Ordered Fluids and Liquid Crystals

14

Downloaded by UNIV OF TEXAS AT DALLAS on July 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0063.ch014

Helix-Coil Transition i n Deuterated Poly-γ-Benzyl-L-Glutamate

F. E. KARASZ and

J.

M.

O'REILLY

General Electric Research and Development Center, Schenectady, Ν. Y.

Our present knowledge of the helix-coil polypeptides,

with particular

transition

-glutamate-dichloroacetic acid-1,2-dichloroethane ly reviewed. position

Recent results concerning

and of polypeptide

modynamic

properties

modynamics transition ously

synthetic

system, is brief

the effect of solvent com

and solvent deuteration

of the transition

and presumably are generally

in

reference to the poly-γ-benzyl-L

on the ther

show that both the ther

the molecular

more complicated

mechanism

of the

than had been previ

supposed.

/ C e r t a i n s y n t h e t i c p o l y p e p t i d e s i n d i l u t e s o l u t i o n c a n undergo a r e v e r s i b l e c o o p e r a t i v e t r a n s i t i o n f r o m a h e l i c a l t o a r a n d o m l y coiled c o n f i g u r a t i o n (16).

F o r p o l y p e p t i d e s i n o r g a n i c s o l v e n t s t h e t r a n s i t i o n c a n be i n

d u c e d b y c h a n g i n g t h e t e m p e r a t u r e of t h e s o l u t i o n o r t h e c o m p o s i t i o n of t h e solvent.

O f t h e s e v e r a l systems t h a t h a v e been i n v e s t i g a t e d , t h e m o s t t h o r

o u g h l y s t u d i e d h a v e been s o l u t i o n s of p o l y - 7 - b e n z y l - L - g l u t a m a t e ( P B G ) i n m i x t u r e s of d i c h l o r o a c e t i c a c i d ( D C A ) a n d e i t h e r ( D C E ) o r c h l o r o f o r m (17).

1,2-dichloroethane

T h e w o r k presented here deals w i t h t h e P B G -

D C A - D C E system. T h e t r a n s i t i o n c a n p e r h a p s m o s t c o n v e n i e n t l y be f o l l o w e d p o l a r i m e t rically.

F i g u r e 1 shows t h e change i n specific o p t i c a l r o t a t i o n of a 3 % P B G

s o l u t i o n (solvent, 70 v o l u m e % D C A - 3 0 v o l u m e % D C E ) as t h e t e m p e r a t u r e is v a r i e d t h r o u g h t h e t r a n s i t i o n range.

F r o m optical rotatory dis

p e r s i o n m e a s u r e m e n t s one m a y o b t a i n t h e M o f f i t t p a r a m e t e r , b , a n d f r o m 0

t h i s i t has been s h o w n t h a t for P B G t h e h i g h t e m p e r a t u r e f o r m ( w i t h p o s i t i v e [a] ) D

corresponds t o t h e h e l i c a l c o n f o r m a t i o n of t h e p o l y p e p t i d e 180

Porter and Johnson; Ordered Fluids and Liquid Crystals Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

(12).

14.

KARASZ AND

Helix-Coil

O'REILLY

181

Transition


I5i

H5h 14

18

Figure 1.

22

26

30

34

38

Specific optical rotation of a 3% PBG solution as a function of temperature

T h e changes w h i c h o c c u r i n s o l u t i o n a r o u n d t h e t r a n s i t i o n t e m p e r a t u r e , T

C.,

c a n be w r i t t e n s c h e m a t i c a l l y a n d o n l y t o a first a p p r o x i m a t i o n , as f o l

lows:

(1)

