y o t r o p i c l i q u i d crystals r e q u i r e s a h y d r o c a r b o n r e g i o n f o r m e d b y flexible chains i n a l i q u i d l i k e state. T h e s u l fate w a s so u n s t a b l e c h e m i c a l l y that i t w a s i m p o s s i b l e t o o b t a i n phase e q u i l i b r i a at h i g h w a t e r content, b u t t h e g e n e r a l b e h a v i o r w a s t h e same as that of the d i h y d r o g e n m o n o p h o s p h a t e . T h e phase d i a g r a m of t h e syst e m c h o l e s t e r o l d i h y d r o g e n p h o s p h a t e - w a t e r is p r e s e n t e d i n F i g u r e 13 (19).

T h e crystals exist b o t h i n a n h y d r o u s f o r m a n d as a h y d r a t e . A q u e -

ous phases w i t h w a t e r exist o n l y at v e r y h i g h w a t e r contents.

Above

a b o u t 85 w t % w a t e r , a stable g e l p h a s e is f o r m e d ; i t exists as a v e r y viscous h o m o g e n e o u s phase u p to a w a t e r content of 99 w t % . A t r a n s i t i o n is o b s e r v e d w h e n t h e g e l phase is h e a t e d ; f r o m x-ray w i d e - a n g l e d i f f r a c t i o n studies, this means that there is i n c r e a s e d d i s o r d e r i n t h e l i p i d bilayers.

I n a n a l o g y w i t h other l i p i d - w a t e r systems, this t r a n s i t i o n is

therefore d e s c r i b e d as a g e l —» l i q u i d c r y s t a l t r a n s i t i o n . A l s o , i n this case i t is p o s s i b l e to e x p l a i n t h e m i n i m u m w a t e r l a y e r thickness r e q u i r e d f o r f o r m a t i o n of t h e g e l phase b y a c r i t i c a l electrostatic r e p u l s i o n t h a t results f r o m i o n i z a t i o n of some p h o s p h a t e g r o u p s . T h e l a m e l l a r s p a c i n g of a m o n o g l y c e r i d e g e l phase as a f u n c t i o n of w a t e r content is p l o t t e d i n F i g u r e 14. T h e g e l phase o f t h e n e u t r a l m o n o g l y c e r i d e has a l i p i d b i l a y e r thickness of 49.5 A , a n d i t swells to a u n i t l a y e r thickness of 64 A (20).

I f a n i o n i c a m p h i p h i l i c substance (e.g. a

soap ) is s o l u b i l i z e d i n t h e l i p i d b i l a y e r , i t is p o s s i b l e to o b t a i n a g e l phase w i t h h i g h w a t e r content.

A s w i t h t h e g e l phases w i t h infinite s w e l l i n g

that w e r e d i s c u s s e d a b o v e , there is, h o w e v e r , a m i n i m u m w a t e r l a y e r thickness w h i c h i n this m o n o g l y c e r i d e g e l is about 40 A . T h e existence of a f o r b i d d e n w a t e r l a y e r thickness range, w h i c h seems to b e a g e n e r a l p h e n o m e n o n w i t h these g e l phases, m i g h t b e r e l e v a n t t o c e l l a d h e s i o n a n d e q u i l i b r i u m distances at c e l l contact.

T h e gel repre-

sents o n e t y p e of l i p i d b i l a y e r structure that occurs i n m e m b r a n e s

(see

b e l o w ) , a n d , b e c a u s e of t h e d o m i n a n c e of n e u t r a l l i p i d m o l e c u l e s , t h e


4.

LARSSON AND LUNDSTROM


55

c h a r g e d e n s i t y c a n b e s i m i l a r to t h a t of the g e l phases d e s c r i b e d here. H y p o t h e t i c a l cells w i t h the same surface s t r u c t u r e as these g e l phases o b v i o u s l y cannot c o m e closer to e a c h other t h a n the l i m i t set b y the m i n i m u m w a t e r l a y e r thickness unless t h e surface structure is c h a n g e d .

The

presence of c o u n t e r i o n s i n t h e w a t e r m e d i u m of the g e l phases, h o w e v e r , has a d r a s t i c effect o n these distances, a n d t h e i r effect o n c e l l c o n t a c t d i s tances is also w e l l r e c o g n i z e d . A s l i t t l e as 0.3 w t % s o d i u m c h l o r i d e i n


t h e a q u e o u s m e d i u m is e n o u g h to p r e v e n t s w e l l i n g of t h e m o n o g l y c e r i d e g e l w i t h s o l u b i l i z e d i o n i c a m p h i p h i l e s a b o v e a w a t e r l a y e r thickness of d(A)

: /

/

/

V

/

/! / /

l o 2o

3o

4o

I 1.5

1

1

1

1

I 2

2.5

Figure 14. X-ray diffraction data for the gel phase of a monostearin sample at 25°C. X X at pH 5-6 the limited swelling of a monoglyceride gel; ana · · : at pH 7 , the swelling up to high water content in the presence of charged groups (sodium stéarate/monoglyceride molecular ratio, 1/60).


56

LYOTROPIC

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CRYSTALS

14 A , a n d , c o n s e q u e n t l y , there is n o m i n i m u m w a t e r l a y e r thickness.

If

the salt c o n c e n t r a t i o n is i n c r e a s e d a b o v e 2 w t % , i t is n o t p o s s i b l e to g e t a n y w a t e r at a l l b e t w e e n t h e l i p i d b i l a y e r s . Membranes and the Significance of the Hydrocarbon

Chain

Structure

A l t h o u g h n u m e r o u s m o d e l s f o r t h e structure o f m e m b r a n e s h a v e b e e n p r o p o s e d , t h e s t r u c t u r a l features w h i c h are g e n e r a l l y a c c e p t e d a t present


are r a t h e r s i m i l a r t o t h e o r i g i n a l D a n i e l l i - D a v s o n m o d e l . T h e r e is c o n v i n c i n g e v i d e n c e that t h e structure is d o m i n a t e d b y l i p i d b i l a y e r s . T h e state of o r d e r of t h e h y d r o c a r b o n chains is n o w b e i n g s t u d i e d e x t e n s i v e l y b y m a n y g r o u p s (see b e l o w ) . L e s s is k n o w n a b o u t t h e p r o t e i n s .

Besides

the proteins that are l o c a t e d o n t h e o u t s i d e a c c o r d i n g to t h e D a n i e l l i D a v s o n m o d e l , there are also p r o t e i n s that are p a r t l y b u r i e d i n t h e h y d r o p h o b i c i n t e r i o r of t h e l i p i d l a y e r ; h o w e v e r , l i t t l e is k n o w n a b o u t t h e l i p i d protein interaction. T h e t r a n s i t i o n b e t w e e n c r y s t a l l i n e a n d m e l t e d h y d r o c a r b o n chains i n m e m b r a n e s w a s s t u d i e d c a l o r i m e t r i c a l l y , a n d t h e p o s s i b i l i t y of v a r y i n g t h e f a t t y a c i d c o m p o s i t i o n a n d therefore t h e phase t r a n s i t i o n t e m p e r a t u r e i n c e r t a i n m i c r o o r g a n i s m s has p r o v i d e d v a l u a b l e i n f o r m a t i o n o n s u c h t r a n s i tions. I t is k n o w n that t h e t h e r m a l t r a n s i t i o n b e t w e e n t h e l a m e l l a r l i q u i d c r y s t a l l i n e phase a n d t h e g e l phase i n aqueous systems has a c o r r e s p o n d ence i n l i p o s o m e s , vesicles, a n d e v e n m e m b r a n e s .

