15 Cholesteric O r d e r in B i o p o l y m e r s Y. BOULIGAND
Downloaded by CORNELL UNIV on October 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch015
Histophysique et Cytophysique, E.P.H.E. et C.N.R.S., 67, rue Maurice-Günsbourg, 94200 Ivry-sur-Seine, France
Many synthetic polymers form cholesteric phases, and even solids showing certain of the fundamental symmetries of cholesteric liquids. The purpose of this paper is to review the main examples of biological polymers assembling into cholesteric liquids or into more or less solid analogues. We will present them according to the main chemical classes of polymers to which they belong. We will also indicate the main forces involved in creating the cholesteric twist. Analogues of liquid crystals are numerous in biological systems (3-6,8-11). Certain fibrous and regularly twisted materials can be considered as polymerized cholesterics. Such twisted fibrous structures are recognizable by electron microscopy and sometimes by light microscopy, by the observation in thin sections of sets of stacked rows of nested arcs (fig. 1). The structure of a cholesteric system is represented in f i g . 2 and the origin of the arced patterns is indicated in f i g . 3. The first sketch (fig. 2) shows a section which is parallel to the twist axis. Fig. 3 corresponds to a section plane which is oblique with respect to the twist axis. This is the case most frequently observed. Pictures of stacked series of nested arcs in biological analogues of cholesterics have been found in numerous invertebrate materials. They have been reviewed in (6). New references can be found in (20) and (21). Twisted fibrous arrangements have been observed in the organic matrix of compact bone in vertebrates (6,23 and Castanet pers. comm.), the connective tissue of certain invertebrates (6,14 and f i g . 1), in numerous animal cuticles (3-6,8-10,20,30), in the body-wall of certain Tunicates (6,15), in the membranes surrounding various animal eggs (6), in various types of cytoplasmic inclusions (6), in different kinds of plant cell walls (6,21,22,27), in certain plant mucilages (J.-C.Thomas, pers. comm.), in the bacterial nucleus (6, Gourret, pers. comm.) and in the chromosomes of certain Protozoa (9, figs 6 and 7). Nucleic acids, proteins and glycoproteins are the main components of the twisted fibrous arrangements. For many of them, biochemical studies are rare or absent. 0-8412-0419-5/78/47-074-237$05.00/0 © 1978 American Chemical Society
Blumstein; Mesomorphic Order in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
Downloaded by CORNELL UNIV on October 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch015
238
M E S O M O R P H I C
ORDER
I N
P O L Y M E R S
Figure 1. Stacked series of nested arcs observed in oblique section in the con nective tissue of Havelockia inermis, Holothuroid, Echinoderm. Phase contrast microscopy, paraffin sec tion, Haematoxylin.
ZZZZZZD
Ο
'czzzzzz:
Ο
Figure 2, Cholesteric architecture ob served in a section plane parallel to the twist axis
Blumstein; Mesomorphic Order in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
15.
BOULiGAND
Cholesteric Order in Biopolymers
239
Downloaded by CORNELL UNIV on October 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch015
1. N u c l e i c a c i d s . The DNA of the D i n o f l a g e l l a t e chromosomes forms a c h o l e s t e r i c network which i s very s i m i l a r to that o f t e n observed i n the b a c t e r i a l nucleus (6,9,18). S i m i l a r s t r u c t u r e s have been observed i n mitochondrial DNA i n c e r t a i n Trypanosomes, a f t e r a treatment with c e r t a i n drugs (review i n 6). DNA i n concentrated aqueous s o l u t i o n s can form c h o l e s t e r i c mesophases (4,18, 24). Remarkable c h o l e s t e r i c s p h e r u l i t e s have been observed i n concentrated ribosomal RNA ( f i r s t considered as t-RNA, 3 3 - 3 5 ) . 2. P r o t e i n s . Synthetic polypeptides can form c h o l e s t e r i c s o l u t i o n s i n s e v e r a l organic solvents (24-26,29). Twisted arrangements of c o l l a g e n f i b r i l s are common i n sponges (Carrière, pers. comm.), i n Holothurians (Echinoderms, 6,14, see f i g . 1) and i n f l a t worms ( 6 ) . Microtubular haemoglobin has been observed i n erythrocytes i n s i c k l e c e l l anaemia. A f t e r deoxygenation and a d d i t i o n of t h e r mal energy, the mutant molecules of haemoglobin (HbS) stack to form monomolecular f i l a m e n t s . Six strands assemble into a hélicoïdal microtubule showing s i x h e l i c e s of long p i t c h (review i n 19). These microtubules form a c h o l e s t e r i c packing ( 6 ) . Superb twisted arrangements have been observed i n l a r v a l haemocytes (oenocytoid) of the silkworm, forming r e g u l a r and conc e n t r i c s e r i e s of arcs (2). P r o t e i n s a l s o are known to form a twisted f i b r o u s system i n the periostracum of c e r t a i n gastropods (review i n 20) and i n the cortex of the oocytes of numerous Teleost f i s h e s , review i n ( 6 ) . 