A s the t e m p e r a t u r e is raised, the D C A b o u n d t o t h e c o i l is released a n d d i m e r i z e s , p e r m i t t i n g t h e p o l y p e p t i d e α-helix t o f o r m , t h e e q u i l i b r i u m t h u s m o v i n g to t h e r i g h t . I t is i m p o r t a n t t o realize t h a t i n t h i s a p p r o x i m a t i o n t h e t o t a l n u m b e r of h y d r o g e n b o n d s i n t h e s y s t e m remains u n c h a n g e d ; therefore i t is c l e a r l y t h e r e l a t i v e l y s m a l l differences i n t h e enthalpies a n d entropies of t h e b o n d s t h a t are p e r t i n e n t i n a n y q u a n t i t a t i v e discussion. A s s u c h differences are n o t calculable w i t h a n y confidence, i t is n o t possible t o decide a priori w h e t h e r , for example, t h e h e l i c a l or t h e r a n d o m l y coiled c o n f i g u r a t i o n is stable at h i g h t e m p e r a t u r e s i n a n y g i v e n s y s t e m . F r o m p o l a r i m e t r i c d a t a , s u c h as are s h o w n i n F i g u r e 1, t h e f r a c t i o n a l h e l i c a l content, f , of t h e p o l y p e p t i d e m a y be c a l c u l a t e d as a f u n c t i o n of t e m p e r a t u r e , a n d b y t h e f o r m a l a p p l i c a t i o n of the v a n ' t H o f f e q u a t i o n , a n a p p a r e n t heat of t r a n s i t i o n , AH, is d e r i v e d . F i g u r e 2 shows s u c h a p l o t [slightly m o d i f i e d b y u s i n g f values i n s t e a d of l o g ( e q u i l i b r i u m constants) i n t h e ordinate) (#)] for t h e d a t a s h o w n i n F i g u r e 1. F o r P B G u n d e r these H

H


182

ORDERED

FLUIDS AND

LIQUID

CRYSTALS

conditions, the v a n ' t H o f f heat, AH = 80 k c a l . / m o l e . A n o b v i o u s q u e s t i o n is: t o w h a t p h y s i c a l process does t h i s r e l a t i v e l y large e n t h a l p y v a l u e relate, or, e q u i v a l e n t l y , w h a t is the significance, i n t h i s context, of a mole? I t is t h e differences i n h y d r o g e n b o n d strengths t h a t are r e l e v a n t . T h e s e m i g h t be of the order of a few h u n d r e d calories per m o l e or less; t h u s 80 k c a l . is e q u i v a l e n t to u p w a r d s of 10 o r 10 of s u c h bonds. A s s h o w n q u a n t i t a t i v e l y below, AH is related to the s t a t i s t i c a l n u m b e r of n e i g h b o r i n g 2

3


p o l y p e p t i d e residues w h i c h c o o p e r a t i v e l y change t h e i r c o n f o r m a t i o n a l state i n t h e course of the t r a n s i t i o n . '.Or--

o.eh

3.24

3.32

3.40

3.48

l/T, •Κ." χ 10* 1

Figure ;

Fractional helical content, f , of PBG as a function of reciprocal absolute temperature H

P o l a r i m e t r i c (or indeed, a n y optical) d a t a alone c a n y i e l d o n l y t w o t h e r m o d y n a m i c parameters d e s c r i b i n g the t h e r m a l t r a n s i t i o n , T a n d AH. To c

proceed f u r t h e r , i t becomes desirable to use one of t h e s t a t i s t i c a l m e c h a n i c a l t r e a t m e n t s of o n e - d i m e n s i o n a l cooperative t r a n s i t i o n s w h i c h h a v e been r e c e n t l y f o r m u l a t e d to treat t h i s a n d m o r e generalized p r o b l e m s (8, 15). In the present w o r k we use t h e t h e o r y of Z i m m a n d B r a g g (18, 19) a n d ex tensions of t h i s b y A p p l e q u i s t (2). T h e t h e o r y p r e d i c t s a l l the m a j o r fea tures of t h e t r a n s i t i o n f o u n d e x p e r i m e n t a l l y i n terms of a p a r a m e t e r , σ, w h i c h i n our n o m e n c l a t u r e is g i v e n b y

i n w h i c h AH is t h e c a l o r i m e t r i c heat associated w i t h the transfer of a m o l e of a m i n o a c i d residues f r o m t h e r a n d o m c o i l c o n f o r m a t i o n t o t h e e n d of e x i s t i n g h e l i c a l sequences. S u p e r f i c i a l l y , therefore, i t is analogous to t h e heat of fusion of a o n e - c o m p o n e n t s y s t e m a n d as s u c h c a n be o b t a i n e d c a l o r i m e t r i c a l l y b y m e a s u r i n g t h e heat c a p a c i t y of t h e a p p r o p r i a t e p o l y p e p t i d e s o l u t i o n as a f u n c t i o n of t e m p e r a t u r e . T h e result of s u c h a m e a s Q



14.

