Molecular mobility, structure and a new model of the carboxymethylchitin–alkaline-earth metal complex in aqueous systems

Abstract

Complex formation between carboxymethylchitin (CMCh) and alkaline-earth metal ions has been studied using viscoelasticity, SAXS and NMR measurements. Dynamic viscoelastic properties have been measured by means of a cone-and-plate rheometer for aqueous solutions of CMCh in the presence of various alkaline-earth metal salts. The dynamic modulus shifts to a longer time-scale region with addition of the salts, and the frequency-dependent curves of the modulus, which are measured at various salt concentrations, can be superimposed on master curves with only a horizontal shift. This means that the molecular mobility of the CMCh main chain is restricted by complex formation. The degree of restriction does not depend on the choice of alkaline-earth metal ion, but depends only on the number of bound metal ions.The carboxyl carbon is located near the site of complex formation and the radius of gyration of the CMCh molecule is elongated by addition of salt.We propose a new model for complex formation in which a metal ion forms a clamp between a carboxyl group and a hydroxy group of the neighbouring residues.