T h i s t r a n s i t i o n is d e -

s c r i b e d as a t r a n s i t i o n b e t w e e n l i q u i d a n d c r y s t a l l i n e chains. I t s h o u l d b e n o t e d , h o w e v e r , t h a t t h e chains are u s u a l l y n o t t r u l y c r y s t a l l i n e i n t h e g e l state. T h e o c c u r r e n c e of a single x-ray short s p a c i n g at 4.15 A , w h i c h is u s e d f o r i d e n t i f i c a t i o n , shows that t h e chains a r e a r r a n g e d i n a h e x a g o n a l structure w i t h r o t a t i o n a l o r o s c i l l a t i o n a l d i s o r d e r of t h e c h a i n s . T h e r e are n o r e p o r t e d observations of p e r f e c t l y c r y s t a l l i n e chains i n g e l phases i n aqueous systems of c o m p l e x l i p i d s of t h e t y p e that occurs i n membranes.

T h e f a c t that t h e chains i n m e m b r a n e s a r e h i g h l y d i s o r d e r e d

e v e n w h e n t h e y a r e d e s c r i b e d as c r y s t a l l i n e h a s c e r t a i n f u n c t i o n a l aspects.

T h e c o r r e s p o n d i n g h e x a g o n a l a r r a n g e m e n t of e x t e n d e d h y d r o c a r -

b o n chains also exists i n m o n o m o l e c u l a r films at t h e a i r - w a t e r i n t e r f a c e , a n d , because of its l i q u i d l i k e p r o p e r t i e s , i t w a s classified as a l i q u i d phase i n t h e m o n o l a y e r phase n o m e n c l a t u r e b y H a r k i n s (21).

This should be

k e p t i n m i n d w h e n l a t e r a l d i f f u s i o n i n m e m b r a n e s is c o n s i d e r e d .

When

m e m b r a n e l i p i d s a r e i n a s o - c a l l e d c r y s t a l l i n e state, t h e h e x a g o n a l c h a i n arrangement

a l l o w s a c o n s i d e r a b l e degree o f l a t e r a l m o v e m e n t o f t h e

m o l e c u l e s . M a n y r e c e n t studies i n d i c a t e that a c e r t a i n p o r t i o n o f t h e h y d r o c a r b o n chains i n m e m b r a n e s are i n a c r y s t a l l i n e state (cf.

R é f . 10).

T h e segregation of chains i n t o o r d e r e d a n d d i s o r d e r e d regions a n d t h e d y n a m i c p r o p e r t i e s i n v o l v e d i n these transitions are therefore i m p o r t a n t i n u n d e r s t a n d i n g t h e structure a n d f u n c t i o n s of b i o m e m b r a n e s .


4.



57

C h o l e s t e r o l has a p r o f o u n d effect o n the t r a n s i t i o n b e t w e e n o r d e r e d and

disordered chains.

C a l o r i m e t r i c measurements of aqueous l e c i t h i n -

c h o l e s t e r o l phases r e v e a l a b r o a d e n i n g i n the t r a n s i t i o n t e m p e r a t u r e r a n g e a n d a r e d u c t i o n i n t r a n s i t i o n e n e r g y w i t h i n c r e a s i n g c h o l e s t e r o l content; at a 1:1 m o l e c u l a r r a t i o , n o t r a n s i t i o n is o b s e r v e d (22).

R a m a n spectro-

s c o p y of the same s y s t e m demonstrates t h a t the change f r o m g a u c h e to trans c o n f o r m a t i o n of t h e chains at the c h a i n c r y s t a l l i z a t i o n t e m p e r a t u r e of l e c i t h i n b i l a y e r s is successively lost w h e n c h o l e s t e r o l is a d d e d ; this c a n Downloaded by NORTH CAROLINA STATE UNIV on September 24, 2012 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch004

b e i n t e r p r e t e d as the f o r m a t i o n of a glassy state w h e n c h o l e s t e r o l is a d d e d (23).

It seems o b v i o u s that the r i g i d c h o l e s t e r o l skeleton m u s t r e d u c e

t h e m o b i l i t y of the h y d r o c a r b o n chains i n the l i q u i d state, a n d also that t h e c h o l e s t e r o l m o l e c u l e s cannot b e a c c o m m o d a t e d i n t o a n y of t h e k n o w n c l o s e - p a c k i n g arrangements of h y d r o c a r b o n chains. T h a t m o l e c u l a r separ a t i o n i n t o c h o l e s t e r o l a n d p h o s p h o l i p i d regions is not o c c u r r i n g at the c h a i n c r y s t a l l i z a t i o n t e m p e r a t u r e m u s t r e s u l t f r o m a strong association of the m o l e c u l e s of the p o l a r h e a d g r o u p s . O n e f u n c t i o n of c h o l e s t e r o l i n m e m b r a n e s m i g h t b e to affect the t r a n s i t i o n c r y s t a l l i n e τ± m e l t e d c h a i n s and

t h e segregation of the l i p i d m o l e c u l e s i n t o o r d e r e d a n d d i s o r d e r e d

regions. I n a recent x - r a y d i f f r a c t i o n s t u d y of o r i e n t e d m u l t i l a y e r s of a l e c i t h i n analog

( l-oleoyl-2-n-hexadecyl-2-deoxyglycero-3-phosphorylcholine )

L e s s l a u e r et al. (25),

by

the p a c k i n g of the h y d r o c a r b o n chains w a s d i s c u s s e d

o n the basis of the o b s e r v e d h i g h - a n g l e d i f f r a c t i o n . T h e a v a i l a b l e i n f o r m a t i o n o n h y d r o c a r b o n c h a i n p a c k i n g i n l i p i d s (cf.

Refs. 26, 27, 28),

how

ever, w a s not t a k e n i n t o c o n s i d e r a t i o n . Since t h e o r d e r i n p h o s p h o l i p i d b i layers is a p r o b l e m of g e n e r a l significance i n c o n n e c t i o n w i t h m e m b r a n e s , w e discuss the structure of the h y d r o c a r b o n r e g i o n i n this p a r t i c u l a r case. T h e o b s e r v e d short spacings ( a t 0 %

r e l a t i v e h u m i d i t y ) at 4.14 a n d

4.64 A a t t r i b u t a b l e t o c h a i n distances i n t h e p l a n e p e r p e n d i c u l a r to t h e c h a i n s w e r e i n t e r p r e t e d as c o r r e s p o n d i n g to a n o r t h o r h o m b i c s u b c e l l w i t h l a t e r a l axes of 8.26 a n d 5.59 A . It c a n b e a s s u m e d that s u c h a c e l l is n o t p o s s i b l e i n the case of c r y s t a l l i n e chains since the cross-sectional area p e r c h a i n is a b o u t 23 A . E v e n i n the loose h e x a g o n a l c h a i n 2

arrangement,

w h e r e t h e chains h a v e r o t a t i o n a l f r e e d o m , the cross-sectional area

per

c h a i n is o n l y a b o u t 19 A . S e v e n d i f f e r e n t m o d e s of p a c k i n g the c h a i n s 2

w e r e o b s e r v e d ; these w e r e d e s i g n a t e d T||, M | | , O l , O ' l , 0||, 0'||, a n d H i n accordance w i t h subcell symmetry and chain plane direction

(29).