KARASZ

AND

I

ι

0

Helix-Coil

O'REILLY

I

ι

I

10

183

Transition

ι

20

I

ι

30

L

40

TEMPERATURE, C. E

Figure 8.

Heat capacity of PBG solution

1 gram of solution contains 0.02002 gram of PBG

u r e m e n t is s h o w n i n F i g u r e 3 ; AH

0

the peak.

F o r P B G , AH

0

(7)

is d i r e c t l y p r o p o r t i o n a l to the area u n d e r

is of the order of 500 c a l . per m o l e of residue

(11)]

therefore σ lies b e t w e e n 10~ a n d 10~ . σ can be i n t e r p r e t e d p h y s i c a l l y i n several essentially e q u i v a l e n t w a y s . I t is f u n d a m e n t a l l y a measure of the cooperativeness of t h e t r a n s i t i o n . I n p a r t i c u l a r , a t T (i.e., a t / / / = 1/2), σ i s e q u a l t o t h e average n u m b e r of residues i n a h e l i c a l sequence; a l o w v a l u e of σ i n d i c a t e s a h i g h degree of cooperation. T h u s σ m a y also be regarded a n a n e q u i l i b r i u m constant for t h e f o r m a t i o n of a n i n t e r r u p t i o n i n a h e l i c a l sequence (2). Other i n t e r p r e t a t i o n s of σ are b r o u g h t o u t below. 4

c

5

_ 1 / 2

W e h a v e been interested i n the b e h a v i o r of σ as a f u n c t i o n of a n u m b e r of v a r i a b l e s . Structure of Polypeptides. I t h a d o r i g i n a l l y been suggested t h a t σ m i g h t be r e l a t i v e l y unaffected b y changes i n t h e c h e m i c a l n a t u r e of t h e a m i n o a c i d side groups or i n t h e solvent s y s t e m . T h i s was based o n the b e lief t h a t as σ was r e l a t e d t o t h e r e l a t i v e d i f f i c u l t y of i n t e r p o s i n g a r a n d o m l y coiled sequence i n a n e x i s t i n g h e l i c a l sequence i n a p o l y p e p t i d e c h a i n , i t s h o u l d be d e t e r m i n e d b y i n t e r a c t i o n s a l o n g t h e p o l y g l y c i n e - t y p e basic s t r u c t u r e of the c h a i n . T h i s has n o w been s h o w n to be too d r a s t i c a n as s u m p t i o n ; i n c h e m i c a l l y r a t h e r s i m i l a r p o l y p e p t i d e s , σ changed b y a factor of ~ o (10). I n d i s s i m i l a r systems ( P B G c o m p a r e d w i t h p o l y g l u t a m i c a c i d


184

ORDERED

FLUIDS A N D

LIQUID CRYSTALS

i n aqueous s o l u t i o n ) orders of m a g n i t u d e differences h a v e been o b s e r v e d (13). Temperature Variation of σ. I f σ were i n fact l a r g e l y d e t e r m i n e d b y t h e c o n f o r m a t i o n a l r e s t r i c t i o n s i m p o s e d o n a d j a c e n t residues i n f o r m i n g a r a n d o m c o i l sequence, as w a s i n effect p o s t u l a t e d a b o v e , t h e n i t m i g h t be expected t o be e n t i r e l y entropie, i n w h i c h case i t c o u l d be w r i t t e n a s : σ = e '


LSi

(3)

