Relation between primary structure and chain flexibility of random coil polysaccharides: calculation and experiment for a range of model carrageenans

Abstract

In an attempt to relate conformational features to overall chain flexibilities of polysaccharides as determined by hydrodynamic volume and (for polyelectrolytes) response to ionic strength, a series of carrageenans was derived from the same alternating copolymeric backbone (2) by controlled chemical intoduction of 3,6-anhydro-D-galactose 2-sulphate residues in place of D-galactose 2,6-disulphate. This change is equivalent to replacement of residues in the ⁴C₁ chair conformation with ‘stiff’ linkages to their neighbours, by those in the ¹C₄ conformation joined by ‘flexible’ linkages. Chain flexibilities are characterised in terms of the parameter B introduced by Smidsrød and Haug, whcih is determined from the response of intrinsic viscosity to ionic strength. Semi-empirical values are calculated for comparison by assuming a chain with fixed, standard bond lengths, angles, and ring conformations, within which motions are constrained only by van der Waals attractions and repulsions. Standard methods of configurational statistics lead to values for the Kuhn persistence length A_m, which were then converted by use of an empirical relationship into B values. Experimetal evidence is given that residue replacement does indeed occur at random, as supposed in the theoretical calculations. Good agreement is found between calculation and experiment and we therefore propose a simple method for semi-quantitative prediction of statistical segment length and response of polysaccharide polyelectrolyte to salt, taking into account (i) stereochemical features which determine local freedom of rotation about glycosidic and aglycone bonds and (ii) geometry of individual sugar residues in the chain.