T h e d i f f e r e n t types of c h a i n p a c k i n g c a n b e i d e n t i f i e d b y t h e i r x-ray shorts p a c i n g diffractions a l t h o u g h the r e l a t e d subcells 0 1 0 ' | I cannot be separated unambiguously (27).

a n d O ' l or 0|| a n d

T h e t w o d o m i n a n t short

s p a c i n g lines at 4.64 a n d 4.14 A d o n o t agree w i t h those of the k n o w n c h a i n - p a c k i n g subcells, b u t this i n d i c a t e s that there are t w o different types of c h a i n arrangement.

T h e m o s t c o m m o n c h a i n - p a c k i n g s u b c e l l f o r sat-



58

LYOTROPIC

LIQUID

CRYSTALS

Figure 15. Hydrocarbon chain structure of 2-oleyl-distearin based on x-ray diffraction. The short spacing data indicate that the two types of chain packing (Tu and O'II) are attributable to separation of the chains into bilayers with saturated chains and monolayers with unsaturated chains. The long spacing data indicate that the unit layer consists of three chain layers. u r a t e d n o r m a l chains is t h e t r i c l i n i c T|| w h i c h has o n e d o m i n a n t shorts p a c i n g l i n e at a b o u t 4.6 A . T|| a n d t h e o r t h o r h o m b i c p a c k i n g O l g i v e the closest c h a i n p a c k i n g . I n t r o d u c t i o n o f i r r e g u l a r i t i e s i n c h a i n structure as w e l l as i m p u r i t i e s i n t h e c h a i n layers r e s u l t i n a l a r g e r cross-sectional area p e r c h a i n , a n d t h e h e x a g o n a l a r r a n g e m e n t

( w i t h one short s p a c i n g

l i n e at a b o u t 4.15 A ) is t h e n f a v o r e d . W i t h l i p i d m o l e c u l e s that c o n t a i n b o t h saturated a n d u n s a t u r a t e d c h a i n s , a c h a i n - s o r t i n g m e c h a n i s m o f t e n results i n s e p a r a t i o n of saturated a n d u n s a t u r a t e d chains i n different layers. N o c o m p l e t e c r y s t a l structure of s u c h a c o m p l e x l i p i d is k n o w n . O n t h e basis o f x-ray p o w d e r d a t a f o r 2 - o l e y l - d i s t e a r i n , i t is possible to d e r i v e t h e p r i n c i p a l m o l e c u l a r arrangem e n t ( F i g u r e 15) (28), a n d t h e s e p a r a t i o n o f s a t u r a t e d a n d u n s a t u r a t e d chains c a n b e seen. W i t h r e g a r d to c h a i n p a c k i n g , there are thus c e r t a i n advantages to a n e x t e n d e d m o l e c u l a r c o n f o r m a t i o n of c r y s t a l l i n e m e m b r a n e p h o s p h o l i p i d s that h a v e o n e s a t u r a t e d a n d o n e u n s a t u r a t e d h y d r o carbon chain.

T h e p o s s i b i l i t y of a l i p i d p h a s e t r a n s i t i o n b e t w e e n t h e

n o r m a l b i l a y e r c o n f o r m a t i o n i n w h i c h t h e p o l a r g r o u p s f o r m t h e outer surfaces a n d a n e x t e n d e d f o r m i n w h i c h t h e p o l a r g r o u p s are i n t h e m i d d l e h a s some s u p p o r t i n t h e m o n o l a y e r b e h a v i o r of d i g l y c e r i d e s

(SO).

S h i e l d i n g of t h e p o l a r h e a d g r o u p s i n a m e m b r a n e r e g i o n w i t h l i p i d b i layers a n d l i q u i d chains so t h a t t h e i r e n v i r o n m e n t b e c o m e s less p o l a r m i g h t r e s u l t i n a phase t r a n s i t i o n i n t o t h e e x t e n d e d f o r m w i t h c r y s t a l l i n e chains. R e p e a t e d transitions of this t y p e a r e e q u i v a l e n t to flip-flop m o v e m e n t of a c e r t a i n p o r t i o n of t h e l i p i d m o l e c u l e s , a n d t r a n s p o r t of a d s o r b e d


4.



59

ions o r s m a l l m o l e c u l e s m i g h t b e a c h i e v e d i n this w a y . T h e r e is r e c e n t e x p e r i m e n t a l e v i d e n c e that t r a n s p o r t of ions across l i p i d b i l a y e r s c a n b e effected b y p h o s p h o l i p i d m o l e c u l e s Electrical

(31).

Properties

T h e e l e c t r i c a l p r o p e r t i e s of b l a c k l i p i d m e m b r a n e s

( B L M ' s ) have

p r o b a b l y b e e n s t u d i e d m o r e t h a n those of other l i p i d systems because of Downloaded by NORTH CAROLINA STATE UNIV on September 24, 2012 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch004

the great s i m i l a r i t y b e t w e e n B L M ' s a n d c e l l m e m b r a n e s .

T h e electrical

p r o p e r t i e s of B L M ' s w e r e r e v i e w e d e x t e n s i v e l y b y other authors (32, 34, 35),

33,

a n d w e s h a l l therefore d e s c r i b e the e l e c t r i c a l a n d p h y s i c a l p r o p

erties of l i p i d s w h i c h are n o t g e n e r a l l y t o u c h e d u p o n i n c o n n e c t i o n w i t h BLM's.

W e also concentrate o n those p r o p e r t i e s w h i c h are i n t i m a t e l y

r e l a t e d to the different states of o r d e r i n l i p i d systems. Electrical Properties of L i p i d - W a t e r Systems. T h e e l e c t r i c a l p r o p e r ties of different mesophases

were recently reviewed b y W i n s o r

Measurements demonstrated for example (a)

(36).

that phase transitions are

seen as changes i n e l e c t r i c a l c o n d u c t i v i t y a n d ( b ) t h a t the c o n d u c t i v i t y i n a m p h i p h i l i c mesophases m a y s h o w a l a r g e a n i s o t r o p y d e p e n d i n g o n t h e structure of the mesophase.

W e m a d e some measurements o n l a m e l l a r

a e r o s o l - O T ( A - O T ) w a t e r systems i n o r d e r to o b t a i n i n f o r m a t i o n a b o u t t h e c o n d u c t a n c e of the rather t h i n w a t e r layers i n these systems

(37).

O r i e n t e d samples w e r e p r e p a r e d b y p l a c i n g a s m a l l a m o u n t of A - O T b e t w e e n t w o glass slides a n d u s i n g t h i n g o l d w i r e s (50 μία) electrodes.

as spacers a n d

I n o r d e r to a v o i d p o l a r i z a t i o n effects, c o n d u c t i v i t y w a s meas

u r e d b y p u t t i n g a square c u r r e n t t h r o u g h the s a m p l e a n d d i s p l a y i n g the v o l t a g e d r o p across a series resistor o n a n oscilloscope. T h e v o l t a g e d r o p at t =

0 ( w h i c h corresponds t o i n f i n i t e f r e q u e n c i e s ) contains the i n f o r

m a t i o n a b o u t the t r u e c o n d u c t a n c e of the s a m p l e . T h e d a t a f r o m these measurements are p r e s e n t e d i n F i g u r e 16; the c o n d u c t i v i t y is p l o t t e d vs. 1/d

w h e r e d is t h e l a m e l l a r r e p e a t distance (38).

