Structure and Properties of Polyglutamates in Concentrated Solutions

11 Structure and Properties of Polyglutamates in Concentrated Solutions Y. UEMATSU Department of Industrial Chemistry, Tokyo Institute of Polytechnics, Atsugi, Kanagawa, 243-02, Japan

Downloaded by CORNELL UNIV on October 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch011

I. UEMATSU Tokyo Institute of Technology, Megro, Tokyo, 152, Japan

It is well known that some polypeptides form lyotropic liquid crystals at sufficiently high concentrarion of the polymers and exhibit the optical properties characteristic of the cholesteric mesophase. The solutions show the regular striation arranged in a finger - printed pattern under a polarization microscope. Robinson et at have reported that the distance between the striation,S, which is the half pitch of the twisted structure, varies with polymer concentration in an inverse manner. They also discussed the molecular arrangement in the twisted structure in the light of X-ray and other measurements. They suggested that the long-range, electric dipole-dipole interactio plays an important role in the formation of the cholesteric structure, and the helical arrangement of dipoles in the polypeptide molecules would tend to impose a unidirectional twist. From the smallness of the twist angle and the fact that it decreases on dilution, they pointed out that the twistangle arises from a dynamic equilibrium. Samulski et al suggested that an asymetric helical molecular conformation may provide an explanation f o r the c h o l e s t e r i c s t r u c t u r e , and very small oscillations of h e l i c e s r e l a t i v e to t h e i r neighbors in the liquid c r y s t a l would be biased to one side of the parallel p o s i t i o n because of the chirality of the van der Waals surface of the polypeptide molecules. Further, they found a l i n e a r r e l a t i o n s h i p between the p i t c h and the r e c i p r o c a l of temperature. This r e l a t i o n holds quite w e l l f o r a numb er of thermotropic l i q u i d crys t a l sys t ems and was explained t h e o r e t i c a l l y by Keating!) , who treated the macroscopic twis t as a r o t a t i o n a l analogue of thermal expansion with the dominant anharmonic forces coming from nearest neighI)

2)

0-8412-0419-5/78/47-074-136$05.50/0 © 1978 American Chemical Society Blumstein; Mesomorphic Order in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

11.

U E M A T S U

bors.

A N D

U E M A T S U

The t h e o r y

Poly glutamate s in Concentrated Solutions

gives

the f o l l o w i n g

137

relationship:


^dlw AkT where k i s the Boltzmann c o n s t a n t , d , the s p a c i n g b e t w e e n n e i g h b o r i n g p l a n e s o f m o l e c u l e s , w, t h e f r e q u e n c y o f t h e e x c i t e d t w i s t m o d e s , I , t h e moment o f i n e r t i a o f t h e m o l e c u l e , and A , t h e c o n s t a n t i n t h e c u b i c a n h a r m o n i c i t y term of the anharmonic e q u a t i o n of m o t i o n d e s c r i b i n g t h e s y s t e m . However, the p i t c h for the p o l y p e p t i d e l i q u i d c r y s t a l s increases with temperature, w h i l e i t decreases f o r the thermotropic liquid crystals. Recently, there are several report discuss ins t h e t w i s t mode o f t h e b i n a r y s y s t e m o f t h e t h e r m o t r o pic liquid crystals composed of a n e m a t i c s o l v e n t and a c h o l e s t e r i c s o l u t e . Another i n t e r e s t i s i n a t t e m p t i n g to pursue the s t r u c t u r a l s i m i l a r i t y between t h e r m o t r o p i c l i q u i d c r y s t a l s and l y o t r o p i c o n e , e s p e c i a l l y f o r t h e d e p e n d e n c e o f c h o l e s t e r i c p i t c h on t h e temperature. One o f t h e b e s t m e a s u r e s o f t w i s t i s t h o u g h t t o be t h e c h o l e s t e r i c p i t c h . The t e m p e r a t u r e dependence of c h o i e s t e r i c p i t c h i s t h e n m e a s u r e d f o r p o l y - γ b e n z y l - L - , and p o l y - y - b e n z y 1 - D - g l u t a m a t e s ( PBLG, and PBDG ) a n d p o l y - y - a l k y 1 - L - g l u t a m a t e s , i n v a r i o u s s o l v e n t s y s t ems. T h e t e m p e r a t u r e d e p e n d e n c e o f some p h y s i c a l p r o p e r t i e s were a l s o m e a s u r e d . Experimental. PBLG ( mw: 2 0 1 , 0 0 0 ) a n d PBDG ( mw: 1 9 7 , 0 0 0 ) u s e d i n t h i s s t u d y were sup 1 i e d b y A j inomo t ο C o . L t d . J a p a n , a n d some s p e c i m e n s o f PBLG w e r e p r e p a r e d b y p o l y m e r i z a t i o n of N - c a r b o x y a n h y d r i d e of γ - b e n z y 1 - L glutamate. P o l y - y - a l k y 1 - L - g l u t a m a t e s ( PALG ) w e r e p r e p a r e d f r o m p o l y - y - m e t h y 1 - L - g l u t a m a t e ( mw: 2 5 0 , 0 0 0 ) by a l c o h o l y s i s by w h i ch a l l m e t h y l r a d i c a l s i n t h e s i d e c h a i n s c a n be r e p l a c e d w i t h t h e d e s i r e d a l k y 1 radicals. P o l y - y - a l k y 1 - L - g l u t amat es u s e d i n t h i s s t u d y w e r e w i t h a l k y 1 g r o u p s s u c h as e t h y l , n - p r o p y 1 , n - b u t y 1 , η - a m y 1 , n - h exy1 , n - h e p t y 1 , and n - o c t y 1 . From t h e v i s c o s i t y measurement s i n d i c h l o r o a c e t i c acid t h e m o l e c u l a r w e i g h t s o f p o l y g l u t a m a t e s were est imat e d . T h e s o l u t i o n s were p r e p a r e d b y w e i g h i n g the p o l y m e r and s o l v e n t i n a s t o p p e r e d b o t t l e . A f t e r s h a k i n g s l o w l y on a magnet i c s t i r r e r f o r long time, t h e s o l u t i o n s were p u t c a r e f u l l y i n t o t h e c e l l , w h i c h was h a v i n g p a r a l l e l s i d e s 2mm a p a r t . Then t h e top o f t h e c a p i l l a r y o f t h e c e l l was s e a l e d t o p r e v e n t t h e