R

T h e large n e g a t i v e v a l u e of ASi ( ~ —19 e.u. p e r m o l e of i n t e r r u p t i o n s for P B G ) i m p l i e d b y t h i s r e l a t i o n s h i p h a s been f a i r l y w e l l a c c o u n t e d for b y c a l c u l a t i n g t h e r e d u c t i o n i n t h e n u m b e r of accessible configurations (19). T h e t e m p e r a t u r e independence of σ, also i m p l i e d i n E q u a t i o n 3, h a d n o t u p t o n o w been d i r e c t l y v e r i f i e d for a n y s y s t e m , t h o u g h b o t h t h e o r e t i c a l (18,19) a n d some r a t h e r i n d i r e c t e x p e r i m e n t a l evidence (13) h a d suggested t h a t a n y t e m p e r a t u r e dependence m i g h t be s m a l l . R e c e n t l y i t has been suggested, h o w e v e r , t h a t d i p o l e i n t e r a c t i o n s a l o n g t h e p o l y p e p t i d e c h a i n m i g h t be i m p o r t a n t i n b o t h t h e r a n d o m - c o i l a n d h e l i c a l configurations. I f t h i s i s t h e case, i t w o u l d r e s u l t i n a significant e n t h a l p i c t e r m i n σ (4). Deuteration of Polypeptide and Solvent. A l t h o u g h i t h a d been p r e v i o u s l y s h o w n t h a t σ w a s s o m e w h a t affected b y v a r i a t i o n s i n side g r o u p i n t e r a c t i o n s , i t m i g h t be p o s t u l a t e d t h a t σ w o u l d be i n v a r i a n t t o t h e s u b s t i t u t i o n of deuterons for t h e l a b i l e h y d r o g e n - b o n d i n g p r o t o n s i n t h e p o l y p e p t i d e a n d t h e D C A . I n s u c h a n exchange side g r o u p i n t e r a c t i o n s w o u l d be m i n i m a l l y affected; f u r t h e r m o r e one w o u l d s i m i l a r l y p r e d i c t r a t h e r s m a l l entropie (as d i s t i n c t f r o m e n t h a l p i c ) p e r t u r b a t i o n s of t h e o v e r - a l l r e a c t i o n . T h e test of t h i s h y p o t h e s i s f o r m e d t h e o r i g i n a l basis of t h e w o r k d e s c r i b e d below. Other Variables. T h e effect of the p o l y p e p t i d e m o l e c u l a r w e i g h t u p o n σ h a s n o t y e t been c o n c l u s i v e l y e s t a b l i s h e d for a n y p o l y p e p t i d e - o r g a n i c s o l v e n t s y s t e m . R e c e n t extensive studies of t h e charge-induced t r a n s i t i o n of p o l y g l u t a m i c a c i d i n aqueous s o l u t i o n h a v e s h o w n a n increase i n σ w i t h a r e d u c t i o n i n m o l e c u l a r w e i g h t (15). S i m i l a r l y t h e influence of p o l y p e p t i d e solute c o n c e n t r a t i o n i s n o t y e t clear, t h o u g h A c k e r m a n n a n d R u t e r j a n s (1) h a v e d e m o n s t r a t e d a r e m a r k a b l y large effect of t h i s v a r i a b l e u p o n AH i n PBG. N e i t h e r of these p o i n t s is discussed f u r t h e r i n t h e present p a p e r . 0

Deuterated

PBG

C a l v i n , H e r m a n s , a n d S c h e r a g a (5) h a v e s h o w n p o l a r i m e t r i c a l l y , for P B G i n a s o l v e n t of c o n s t a n t c o m p o s i t i o n (80 v o l u m e % D C A - 2 0 v o l u m e % D C E ) , t h a t i n t h e d e u t e r a t e d s y s t e m T decreased f r o m 40° t o 29°C., a n d t h e c o r r e s p o n d i n g AH's rose f r o m 65 ( p r o t o n a t e d P B G ) t o 83 k c a l . / m o l e (deuterated P B G ) . T h e r e f o r e i f σ were to d i s p l a y no isotope effect (see a b o v e ) , t h e n a c c o r d i n g t o E q u a t i o n 2, AH w o u l d h a v e t o u n d e r g o a c o n c o m i t a n t change. c

0


14.

