Conformational analysis of polysaccharides. Part II. Alternating copolymers of the agar–carrageenan–chondroitin type by model building in the computer with calculation of helical parameters
Model building in the computer has been used to analyse the conformational effects of steric interactions between atoms of the polymer skeleton for κ-carrageenan, ι-carrageenan, λ-carrageenan, agar, chondroitin, chondroitin sulphates, dermatan sulphate, keratan sulphate, hyaluronic acid, and related polysaccharides. Over 99% of the conformations are thus excluded and virtually all of the remainder for each polysaccharide lie close together. Predictions have also been made from disaccharide crystal structures and checked against experimental results for polysaccharides where these are available. Geometrical properties of all the allowed conformations were calculated and compared, and the ranges show a remarkable similarity within the entire group of seaweed and animal polysaccharides. Steric restrictions cause all the polysaccharides to have rather extended conformations. The main distinction is in chain flexibility as indicated by the proportion of allowed conformations. This tends to be lower for the glycosaminoglycans because they have more large equatorial groups adjacent to each glycosidic oxygen. It is suggested that a common stereochemical theme underlies some of the properties and functions of all these polysaccharides.