Blumstein; Mesomorphic Order in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


138

M E S O M O R P H I C

ORDER

I N

P O L Y M E R S

l o s s o f s o l v e n t by v a p o r i z a t i o n . The c o n c e n t r a t i o n was c a l c u l a t e d by u s i n g 0 . 7 8 7 m l / g as t h e s p e c i f i c v o l u m e o f P B G , and 0 . 8 3 m l / g as t h a t o f PALG s , and e x p r e s s e d i n the u n i t s v o l / v o l . A f t e r on s t a n d i n g , u n t i l a r e g u l a r s t r u c t u r e a p p e a r e d , we m e a s u r e d t h e half p i t c h with a p o l a r i z a t i o n microscope. The t e m p e r a t u r e o f t h e s a m p l e was r e g u l a t e d w i t h i n + 0 . 2 ° C by u s e o f a c o n s t a n t t e m p e r a t u r e c i r c u l a t o r . The measurements were c a r r i e d out a f t e r m a i n t a i n i n g the s p e c i m e n s f o r 12 h r s a t e a c h temperature. D i l a t o m e t r i c measurements were c a r r i e d out u s i n g Hg as a c o n f i n i n g l i q u i d , w h i c h was t r a n s f e r r e d i n t o the d i l a t o m e t e r a f t e r s o l i d i f y i n g the sample s o l u t i o n by l i q u i d n i t r o g e n . CD s p e c t r a , were m e a s u r e d w i t h JASCO O R D / U V - 5 0 o v e r t h e r e g i o n f r o m 220 t o 300nm. NMR s p e c t r a w e r e m e a s u r e d by u s e o f J E O L M H - 1 0 0 . The t i m e r e q u i r e d t o a r r i v e a t e q u i l i b r i u m v a r i e d f r o m one t o s e v e r a l h o u r s . The s o l v e n t s u s e d h e r e were p u r i f i e d by s t a n d a r d m e t h o d s . R e s u l t s and d i s c u s s i o n . In g e n e r a l the c h o l e s t e r i c p i t c h of t h e r m o t r o p i c l i q u i d c r y s t a l s decreases with temperature. The theore t i c a l treatments f o r t h i s b e h a v i o r have been r e ported3,4). H o w e v e r , i t was f o u n d t h a t f o r a b i n a r y s y s t e m , t h e d e p e n d e n c e o f t h e c h o l e s t e r i c p i t c h on t e m p e r a t u r e changes f r o m ρ o s i t i v e to n e g a t i v e at a c r i t i c a l compos i t i o n . F o r the b i n a r y s y s t e m of t h e r m o t r o p i c l i q u i d c r y s t a l s , c o m p o s e d o f c h o i e s t e r i c and nemat i c s u b s t a n c e s , d S / d T i s g r e a t e r t h a n z e r o i n t h e nemat i c r i c h r e g i o n , and d S / d T i s l e s s t h a n z e r o i n the c h o l e s t e r i c r i c h r e g i o n . I n t h i s p a p e r , we h a v e i n v e s t i g a t ed t h e t e m p e r a t u r e d e p e n d e n c e o f cholesteric p i t c h f o r the p o l y p e p t i d e l i q u i d crystals. R e c e n t l y DuPre et a l l } reported that , S increases l i n e a r l y with temperature r i s e . Qualitatively, their r e s u l t s are c o n s i s t e n t w i t h o u r s . However, the time r e q u i r e d to r e a c h the e q u i l i b r i u m p i t c h , v a r i e d w i t h temperature, t h e c o n c e n t r a t i o n o f p o l y m e r , and a l s o the t h e r m a l h i s t o r y . F o r PBLG s o l u t i o n i n d i c h l o r o e t h a n e (EDC) , w h i c h c o n c e n t r â t i o n i s 0 . 1 2 v o 1 / v o 1 , the v a r i a t i o n o f p i t c h w i t h t i m e was m e a s u r e d a t a c o n s t a n t t e m p e r a t u r e by T - j ump m e t h o d . F i g . 1 shows t h e t i m e d e p e n d e n c e o f c h o i e s t e r i c p i t c h by t h e T - j ump m e t h o d f r o m - 2 ° C t o + 3 0 ° C , 4 0 ° C and 50°C r e s p e c t i v e l y . I t i s c l e a r t h a t the time r e q u i r e d to a r r i v e at the equilibrium pitch i s s h o r t e r at h i g h e r temperature but i s s t i l l over s e v e r a l h o u r s . T h e r e f o r e , the e q u i l i b r i u m p i t c h mu s t be m e a s u r e d a f t e r p r o l o n g e d a g i n g at each m e a s u r i n g t e m p e r a t u r e . 11 was f o u n d t h a t the