A r r h e n i u s plots have a

b r e a k i n g p o i n t at a c e r t a i n t e m p e r a t u r e f o r some of the samples. I t w a s c o n c l u d e d i n R e f . 37 that the c o n d u c t a n c e of the w a t e r l a m e l l a e c a n n o t b e e x p l a i n e d b y n o r m a l s o d i u m c o n d u c t a n c e a n d that the n a r r o w w a t e r channels affect either the m o b i l i t y of s o d i u m ions a n d / o r the n u m b e r of free s o d i u m ions.

O t h e r c o n d u c t i o n m e c h a n i s m s (e.g.

proton jumping

a l o n g a n o r d e r e d w a t e r structure ) are also p o s s i b i l e . S o m e measurements w e r e also m a d e o n l a m e l l a r t e t r a d e c y l a m i n e w a t e r systems (see

section o n T h e G e l State i n L i p i d - W a t e r Systems)

w i t h 8 8 - 9 5 % w a t e r u s i n g the m e t h o d d e s c r i b e d i n R e f . 37.

I n this case,

the w a t e r l a y e r w a s a b o u t 1 5 0 0 - 1 6 0 0 A t h i c k , a n d n o c o u n t e r i o n s w e r e present.

C o n d u c t i v i t y i n the l a m e l l a r r e g i o n is therefore e x p e c t e d to b e

c a u s e d b y p r o t o n s j u m p i n g b e t w e e n the a m i n e g r o u p s of the l i p i d . F i g u r e



60

LYOTROPIC

g g

Figure 16. Plot of the conductivity of aerosol-OT (A-OT) samples vs. I / d where d is the lameUar repeat distance (38). Samples with 65.1 % A-OT were sometimes bistable, switching between high and low conductivity.

LIQUID

CRYSTALS

ι.

Ο ΟΟ (LAMELLAR REPEAT DISTANCE)"

1

[I/A]

Journal of Colloid and Interface Science

17, a n A r r h e n i u s p l o t of the n a t u r a l l o g a r i t h m of the c o n d u c t i v i t y vs.

1/T

f o r t w o samples, i n d i c a t e s w h a t m i g h t h a p p e n at a phase t r a n s i t i o n .

The

c o n d u c t i v i t y changes as w e l l as the slope of the 1/T

plot. Furthermore,

it w a s o b s e r v e d that w h e n the t e m p e r a t u r e w a s i n c r e a s e d , the c o n d u c t i v i t y first decreased r a p i d l y at t h e p h a s e t r a n s i t i o n a n d t h e n i n c r e a s e d to the v a l u e i n d i c a t e d i n t h e a l l samples tested.

figure.

C o n d u c t i v i t y b e h a v i o r w a s s i m i l a r for

R o o m t e m p e r a t u r e c o n d u c t i v i t y w a s also a b o u t t h e

same (0.005-0.008 1 / O m ) ; a c t i v a t i o n energy of c o n d u c t i v i t y w a s a b o u t 0.09-0.12 e V b e l o w t h e phase t r a n s i t i o n a n d i n the o r d e r of 0.2-0.4 e V a b o v e the phase t r a n s i t i o n . If w e assume that the c o n d u c t i v i t y b e f o r e t h e phase t r a n s i t i o n is a t t r i b u t a b l e to p r o t o n s j u m p i n g a l o n g the l i p i d layers a n d t h a t t h e n u m b e r of protons p e r l i p i d l a y e r is g i v e n b y the n u m b e r of i o n i z e d a m i n e groups (see

a b o v e ) , t h e n the m o b i l i t y of the p r o t o n s is a b o u t o n e - t e n t h that of

p r o t o n s i n w a t e r a n d the a c t i v a t i o n energy is s i m i l a r to that of protons i n w a t e r ( i n the same t e m p e r a t u r e r e g i o n ) . A n o t h e r p o s s i b i l i t y is t h a t o n l y a b o u t 1 0 % of the i o n i z e d a m i n e groups act as p r o t o n d o n o r s . If this i n t e r p r e t a t i o n is correct, t h e n p r o t o n s m a y j u m p a l o n g the l i p i d - w a t e r i n t e r face. C o m p a r i n g these findings w i t h those f o r A - O T , w e find that w a t e r structure b e c o m e s i m p o r t a n t i n d e t e r m i n i n g c o n d u c t i v i t y w h e n the t h i c k ness of the w a t e r l a y e r is s m a l l . E l e c t r i c a l P r o p e r t i e s o f L i p i d M u l t i l a y e r s . B y the L a n g m u i r - B l o d gett t e c h n i q u e (39, 40),

i t is p o s s i b l e to o b t a i n w e l l o r d e r e d m u l t i l a y e r s


4.

L A R S S O N A N D LUNDSTRÔM

61


of l i p i d s o n s o l i d s u p p o r t s l i k e glass, m e t a l s , a n d s e m i c o n d u c t o r s . A n u m b e r o f p h y s i c a l investigations o f these types o f films h a v e b e e n some of w h i c h are r e v i e w e d b r i e f l y here.

B l o d g e t t (41)

made,

u s e d this t e c h -

n i q u e t o p r e p a r e o r g a n i c anti-reflection coatings o n glass slides. M a n y of the p h y s i c a l investigations o f f a t t y a c i d m u l t i l a y e r s w e r e r e v i e w e d b y K u h n a n d M o b i u s (42).

K u h n a n d c o - w o r k e r s i n recent years u s e d t h e

L a n g m u i r - B l o d g e t t technique i n very ingenious ways (they incorporated layers of d y e m o l e c u l e s i n f a t t y a c i d m u l t i l a y e r s ) to s t u d y , a m o n g other Downloaded by NORTH CAROLINA STATE UNIV on September 24, 2012 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch004

things, h o w e x c i t a t i o n e n e r g y is t r a n s f e r r e d b e t w e e n m o l e c u l e s (42, M a n n et al. (44)

43).

i n v e s t i g a t e d t h e t u n n e l i n g of electrons t h r o u g h f a t t y

a c i d m o n o l a y e r s ; t h e y f o u n d that t h e e x p e r i m e n t a l d a t a a g r e e d w i t h t h e e x p o n e n t i a l decrease i n c o n d u c t a n c e vs. thickness that is p r e d i c t e d b y tunnel theory. A n u m b e r of o t h e r investigations of t h e e l e c t r i c a l p r o p e r t i e s o f l i p i d m o n o - a n d m u l t i l a y e r s w e r e p u b l i s h e d r e c e n t l y . I t is o b v i o u s f r o m studies of t h e c o n d u c t i v i t y of t h i n L a n g m u i r films that t h e e l e c t r i c a l p r o p e r t i e s o f m e t a l - o r g a n i c l a y e r - m e t a l structures c a n b e d e s c r i b e d b y w e l l k n o w n concepts f r o m s o l i d state p h y s i c s , l i k e S c h o t t k y i n j e c t i o n o f electrons f r o m the m e t a l i n t o t h e l i p i d

film

(45, 46, 47).

M e a s u r e m e n t s of d i e l e c t r i c

losses i n c a l c i u m stéarate a n d b e h e n a t e i n d i c a t e t h e p r e s e n c e o f m o v e ments of d i p o l e s i n t h e o r g a n i c m o l e c u l e s , a n d loss peaks c o n n e c t e d w i t h t h e a m o r p h o u s a n d c r y s t a l l i n e parts o f t h e layers w e r e i d e n t i f i e d

-i

3.0

1

3.1

1

3.2

ι/τ no- /*]

3.3

r—'

3.4

(48).

