Biopolymers. 9 Springer-Verlag 1986. Optical rotation of branched polysaccharides. The galactomannan case. Oliver Noble, Serge Perez*, Cyrllle Rochas, and ...
Polymer Bulletin 16, 175-180 (1986)
Polymer Bulletin 9 Springer-Verlag 1986
Biopolymers Optical rotation of branched polysaccharides The galactomannan case Oliver Noble, Serge Perez*, Cyrllle Rochas, and Fran(~ols Taravel Centre de Recherche sur les Macromol~cules V~gdtales, C.N.R.S., BP 68, F-38402 Saint Martin d'Heres, France
SUMMARY
Correlation between optical activity and conformational behavior is attempted for the series of oligomannosides and p u r e mannan polysaceharide. Then, the principle of decomposing the contribution to optical rotation into some basic components is extended to the case of b r a n c h e d polymers. It is shown that the derived relationship fits the experimental rotation data g a t h e r e d for this series. Conclusions about the influence of b r a n c h i n g on solution behavior of galactomannan chains are drawn. INTRODUCTION Crystallographic investigations of oligosaccharides s t r u c t u r e s , and the extension of such studies to the elucidation of the solid state structures of p o l y s a c c h a r i d e s , have led to the conclusion that separation of the polymeric backbone into rigid entities (bond l e n g t h s , bond angles and s u g a r r i n g conformation) alternatively oriented about flexible linkages (the glycosidic torsion angles) is valid. Because of the spatial separation afforded b y the rigid s u g a r residues which are interpolated between the flexible linkages, the independence of the sets of glycosidic torsional angles from neighboring sets within the polysaceharide chain has been shown to be a valid model, at least in a first approximation. As a consequence, the principle of decomposing a given observable into additive contributions of adequately choosen segments has emerged. Following the work of Whiffen (1) and Brewster (2) in the late 50's, Rees and collaborators (3,4) have pionnered the field of c o n n e c t i n g information derived from optical rotation measurements to eonformational behavior of oligo and p o l y s a c c h a r i d e s . These attempts follow the established tradition of c a r b o h y d r a t e chemistry, where optical activity has been used as an index to monitor the spatial relationships between c o n s t i t u e n t s . Many systems of rules have been p r o p o s e d to relate monochromatic optical rotations, usually at the sodium D-line (589nm), to stereoehemical f e a t u r e s . Optical aetivity of p o l y s a c c h a r i d e s has been shown to be sensitive to chain conformation, indicating that contributions to total activity can arise not only from the s t r u c t u r e and conformation of individual c a r b o h y d r a t e moieties, along with their configurations at the anomeric c e n t e r s , but also from torsion angles about the glycosidic linkages. * To whom offprint requests should be sent
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Based upon the principle of decomposing the contribution to optical rotation into some basic components, the concept of linkage rotation IAI was i n t r o d u c e d ( 3 ) . This r e p r e s e n t s the contribution of the glycosidic linkage to the molar rotation of a disaccharide s e g m e n t . In what follows, a somewhat simplified definition will be u s e d . For a d i s a c c h a r i d e , the linkage rotation IAI is defined as follows :
IAI
--
IMID
where I M I D
-
, I IlR
(IMIR
§
and I zvil N represent the mo,ar rotation of the
disaccharide s e g m e n t , the r e d u c i n g r e s i d u e , and the n o n - r e d u c i n g r e s i d u e , r e s p e c t i v e l y . In o r d e r to u s e in an efficient way I M |~T s h o u l d h a v e the same anomeric configuration t h a n the one at t ~ glycosidic linkage in the disaccharide. Equation can be e x t e n d e d to the more complex c a s e s homopolysaccharides h a v i n g d e g r e e of polymerization (DP) = n.
IPln = where