KARASZ A N D O'REILLY

Helix-Coil

185

Transition

I n the course of t h i s w o r k i t developed, h o w e v e r , t h a t t h i s change i n AH w a s m o r e d i r e c t l y r e l a t e d t o t h e change i n T i n t h e d e u t e r a t e d , as c o m p a r e d w i t h t h e p r o t o n a t e d , s y s t e m , a n d a s t u d y of AH vs. T for b o t h s y s tems w a s u n d e r t a k e n . T w a s v a r i e d o v e r t h e range 5 ° - 5 0 ° C . b y v a r y i n g t h e c o m p o s i t i o n of t h e solvent f r o m a b o u t 62 t o 84 v o l u m e % D C A (the other c o m p o n e n t being D C E ) . E x p e r i m e n t a l details are g i v e n elsewhere (9). T h e results are s h o w n i n F i g u r e 4. I n b o t h cases T rises w i t h D C A c o n c e n t r a t i o n ; t h i s agrees w i t h t h e s c h e m a t i c r e a c t i o n m e c h a n i s m ( E q u a t i o n 1), w h i c h shows t h a t t h e presence of D C A f a v o r s t h e coiled o r l o w t e m p e r a t u r e f o r m . T h e r e f o r e b y a p p l i c a t i o n of L e C h a t e l i e r ' s p r i n c i p l e , a h i g h e r t e m p e r a t u r e is needed t o i n d u c e t h e e n d o t h e r m i c r e a c t i o n . F i g u r e 4 c

c

c


c

50

40

30 T /C., c

». 10

°60

64

68 — •

72 76 VOLUME V· OCA

80

Figure J+. T as a function of solvent composition for PBG(H) PBG(D) solutions c

84 and

also confirms t h e fact t h a t d e u t e r a t i o n lowers T for a g i v e n solvent c o m p o s i t i o n , b y f r o m 5° t o 10°C. T h e s e results are i n good q u a n t i t a t i v e agree m e n t w i t h those of C a l v i n , H e r m a n s , a n d Scheraga (5). C.,

F i g u r e 5 shows t h e AH's c a l c u l a t e d f r o m p o l a r i m e t r i c d a t a for t h e v a r i o u s solutions, as a f u n c t i o n of T . W i t h i n t h e r a t h e r sizable e x p e r i m e n t a l errors t h e d a t a for b o t h t h e p r o t o n a t e d a n d d e u t e r a t e d solutions c a n be represented b y a single c u r v e . T h e i m p l i c a t i o n is therefore t h a t differc

ences i n AH f o r P B G ( H ) a n d P B G ( D ) solutions w i t h i d e n t i c a l solvent compositions are reflections of changes i n T r a t h e r t h a n a result of d e u t e r a t i o n p e r se. c

T h e c a l o r i m e t r i c heats, AH , for t w o d e u t e r a t e d solutions w i t h T 's of 40° a n d 8.5°C. are g i v e n , together w i t h t h e c o r r e s p o n d i n g AH's, i n T a b l e I . W h i l e b o t h AH a n d AH decrease w i t h i n c r e a s i n g T t h e rates are n o t 0

0

c

C.,



186

ORDERED

I

20I 0

I

10

20

— Figure 5.

equal.

FLUIDS AND

LIQUID

CRYSTALS

1

I

I

30

40

50

V

e

AH as a function of T for PBG(H) solutions c

and

PBG(D)

B e c a u s e of the e x p e r i m e n t a l errors a r i s i n g i n b o t h AH

0

a n d AH, the

a c c u r a c y w i t h w h i c h σ c a n be d e t e r m i n e d at present is r a t h e r l o w .

Never

theless, t h e c a l c u l a t e d σ'β (last c o l u m n i n T a b l e I) show a significant v a r i ation.

T h i s is t h e first d i r e c t i n d i c a t i o n t h a t , c o n t r a r y to earlier s u p p o s i

t i o n , σ is s u b s t a n t i a l l y temperature-dependent,

at least i n P B G .