H

3.5

3

Figure 17. Arrhenius plot of conductivity 1/T for a tetradecylaminewater system. Conductivity was measured during a slow increase in temperature. Φ: 92% water; and O: 88% water.


62


960

LYOTROPIC LIQUID CRYSTALS

-10 volts seconds 120 30 10

10 volts

Figure 18. C(V) curves for a metal-Ba stéarate semiconductor structure (multilayer thickness, ~1000 A). Capacitance levels are indicated; the max/min ratio depends on the parameters of the structure, and the absolute values of the capacitance depend of course on the area of the metal contact (a mercury probe). Different areas of the same sample were used to obtain the curves in the top and bottom figures. : an ideal, theoretical C(V) curve. a: curve E: the experimental equilibrium C(V) curve that was obtained when the structure was kept for a long time without voltage being applied. The curves displaced to the left of Curve Ε were obtained by applying negative voltage (10 V) pulses (metal negative) across the structure for the indicated periods of time. The curves displaced to right of Curve Ε were obtained by applying positive voltage (10 V) pulses. These curves indicate that, during negative voltage pulses, holes are injected into the fatty acid whereas electrons are injected during positive voltage pulses; the shift along the voltage axis from Curve Ε is a direct measure of the amount of charge injected. P r o c a r i o n e a n d K a u f m a n n (49)

studied the electrical properties of

p h o s p h o l i p i d b i l a y e r s b e t w e e n m e t a l contacts.

T h e y o b s e r v e d , f o r ex

a m p l e , i r r e g u l a r i t i e s i n c u r r e n t a n d c a p a c i t a n c e vs. t e m p e r a t u r e

data

w h i c h m a y b e t h e r e s u l t o f p h a s e transitions i n t h e l i p i d b i l a y e r .

They

also o b s e r v e d t h a t b o t h t e m p e r a t u r e - i n d e p e n d e n t ( t u n n e l i n g ) a n d t e m p e r a t u r e - d e p e n d e n t c o n d u c t i o n processes w i t h a n a c t i v a t i o n e n e r g y of 0.65 e V w e r e i m p o r t a n t . R e c e n t l y t h e s o - c a l l e d C ( V ) t e c h n i q u e w a s i n t r o d u c e d to t h e s t u d y of l i p i d s ( 5 0 , 5 1 , 5 2 ) . T h i s is a m e t h o d b o r r o w e d f r o m s e m i c o n d u c t o r



4.


Phases in Biologicdl Model Systems

63

surface studies, a n d i t enables one to s t u d y t h e a m o u n t of c h a r g e s t o r e d or i n t r o d u c e d i n a n i n s u l a t o r o n t o p o f a s e m i c o n d u c t o r .

T h e technique

is d e s c r i b e d i n R e f s . 50 a n d 52. I n p r i n c i p l e , t h e c a p a c i t a n c e of a m e t a l o r g a n i c l a y e r s e m i c o n d u c t o r structure is m e a s u r e d as a f u n c t i o n of t h e p o t e n t i a l difference b e t w e e n t h e m e t a l a n d t h e s e m i c o n d u c t o r .

A theo

r e t i c a l i d e a l c u r v e is r e p r e s e n t e d b y the d a s h e d l i n e i n F i g u r e 18a. F o r an η-type s e m i c o n d u c t o r , a p o s i t i v e p o t e n t i a l o n the m e t a l electrode at tracts electrons to t h e s e m i c o n d u c t o r surface, a n d w e m e a s u r e the c a p a c i t a n c e of t h e i n s u l a t o r itself. I f t h e p o t e n t i a l b e c o m e s n e g a t i v e , a r e g i o n d e p l e t e d of electrons is c r e a t e d at t h e s e m i c o n d u c t o r surface; w e measure the i n s u l a t o r c a p a c i t a n c e i n series w i t h t h e c a p a c i t a n c e of t h e d e p l e t e d r e g i o n , a n d the t o t a l c a p a c i t a n c e decreases. A t l a r g e n e g a t i v e p o t e n t i a l s , the w i d t h of t h e d e p l e t e d r e g i o n b e c o m e s constant, i n d e p e n d e n t of v o l t age, a n d w e m e a s u r e a constant ( s m a l l ) c a p a c i t a n c e .

I f there are charges

i n t h e i n s u l a t o r , h o w e v e r , t h e c a p a c i t a n c e c u r v e is d i s p l a c e d i n v o l t a g e . I t is easy t o u n d e r s t a n d t h a t p o s i t i v e charges i n t h e i n s u l a t o r m e a n that a l a r g e r n e g a t i v e p o t e n t i a l c a n b e a p p l i e d o n t h e m e t a l b e f o r e elec trons are r e p e l l e d f r o m t h e s e m i c o n d u c t o r surface, a n d therefore t h e C(V)

c u r v e is d i s p l a c e d to t h e left. O b v i o u s l y , n e g a t i v e charges i n t h e

insulator displace the curve to the right. B y m a k i n g metal-stearate henate, e t c . ) s e m i c o n d u c t o r structures

(be-

( w i t h a n i n s u l a t o r thickness o f


64

L Y O T R O P I C LIQUID

CRYSTALS

5 0 0 - 1 0 0 A ) b y the L a n g m u i r - B l o d g e t t t e c h n i q u e , w e w e r e a b l e to d e m o n strate that electrons a n d holes c a n b e i n t r o d u c e d i n t o f a t t y a c i d m u l t i layers b y a p p l y i n g large v o l t a g e pulses across the structure (50, 5 2 ) . curves i n F i g u r e 18a w e r e o b t a i n e d f r o m s u c h a n e x p e r i m e n t :

The

negative

v o l t a g e pulses i n t r o d u c e d p o s i t i v e charges ( h o l e s ) i n the f a t t y a c i d m u l t i layers a n d p o s i t i v e v o l t a g e pulses i n t r o d u c e d n e g a t i v e charges ( electrons ) i n the m u l t i l a y e r s . M e a s u r e m e n t s at d i f f e r e n t temperatures r e v e a l e d t h a t the i n j e c t i o n Downloaded by NORTH CAROLINA STATE UNIV on September 24, 2012 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch004

process w a s t e m p e r a t u r e - d e p e n d e n t w i t h a n a c t i v a t i o n energy of a b o u t 0.65

eV

(52).

measurements

T h i s a c t i v a t i o n e n e r g y is consistent w i t h c o n d u c t i v i t y o n m e t a l - f a t t y a c i d - m e t a l structures

(46,

w i t h t h e a c t i v a t i o n e n e r g y of t h e ( h i g h t e m p e r a t u r e ) p h o s p h o l i p i d b i l a y e r s (49).

47)

a n d also

conductance

of

T h e a c t i v a t i o n e n e r g y m a y b e the d i f f e r e n c e

b e t w e e n the s e m i c o n d u c t o r F e r m i l e v e l a n d t h e v a l e n c e ( a n d c o n d u c t i o n ) b a n d edges i n the f a t t y a c i d , or i t m a y b e the e n e r g y difference b e t w e e n some l o c a l i z e d e l e c t r o n i c states i n the f o r b i d d e n b a n d gap of the f a t t y a c i d a n d the b a n d edges. W e also s t u d i e d (50, 52) h o w l o n g the i n t r o d u c e d charges s t a y e d i n the f a t t y a c i d . It w a s c o n c l u d e d t h a t m e m o r y times i n the o r d e r of tenths of seconds c o u l d b e o b t a i n e d at r o o m t e m p e r a t u r e

(Figure 18b).