11.

U E M A T S U

A N D U E M A T S U

Poly glutamates in Concentrated Solutions

139

h a l f p i t c h , S , i n c r e a s e d mono t o n i c a l l y w i t h t e m p e r a t u r e , and f i n a l l y , r e t a r d a t i o n l i n e d i s a p p e a r e d a t a c r i t i c a l t e m p e r a t u r e , and t h e b i r e f r i n g e n c e c h a r a c t e r i s t i c of t h e nemat i c s t r u c t u r e was ob s e r v e d . P l o t o f S v e r s u s t e m p e r a t u r e f o r PBDG i n c h l o r o f o r m i s shown i n f i g . 2 . T h i s b e h a v i o r c o r r e s p o n d s to the b e h a v i o r of b i n a r y m i x t u r e of t h e r m o t r o p i c l i q u i d c r y s t a l s i n t h e nemat i c r i c h r e g i o n . As shown i n f i g . 3, t h e r e c i p r o c a l o f t h e h a l f p i t c h wh i c h i s p r o p o r t i o n a l to the a n g l e of t w i s t between ( 10Ï0) ρ l a n e s , i s a l i n e a r f une t i o n o f t e m p e r a t u r e . The critical t e m p e r a t u r e , w h e r e 1/S = 0, was ob t a i n e d f r o m t h e e x t r a p o l a t i o n o f t h i s c u r v e , and was d e f i n e d as t h e n e m a t i c t e m p e r a t u r e , T^. The o p t i c a l p a t t e r n c h a r a c t e r i s t i c o f t h e nemat i c s t a t e was o b s e r v e d i n t h i s r e g i o n under the p o l a r i z a t i o n m i c r o s c o p e . DuPre et a l def i n e d t h i s t e m p e r a t u r e as t h e c l e a r i n g p o i n t ' ,s i n c e the t w i s t e l a s t i c c o n s t a n t k22> o b t a i n e d f r o m t h e o r i e n t a t i o n i n a magnet i c f i e l d t e n d s t o w a r d z e r o a t t h i s temperature. H o w e v e r , f r o m t h e ob s e r v a t i o n by a p o l a r i z a t i o n m i c r o s c o p e , i t i s o b v i o u s t h a t the newly ap p e a r e d p h a s e i s n o t an i s o t r o p i c o n e . The f a c t t h a t k22 t e n d s t o w a r d z e r ο , c a n be e x p l a i n e d w i t h t h e p r e sence of the s i z e of the c l u s t e r of c o h e r e n t l e n g t h , i n w h i c h t h e m o l e c u l e s w o u l d be a r r a n g e d p a r a l l e l . In o r d e r t o o b t a i n t h e r e l a t i o n s h i p b e t w e e n S and the c o n c e n t r a t i o n C, a d o u b l e l o g a r i t h m i c p l o t o f S a g a i n s t C a t e a c h t e m p e r a t u r e was made as shown i n f i g . 4. I t c a n be s e e n t h a t l o g S i s p r o p o r t i o n a l t o l o g C a n d , i t s s l o p e n i s an i n c r e a s i n g f u n e t i o n o f t e m p e r a t u r e , c h a r a c t e r i s t i c of the specimens used. The e x t r a p o l a t e d l i n e o f l o g S - l o g C c r o s s e d e a c h other at a c r i t i c a l c o n c e n t r a t i o n C a t wh i c h S s t a y s c o n s t a n t and i n d e p e n d e n t ο f t e m p e r a t u r e . These r e s u l t s sugges t t h a t the temperature dependence of the c h o l e s t e r i c p i t c h would i n f l e c t at the concentrât i o n higher than C . T h i s i s analogous to the b e h a v i o r of t h e r m o t r o p i c l i q u i d c r y s t a l s comp o s ed o f c h o i e s t er i c s o l u t e and nemat i c s o l v e n t , w h e r e t h e s i g n o f dS/dT r e v e r s e s a t a c r i t i c a l c o n c e n t r a t i o n . 11 i s u n d e r s t o o d t h a t t h e b e h a v i o r o f b o t h t h e r m o t r o p i c and l y o t r o p i c l i q u i d c r y s t a l s i s comparable p r o v i d e d that the nemat i c s ub s t a n c e s o f t h e f o rmer a r e s u b s t i t u t e d w i t h the s o l v e n t s of the l a t t e r . The c r i t i c a l c o n centration C i s a b o u t 0.41 v o l / v o l and t h i s v a l u e i s v e r y c l o s e to the c o n c e n t r a t i o n at which the s i d e c h a i n s on ne i g h b o r i n g m o l e c u l e s o f t h e p o l y m e r come to c o n t a c t e a c h o t h e r ( r e f e r t o f i g . 5 ) . From t h e s e r e s u l t s , i t i s e x p e c t e d t h a t t h e o r i g i n o r me c h a n i s m of t w i s t w o u l d c h a n g e a t t h i s c o n c e n t r a t i o n C . The T