I f we

w r i t e σ i n the e x p a n d e d f o r m , (AH,

ASA

f

.

we find AHi = —3700 c a l . a n d AS ι = —33.1 e.u., b o t h per mole of i n t e r r u p t i o n s . I n a d d i t i o n , b y c o m p a r i n g the values of σ for the d e u t e r a t e d s o l u t i o n s w i t h a p r e v i o u s l y o b t a i n e d result for a p r o t o n - c o n t a i n i n g P B G s o l u t i o n of i n t e r m e d i a t e T (26°C.) (11), i t is f o u n d t h a t w i t h i n e x p e r i m e n t a l error a l l three results f a l l o n t h e same l i n e w h e n p l o t t e d as a f u n c t i o n of T . T h i s fact appears to c o n f i r m o u r o r i g i n a l h y p o t h e s i s t h a t d e u t e r a t i o n i n itself does n o t affect σ. I t does h a v e a n i n d i r e c t effect because of t h e r e s u l t a n t change i n T a n d t h e t e m p e r a t u r e dependence of σ. c

c

C.,

Table I.

Data on Deuterated Solutions

T , °C.

AHo,Cal./ Mole Residues

8.5 40.0 26.0

670 ± 50 380 ± 50 525 ± 80

e

PBG(D) PBG(H)

AH, Kcal./Mois 100 ± 1 5 80 ± 10 95 ± 1 2

σ Χ 10* 4.5+2.0 2.3 ± 1.1 3.1 ± 1.4


14.

KARASZ AND o ' R E I L L Y

Helix-Coil

187

Transition

Discussion A n u m b e r of i m p l i c a t i o n s of these results are discussed i n greater d e t a i l elsewhere (9). H o w e v e r , we t o u c h briefly o n some p o i n t s below. F i r s t , t h e AH ο results i n d i c a t e t h a t a t least f o r m a l l y a large AC t e r m is i n v o l v e d i n the t r a n s i t i o n . F r o m the t e m p e r a t u r e dependence of AH we calculate t h a t t h e heat c a p a c i t y of t h e r a n d o m c o i l c o n f o r m a t i o n exceeds t h a t of the h e l i c a l c o n f o r m a t i o n b y ~ 9 c a l . / d e g . / m o l e of P B G residues, or a p p r o x i m a t e l y 0.04 c a l . / d e g . / g r a m of P B G . T h i s is the order of m a g n i t u d e change f o u n d i n c r y s t a l - l i q u i d t r a n s i t i o n s i n m a n y organic solids a n d m i g h t t h u s be a c c o u n t e d for o n t h i s basis. A AC of s i m i l a r m a g n i t u d e was also f o u n d i n t h e e q u i v a l e n t change i n ribonuclease A (3). T w o reservations m u s t be a t t a c h e d to these considerations, however. F i r s t , the w h o l e s y s t e m has to be t a k e n i n t o account. T h e r e f o r e a n u n k n o w n f r a c t i o n of t h e AC observed m u s t be caused b y changes i n the solvent d u r i n g the t r a n s i t i o n . S e c o n d , the f a c t t h a t the AH measurements refer t o solutions of different solvent c o m p o s i t i o n suggests t h a t i t m a y be necessary t o consider w h e t h e r t h e change i n AH ο w i t h T is a n inherent solvent effect (and not caused b y the existence of a finite AC ). P