No

significant d i f f e r e n c e b e t w e e n f a t t y acids of different h y d r o c a r b o n c h a i n lengths w a s o b s e r v e d i n o u r e x p e r i m e n t s . T h e e x p e r i m e n t a l results c o m p a r e f a v o r a b l y w i t h s i m p l e t h e o r e t i c a l m o d e l s s i m i l a r to those a p p l i e d to a n e w t y p e of s e m i c o n d u c t o r m e m o r y d e v i c e ( 5 3 ) . M o r e i n t e r e s t i n g , h o w e v e r , w a s t h e f a c t that m i n o r changes i n c o m p o s i t i o n a n d s t r u c t u r e of the films c h a n g e d c h a r g e storage

properties

m a r k e d l y . I n t r o d u c t i o n of a s m a l l a m o u n t of e r u c i c a c i d , a n u n s a t u r a t e d f a t t y a c i d , i n t o a b e h e n i c a c i d m u l t i l a y e r c h a n g e d the a c t i v a t i o n e n e r g y to 0.6 e V w h i c h c h a n g e d s i g n i f i c a n t l y the s p e e d of c h a r g e i n j e c t i o n . with

fluorine-substituted

Lipids

h y d r o g e n of t h e m e t h y l g r o u p c o u l d n o t store

c h a r g e to the same extent as t h e p u r e l i p i d s . F u r t h e r m o r e , l i p i d - s o l u b l e gases l i k e ether, i o d i n e , a n d h a l o t h a n e c h a n g e d t h e charge storage p r o p erties m a r k e d l y ; at sufficiently l a r g e c o n c e n t r a t i o n s , c h a r g e storage a b i l i t y almost d i s a p p e a r e d . W a t e r v a p o r , o n the other h a n d , s e e m e d o n l y to increase s o m e w h a t t h e s p e e d of c h a r g e i n j e c t i o n . T h e s e findings suggest that the c h a r g e storage a b i l i t y of f a t t y a c i d m u l t i l a y e r s d e p e n d s o n t h e m e t h y l e n d g r o u p s of the h y d r o c a r b o n c h a i n s , that is, o n the m e t h y l g a p b e t w e e n t w o f a t t y a c i d layers. T h i s also s u g gests t h a t c o n d u c t i v i t y of c r y s t a l l i n e f a t t y acids p r o b a b l y s h o u l d b e greater i n a d i r e c t i o n p e r p e n d i c u l a r to the h y d r o c a r b o n chains t h a n i n a d i r e c t i o n p a r a l l e l to the chains, t h e c o n d u c t i n g charges b e i n g t r a p p e d at the m e t h y l g a p .

A l t h o u g h i t is r a t h e r easy to p r e p a r e t h i n c r y s t a l l i n e

layers f o r measurements a l o n g t h e h y d r o c a r b o n chains, i t is m o r e d i f f i c u l t


4.


Phases in Biological

Model Systems

to p r e p a r e a n d c o n t a c t layers f o r p e r p e n d i c u l a r measurements.

65 I n some

p r e l i m i n a r y studies ( 5 4 ) , w e m a d e t h i n layers b e t w e e n glass slides b y m e l t i n g s m a l l a m o u n t s o f f a t t y acids b e t w e e n t h e m .

B y slowly heating

the s a m p l e f r o m o n e e n d t o t h e other m a n y times ( z o n e r e f i n i n g ) , a r a t h e r good crystalline layer was obtained w i t h the molecules oriented perpendic u l a r to t h e glass slides. G o l d w i r e s w e r e u s e d as spacers a n d contacts f o r measurements p e r p e n d i c u l a r to t h e m o l e c u l e s , e v a p o r a t e d g o l d stripes o n t h e glass slides f o r measurements

along the molecules.

T h e experi-


ments d e m o n s t r a t e d that t h e c o n d u c t i v i t y p e r p e n d i c u l a r t o t h e h y d r o c a r b o n chains w a s a b o u t 2 0 - 1 0 0 times greater t h a n t h a t a l o n g t h e chains b e l o w the m e l t i n g point a n d that the t w o conductivities became equal ( t h e p e r p e n d i c u l a r decreased as the p a r a l l e l i n c r e a s e d )

at t h e m e l t i n g

p o i n t of t h e stearic a c i d . T h e a c t i v a t i o n energy of t h e c o n d u c t i v i t y w a s a b o u t the same p a r a l l e l a n d p e r p e n d i c u l a r to the chains ( 0 . 9 - 1 e V ), a n d i t d i d n o t c h a n g e a p p r e c i a b l y o n m e l t i n g . T h e c o n d u c t i v i t y a l o n g the (unmolten) (~10~

1 2

chains w a s of t h e same o r d e r as t h a t of L a n g m u i r

l/Ωπι

at r o o m t e m p e r a t u r e ) .

E v e n if the prepared

films

samples

w e r e f a r f r o m perfect, the findings c e r t a i n l y i n d i c a t e that t h e c o n d u c t i v i t y p e r p e n d i c u l a r to t h e h y d r o c a r b o n chains is greater t h a n that p a r a l l e l to the chains. I n the p e r p e n d i c u l a r d i r e c t i o n , there m a y b e c o n d u c t i o n a l o n g the p o l a r e n d g r o u p o r b y change transfer b e t w e e n t h e h y d r o c a r b o n chains. P r o t o n c o n d u c t i v i t y is also a p o s s i b i l i t y . N o gas e v a l u a t i o n a n d n o i n crease i n resistance f r o m p o l a r i z a t i o n effects w e r e n o t e d , h o w e v e r , i n o u r p r e l i m i n a r y experiments.

R e c e n t l y w e h a v e started some studies o n elec-

t r o l y t e - l i p i d s e m i c o n d u c t o r structures.

W e observed, for example, inter

esting differences i n the effect of d i v a l e n t ions o n different l i p i d m o n o layers ( 5 5 ) . W e b e l i e v e t h a t t h e t y p e of investigations t h a t are o u t l i n e d b r i e f l y above p r o v i d e interesting n e w information o n the properties of lipids f r o m b o t h t e c h n i c a l a n d b i o p h y s i c a l p o i n t s of v i e w .

Measurements on

e l e c t r o l y t e - l i p i d s e m i c o n d u c t o r systems s h o u l d p r o v i d e u s e f u l i n f o r m a t i o n c o m p l e m e n t a r y t o that o b t a i n e d f r o m B L M investigations.

Further

m o r e , t h e gas s e n s i t i v i t y of t h e e l e c t r i c a l properties of l i p i d s c o u l d b e u t i l i z e d i n p r a c t i c a l devices. Biological Implications of Structural and Electrical Properties of Lipids.

I t is r a t h e r o b v i o u s that t h e structure of l i p i d s is v e r y i m p o r t a n t

i n c o n n e c t i o n w i t h t h e f u n c t i o n of l i v i n g cells since m o s t p h y s i o l o g i c a l processes o c c u r i n l i p i d e n v i r o n m e n t . T h e r e is, f o r e x a m p l e , e v i d e n c e t h a t l i p i d - p r o t e i n complexes are necessary f o r t h e p r o p e r f u n c t i o n i n g of m i t o c h o n d r i a ( 5 6 ) . A l t h o u g h l i p i d s are m o s t i m p o r t a n t i n p r o v i d i n g a suit a b l e m a t e r i a l f o r f u n c t i o n a l complexes ( i o n i c channels, e l e c t r o n t r a n s p o r t systems, r e c e p t o r u n i t s , e t c . ) , t h e i r o w n p h y s i c a l p r o p e r t i e s are c e r t a i n l y


66

L Y O T R O P I C LIQUID

n o t u n r e l a t e d to t h e f u n c t i o n of l i v i n g systems.

