Fragmentation and Modification of r‐Carrageenan and Characterisation of the Polysaccharide Order‐Disorder Transition in Solution

Abstract

A procedure is described by which the algal polysaccharide, l-carrageenan, can be modified and fragmented to products of more regular structure and shorter chain length. Since most chains of this product can form uninterrupted helices, they undergo the disorder-order transition without the gelation that is a feature of the natural polymer. The disorder-order transition becomes much sharper and occurs at higher temperatures with increasing chain length. The sign and magnitude of the accompanying shift in optical rotation can be estimated satisfactorily from the solid-state geometry of the helix using a semi-empirical method for correlating optical rotation with the conformations at glycosidic linkages. In solutions of the helical form, but not the coil form, cationic dyes such as methylene blue or acridine orange have induced optical activity and a changed visible spectrum which is attribute to binding and stacking on the helix. This causes a shift in the temperature of the helix-coil transition. The possibility that the tendency of dyes to stack could drive polysaccharides into conformations that they would not otherwise adopt, with implications for their use as conformational probes, is discussed.