3. Glycoproteins (associated polysaccharides and p r o t e i n s ) . C e l l u l o s e and p r o t e i n s form m i c r o f i b r i l s i n the body-wall of c e r t a i n marine i n v e r t e b r a t e s , the Tunicates. The f i b r o u s network shows a r e g u l a r twist i n one species Halocynthia p a p u l o s a . Cellulose i s an important component of plant c e l l w a l l s and s e r i e s of nested arcs are o f t e n v i s i b l e . I n t e r e s t i n g p i c t u r e s have been discussed with reference to the c h o l e s t e r i c conception (21,22,27) . Plant c e l l w a l l s contain a great v a r i e t y of polysaccharides and they are o f t e n d i f f e r e n t from c e l l u l o s e . The examples i n d i c a t i n g the presence of a twisted a r c h i t e c t u r e are reviewed i n (6,21,22, 27). C e r t a i n blue-green algae ( R i v u l a r i a a t r a and Chroococcus minutus) show remarkable arced patterns i n t h e i r mucilage (J.-C. Thomas, pers. comm. f i g . 8). C h i t i n / P r o t e i n complexes are found i n f u n g i and i n the a r t h ropod c u t i c l e . C h i t i n i s the poly-N-acetyl-D-glucosamine. Twisted arrangements have been observed i n the c e l l w a l l of the spore of Endogone (Mucorale, fungi) and i n v a r i o u s animal c u t i c l e s , namely i n c e r t a i n medusae (6) and i n almost a l l Arthropods : Crustaceans (3-6) , Insects and other groups (review i n 20). 4. Twisted systems due to v i r u s e s . C e r t a i n v i r u s e s i n the form of long c y l i n d e r s can assemble i n t o c h o l e s t e r i c phases (Narcissus mosaic v i r u s , 6,39) . The presence of a v i r u s i n a c e l l o f t e n leads to the d i f f e r e n t i a t i o n of c h o l e s t e r i c networks of f i b r i l s (6,16).
Blumstein; Mesomorphic Order in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
240
M E S O M O R P H I C ORDER IN P O L Y M E R S
Many works deal with the v a r i a t i o n of the h é l i c o ï d a l p i t c h i n c h o l e s t e r i c phases as a f u n c t i o n of temperature and composition. The best way to e l u c i d a t e the o r i g i n of the twist seems to be to compare the p i t c h v a r i a t i o n s i n l y o t r o p i c and i n thermotropic systems. The f i r s t accurate work i n t h i s f i e l d i s due to Robinson (1958-66) who studied PBLG (polybenzyl-L-glutamate) a s y n t h e t i c polypeptide i n organic solvents as dioxane, e t h y l i c a l c o h o l , c h l o roform e t c . and Cano (1967) who made measurements of the p i t c h e s of nematic paraazoxyphenetol with d i f f e r e n t amounts of c h o l e s t e r o l benzoate. The twist i s p r o p o r t i o n a l to the concentration of the t w i s t ing molecules i n Cano s experiments and to the square of the conc e n t r a t i o n i n Robinson s. These observations suggest that t h i s twist i s p r o p o r t i o n a l to the frequency of molecular c o l l i s i o n s between c h o l e s t e r i c and nematic molecules (C and N) i n Cano s work and between two c h o l e s t e r i c molecules (C with C) i n Robinson s. A c c o r d i n g l y , i t appears that the twist can be derived from the molecular concentrations i n an analogous way to that of the chemical k i n e t i c s (Bouligand, 1974) . More g e n e r a l l y , i n a c h o l e s t e r i c phase, d i f f e r e n t kinds of molecules may be involved and belong to four main types : N: molecules able to form by themselves a nematic phase. C: molecules able to form by themselves a c h o l e s t e r i c phase. R: non-mesogenic molecules showing a molecular r o t a t o r y power and able to twist a nematic l i q u i d . S : non-mesogenic solvent without any t w i s t i n g i n f l u e n c e . The c o l l i s i o n s between two molecules s u s c e p t i b l e to create an elementary twist are of the f o l l o w i n g types 1
Downloaded by CORNELL UNIV on October 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch015
T
1
T
N+C; N+R; C+C; C+R. The r e s u l t i n g twist i s a l i n e a r expression of the frequencies of the v a r i o u s kinds of molecular contacts and w i l l be : t
ο
= m [N][C]-hn [N][R]-Hn [C][R]+iPL [C] 1
2
J
3
t
2
(7) ,
where m-^ , m , m , m^ are constants at a given temperature and the brackets i n d i c a t e c o n c e n t r a t i o n s . These formulations allow the i n t e r p r e t a t i o n of more recent r e s u l t s (1). Cano s experiments correspond to 2
3
T
t
= m^CHNl-hiiijC] ,
with [C] + [N] = 1.
2
One has thus : t
= m [C]+(m -m ) [C ] l
l+
1
2
and for the range of concentrations studied by Cano 0