CRYSTALS

Furthermore, different

l i p i d systems m a y serve as m o d e l s f r o m w h i c h extrapolations to a c o m p l e t e l y different s i t u a t i o n c a n b e m a d e . T h e t h i n w a t e r channels i n cert a i n l a m e l l a r systems m a y b e s i m i l a r to the i o n i c channels of n e r v e m e m branes w h e r e the c h a n n e l s m a y a c t u a l l y b e c r e a t e d b y p r o t e i n m o l e c u l e s . P r o t o n c o n d u c t i v i t y a l o n g l i p i d - w a t e r interfaces, i n t e r e s t i n g as s u c h , m a y also m o d e l the p r o t o n c o n d u c t i v i t y of c e r t a i n proteins or p o l y p e p t i d e s w i t h p r o p e r s i d e g r o u p s . T h e e l e c t r o n i c p r o p e r t i e s of l i p i d s m a y b e i n Downloaded by NORTH CAROLINA STATE UNIV on September 24, 2012 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch004

teresting i n c o n n e c t i o n w i t h e l e c t r o n donors a n d e l e c t r o n acceptors o n o p p o s i t e sides of a m e m b r a n e .

Is it, f o r e x a m p l e , p o s s i b l e to t r a n s p o r t

electrons across a l i p i d , t h e r e b y c r e a t i n g a difference i n p o t e n t i a l ? Interesting i n this c o n n e c t i o n is the f a c t t h a t electrons m a y t u n n e l against a p o t e n t i a l g r a d i e n t as l o n g as s u i t a b l e e n e r g y levels are present.

A r e the

e l e c t r o n i c traps that are present i n a m e m b r a n e c a u s e d b y the l i p i d s or b y m a c r o m o l e c u l e s w i t h p r o p e r t i e s s i m i l a r t o those of the l i p i d s w i t h r e g a r d to c h a r g e storage? A n d , i f so, is i t n o t c o n c e i v a b l e t h a t m a n y of the t r i g g e r i n g f u n c t i o n s of c e l l m e m b r a n e s ( n e r v e w a l l s , n e r v e e n d i n g s , r e t i n a l m e m b r a n e , s m e l l r e c e p t o r sites, etc. ) h a v e a n e l e c t r o n i c o r i g i n ? It is n o t p o s s i b l e that c h a r g e storage is a short t e r m m e m o r y u s e d i n some of the processes i n o u r b r a i n ? T h e s e are just a f e w of the questions w h i c h are i n s p i r e d b y the e l e c t r i c a l p r o p e r t i e s of l i p i d s a n d l i p i d - p r o t e i n c o m p l e x e s . T h e first a s s u m p t i o n of e l e c t r o n i c processes i n l i v i n g systems w a s p r o b a b l y that of S z e n t - G y o r g y i a b o u t s e m i c o n d u c t i o n i n proteins ( 5 7 ) , a n d h e a n d others d e m o n s t r a t e d a f t e r w a r d s t h a t p r o t e i n s m a y b e a p a r t of

charge

transfer c o m p l e x e s a n d t h a t t h e i r c o n d u c t i v i t y increases w h e n t h e y are treated suitably ( d o p e d )

(58).

R o s e n b e r g et al. d i s c u s s e d s e m i c o n d u c -

t i o n i n p r o t e i n s i n some d e t a i l (60).

T h e o b s e r v e d p h o t o s e n s i t i v i t y of

B L M ' s that c o n t a i n c h l o r o p h y l l (59, 61, 62) i n d i c a t e s that e l e c t r o n i c p r o c esses o c c u r across the m e m b r a n e .

T h e e v i d e n c e f o r e l e c t r o n i c processes

i n l i v i n g systems is thus v e r y great, a n d of course e l e c t r o n i c processes are k n o w n to take p l a c e i n the chloroplasts a n d i n the m i t o c h o n d r i a . W e r e c e n t l y p r o p o s e d a c o m p l e t e l y e l e c t r o n i c m o d e l f o r the e x c i t a b i l i t y of n e r v e m e m b r a n e s t h a t is b a s e d o n t h e a s s u m p t i o n o f e l e c t r o n - d o n a t i n g , e l e c t r o n - a c c e p t i n g , a n d e l e c t r o n - s t o r i n g p r o p e r t i e s of m a c r o m o l e cules or of p r o t e i n - l i p i d complexes w h i c h constitute the i o n i c channels of t h e n e r v e m e m b r a n e (63). concepts

with

easily

T h i s m o d e l , w h i c h is b a s e d o n s i m p l e p h y s i c a l

d e f i n e d parameters,

reproduces

the

empirical

H o d k g i n - H u x l e y equations rather w e l l a n d also explains h o w different types of d r u g s m a y w o r k o n nerves. T h e m o d e l is easily e x t e n d e d to o t h e r e x c i t a b l e c o m p l e x e s l i k e the r e c e p t o r p r o t e i n c o m p l e x at n e r v e synapses a n d the r o d o p s i n m o l e c u l e s i n the r e t i n a . N o r is i t i n c o n c e i v a b l e to b u i l d a m o d e l f o r the f u n c t i o n of s m e l l t h a t is b a s e d o n e l e c t r o n i c t r i g g e r i n g of i o n i c channels w h i c h are affected b y m o l e c u l e s a d s o r b e d onto or d i s -


4.



s o l v e d i n t h e l i p i d s of a l l m e m b r a n e s .

67

T h i s r a t h e r s p e c u l a t i v e section is

o n l y to r e m i n d us that, e v e n i f m o s t o f t h e l i v i n g processes d o o c c u r i n a w a t e r y e n v i r o n m e n t w i t h ions s e r v i n g as i m p o r t a n t c h a r g e carriers, t h e e l e c t r o n i c p r o p e r t i e s of m a c r o m o l e c u l e s a n d l i p i d b i l a y e r s m a y b e i m p o r tant i n w a y s n o t n o r m a l l y t h o u g h t of. I t is needless t o say t h a t t h e elect r o n i c p r o p e r t i e s are i n t i m a t e l y r e l a t e d t o t h e s t r u c t u r a l properties of t h e macromolecules, the lipids, a n d their complexes.


W e s h a l l e n d this s e c t i o n b y s h o w i n g h o w v e r y g e n e r a l p h y s i c a l phenomena m a y explain a n experimental observation o n nerve m e m branes w h i c h , t o o u r k n o w l e d g e , w a s n o t e x p l a i n e d satisfactorily b e f o r e (64).

W h e n w e s t u d y t h e c u r r e n t t h r o u g h a n e r v e m e m b r a n e at a p o t e n -

t i a l difference so t h a t t h e i o n i c channels are o p e n , w e find a n o i s e i n t h e c u r r e n t w h o s e p o w e r s p e c t r u m is p r o p o r t i o n a l to 1/f w h e r e / is t h e frequency.