Q

Q

0

Q


MESOMORPHIC


140

3

0

0

ORDER IN

Time (min.)

Figure 1. The change of cholesteric pitch with time for PBLG-EDC. Polymer concentration is 0.12 vol/vol.

Figure 2. The change of S with temperature for PBDG-CHCk. Numbers represent the concentration of polymer.


POLYMERS


U E M A T S U

A N D

U E M A T S U

Ρ olij glutamate s in Concentrated Solutions

Figure 3. 1/S vs. temperature for PBDG—EDC. Concentration (from top to bottom) 0.30, 0.23, 0.175, 0.135 (vol/vol).


141

142

M E S O M O R P H I C ORDER IN P O L Y M E R S

e x t r a p o l a t i o n of the p l o t of l o g S v e r s u s l o g C , to C = 1, g i v e s the h a l f p i t c h of pure p o l y m e r , which i s a d e c r e a s i n g f u n c t i o n of t e m p e r a t u r e ; and t h i s b e h a v i o r i s a n a l o g o u s to t h a t of t h e r m o t r o p i c cholesteric liquid crystals. I t i s c o n v e n i e n t to use the r e d u c e d temperature T/T^j, where i s the nematic t e m p e r a t u r e , i n s t e a d of temperature T , i n the temperature range near T . Fig. 6 shows t h e o b s e r v e d v a l u e s o f 1/S v e r s u s Τ / T , f o r PBDG i n c h l o r o f o r m , a t v a r i o u s concentrations. The s l o p e Β d e p e n d s on t h e c o n c e n t r a t i o n , and i t i n c reases with concentration. If the c h o l e s t e r i c p i t c h was m e a s u r e d a t c o n s t a n t c o n c e n t r a t i o n , t h e f o l l o w i n g equation holds: N


N

1/S

= Β (

1 -

T/T

N

)

The d e p e n d e n c e o f Β on t h e c o n c e n t r a t i o n o f p o l y m e r was o b t a i n e d f r o m t h e p l o t o f l o g 1/P o r 1/S against l o g C , at each T / T . The v a l u e s o f t h e s l o p e η a r e constant and i n d e p e n d e n t o f t e m p e r a t u r e ( f i g . 7 ). A c c o r d i n g t o Rob i n s o n i ) , t h e f o l l o w i n g r e l a t i o n holds : N

θ

Hence

=

Ιττά/Έ

or

ud/S

θ = ( A/d2n-l)(

and

l -

T

C

Structure and Properties of Polyglutamates in Concentrated Solutions

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