0


P

P

0

c

V

T h e appearance of a large e n t h a l p y t e r m i n σ is unexpected f r o m the basis of the Z i m m - B r a g g t h e o r y . I n p h y s i c a l t e r m s t h e i m p l i c a t i o n is t h a t t h e i n t e r p o s i t i o n of a r a n d o m c o i l sequence i n a h e l i c a l sequence, as w e l l as i n v o l v i n g significant entropie r e s t r a i n t s o n t h e adjacent residues, also i n v o l v e s c o m p a r a t i v e l y large energetic changes. T h e m a g n i t u d e of t h e l a t t e r is s u c h as t o m a k e t h e suggestion t h a t t h e y s t e m e x c l u s i v e l y f r o m changes i n h y d r o g e n b o n d i n g d o u b t f u l . F u r t h e r m o r e , whereas i t was possible to e x p l a i n a n e n t r o p y t e r m i n σ of ^ — 1 9 e.u. per mole of i n t e r r u p t i o n s [a n e t of t w o a d d i t i o n a l residues is i n v o l v e d per i n t e r r u p t i o n (19)], a v a l u e as h i g h as 33 e.u. suggests t h a t f u r t h e r o r d e r i n g w i t h i n t h e s y s t e m a c c o m panies t h e change. I n t h i s connection, therefore, i t is p e r t i n e n t to consider t h e recent results of H a n l o n a n d K l o t z (7) a n d H a n l o n (6), w h o f o u n d , i n P B G a n d other p o l y p e p t i d e s , t h a t t h e t r a n s i t i o n i n v o l v e d changes i n p r o t o n a t e d as w e l l as h y d r o g e n - b o n d e d species. S u c h changes m a y be ex p e c t e d t o i n v o l v e e n t h a l p i e s closer t o t h e m a g n i t u d e of those f o u n d here. A n o t h e r possible e n t h a l p i c c o n t r i b u t i o n stems f r o m the dipole i n t e r a c t i o n between p e p t i d e u n i t s (4), w h i l e a t h i r d is t h e changes i n n o n c o v a l e n t b o n d i n g between t h e a m i n o a c i d side groups as a r e s u l t of changes i n c o n f i g u r a t i o n (14)Conclusions T h e present results d e m o n s t r a t e o n a t h e r m o d y n a m i c basis t h a t the h e l i x - c o i l t r a n s i t i o n i n P B G i s generally m o r e c o m p l i c a t e d t h a n p r e v i o u s l y supposed.

I n p a r t i c u l a r , t h e large t e m p e r a t u r e dependencies of b o t h

a n d AH ο c a r r y new i m p l i c a t i o n s .

AH

E x p e r i m e n t a l l y i t w o u l d seem h i g h l y d e -


188

ORDERED FLUIDS A N D LIQUID CRYSTALS

sirable to reconcile all the thermodynamic parameters involved with the molecular changes that occur i n solution during the transition.


Literature

Cited

(1) Ackermann, T., Rüterjans, H., Z. Physik. Chem. 41, 116 (1964). (2) Applequist, J., J. Chem. Phys. 38, 934 (1963). (3) Beck, K., Gill, S. J., Downing, M . , J. Am. Chem. Soc. 87, 901 (1965). (4) Brant, D . Α., Flory, P. J., J. Am. Chem. Soc. 87, 663 (1965). (5) Calvin, M . , Hermans, J., Jr., Scheraga, Η. Α., J. Am. Chem. Soc. 81, 5048 (1959). (6) Hanlon, S., "Abstracts of Papers," 150th Meeting, ACS, September 1965, 105C. (7) Hanlon, S., Klotz, I. M . , Biochemistry 4, 37 (1965). (8) Hill, T. L., J. Chem. Phys. 30, 383 (1959). (9) Karasz, F. E., O'Reilly, J. M . , Biopolymers, in press. (10) Karasz, F. E., O'Reilly, J. M . , Bair, Η. E., Biopolymers 3, 241 (1965). (11) Karasz, F. E., O'Reilly, J. M . , Bair, Η. E., Nature 202, 693 (1964). (12) Moffitt, W., Yang, J. T., Proc. Natl. Acad. Sci. U. S. 42, 596 (1956). (13) Rifkind, J., Applequist, J., J. Am. Chem. Soc. 86, 4207 (1964). (14) Scheraga, Η. Α., 'The Proteins," 2nd ed., H . Neurath, ed., Vol. 1, Academic Press, New York, 1963. (15) Snipp, R. L., Miller, W. G., Nylund, R. E., J. Am. Chem. Soc. 87, 3547 (1965). (16) Urnes, P., Doty, P., Advan. Protein Chem. 16, 401 (1961). (17) Ibid., 16, 434, 474 (1961). (18) Zimm, Β. H., Bragg, J. K., J. Chem. Phys. 31, 526 (1959). (19) Zimm, Β. H., Doty, P., Iso, K., Proc. Natl. Acad. Sci. U. S. 45, 1601 (1959). RECEIVED February 11, 1966.


Ordered Fluids and Liquid Crystals

Recommend Documents