T h i s 1/f

noise, w h i c h c a n b e o b s e r v e d d o w n to v e r y l o w f r e -

quencies ( ^ 10 H z ) , is r a t h e r p u z z l i n g since i t is k n o w n that ( a ) the gati n g processes ( o p e n i n g of i o n i c c h a n n e l s ) h a v e t i m e constants i n t h e o r d e r of 0 . 1 - 1 msec, a n d ( b ) t h e ions flowing t h r o u g h a n o p e n c h a n n e l s p e n d a b o u t 0.1 msec i n the c h a n n e l . H e n c e , these c o n d i t i o n s d o n o t p r o d u c e the l a r g e t i m e constants that are necessary i n o r d e r to e x p l a i n 1/f

noise

d o w n to 10 H z ( t i m e constants a b o u t 0 . 1 - 1 s e c ) . T h e 1/f

noise is o f t e n a t t r i b u t e d t o t h e p o t a s s i u m c u r r e n t t h r o u g h

the n e r v e m e m b r a n e , a n d i t is t h e n r e l a t e d t o t h e n u m b e r of ions t h a t pass a p o t a s s i u m c h a n n e l p e r u n i t t i m e , i.e. t h e c o n d u c t a n c e of t h e ( o p e n ) c h a n n e l . It w a s r e c e n t l y d e m o n s t r a t e d that 1/f noise c a n b e e x p l a i n e d b y s i m p l e assumptions a b o u t t h e p r o p e r t i e s of t h e l i p i d s a n d t h e l i p i d c h a n n e l m o l e c u l e complexes i n nerve m e m b r a n e s ( 6 5 , 6 6 ) . F i r s t of a l l , w e assume that t h e l i p i d b i l a y e r o r t h e h y d r o c a r b o n chains w i t h i n i t h a v e a l i q u i d crystalline property i n that the free energy of the bilayer depends o n t h e g r a d i e n t of t h e d i r e c t i o n of t h e m o l e c u l e s o r segments of t h e m o l e cules.

S e c o n d l y , t h e m o v e m e n t of t h e chains ( o r segments of t h e m ) is

d e s c r i b e d b y a d i f f u s i o n l i k e e q u a t i o n that is u s e d to d e s c r i b e

fluctuations

i n l i q u i d crystals. F i n a l l y , w e assume that t h e fluctuations i n t h e d i r e c t i o n of t h e l i p i d m o l e c u l e s ( o r segments of t h e m ) close t o a n i o n i c c h a n n e l m o d u l a t e t h e c o n d u c t i v i t y of t h e c h a n n e l . F i g u r e 19 is a v e r y s i m p l e d i a g r a m of this m o d e l . T h e m o v e m e n t s of the l i p i d m o l e c u l e s m a y , f o r e x a m p l e , affect the c h a n n e l g e o m e t r i c a l l y , o r t h e y m a y m o v e c h a r g e d g r o u p s , t h e r e b y c h a n g i n g t h e electrostatic p o t e n t i a l seen b y ions m o v i n g t h r o u g h t h e c h a n n e l . B y u s i n g these rather s i m p l e assumptions, i t w a s p o s s i b l e t o d e t e r m i n e 1/f noise d o w n to l o w frequencies. T h e c a l c u l a t i o n of t h e noise s p e c t r u m is f o u n d i n Refs. 65 a n d 67. O u r p h y s i c a l m o d e l is a n a l t e r n a t i v e t o m o d e l s b a s e d o n c h a n n e l statistics; i t p r o v i d e s a basis f o r f u r t h e r studies of 1/f noise i n n e r v e m e m b r a n e s .

A l t h o u g h i t has a l l t h e g e n e r a l features



68

LYOTROPIC

LIQUID

CRYSTALS

(a)

Figure 19. Schematic of how an ionic channel in a nerve membrane can be affected by the direction of the lipid molecules (a) or of their segments (b). The direction of the molecules can determine the geometrical configuration of the channel and/or change the position of the charged groups that affect the potential seen by a positive ion, thereby affecting the mobility of a channel. o f t h e e x p e r i m e n t a l l y o b s e r v e d noise, t h e r e are several p o s s i b i l i t i e s w h i c h w e r e n o t a c c o u n t e d f o r . F o r e x a m p l e , d o t h e i o n i c channels ( a n d t h e c u r r e n t t h r o u g h t h e m ) affect t h e m o v e m e n t s o f t h e l i p i d m o l e c u l e s ? I n o u r s i m p l e m o d e l , t h e noise p o w e r is p r o p o r t i o n a l to IK w h e r e I 2

s i u m current.

K

is t h e potas-

I f t h e p o t a s s i u m c u r r e n t consists of t h e s u m of t w o i n d e -

p e n d e n t currents, one o u t w a r d a n d o n e i n w a r d ( b e c a u s e o f active transp o r t ) , d o w e get a 1/f noise e v e n w h e n I

K

=

0? T h e tentative a n s w e r i s

yes, b u t this r e m a i n s o n e o f t h e i n t e r e s t i n g questions w h i c h s h o u l d b e a n s w e r e d b y f u t u r e studies. A n i n t e r e s t i n g e x p e r i m e n t i n c o n n e c t i o n w i t h 1/f noise i n n e r v e m e m branes w o u l d b e to s t u d y t h e noise i n c u r r e n t t h r o u g h B L M ' s that c o n t a i n a n i o n o p h o r e l i k e g r a m i c i d i n . I t is k n o w n that t h e c o n d u c t i v i t y of B L M ' s c o n t a i n i n g g r a m i c i d i n does n o t c h a n g e a p p r e c i a b l y at t h e m e l t i n g p o i n t of t h e h y d r o c a r b o n chains of t h e B L M , w h i c h i n d i c a t e s that g r a m i c i d i n acts as a n i o n i c c h a n n e l ( 6 8 ) . I f t h e m o v e m e n t of t h e h y d r o c a r b o n c h a i n s m o d u l a t e s t h e c o n d u c t i v i t y of t h e g r a m i c i d i n i o n i c c h a n n e l , w e w o u l d exp e c t , h o w e v e r , t h e m a g n i t u d e of t h e noise to increase at t h e m e l t i n g o f the h y d r o c a r b o n chains.


4.



69

Final Remarks W e h a v e s u m m a r i z e d some o f t h e present k n o w l e d g e o f t h e s t r u c t u r a l a n d p h y s i c a l p r o p e r t i e s of l i q u i d c r y s t a l l i n e systems. W e a r e a w a r e t h a t a n u m b e r o f i n t e r e s t i n g findings a n d m e t h o d s o f investigations w e r e n o t mentioned.

T h e m a i n p u r p o s e o f this c o m m u n i c a t i o n h o w e v e r , w a s t o

discuss b r i e f l y o u r o w n a n d closely r e l a t e d f u n d i n g s a n d ideas. W e h a v e also s p e c u l a t e d a b o u t t h e p o s s i b l e b i o l o g i c a l significance o f p r o p e r t i e s o f Downloaded by NORTH CAROLINA STATE UNIV on September 24, 2012 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch004

l i p i d s a n d l i p i d - w a t e r systems.

I t is e v i d e n t that m o d e l systems a n d

p h y s i c a l p r o p e r t i e s o f l i p i d s t h a t are n o t g e n e r a l l y t h o u g h t o f m a y b e o f interest i n c o n n e c t i o n w i t h t h e m o d e l l i n g a n d t h e u n d e r s t a n d i n g o f c e l l membranes.

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Lyotropic Liquid Crystals - American Chemical Society

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