Complex Formation of Gelatin and Chondroitin Sulfate and the Effects on H+ Ion Binding

Soluble and insoluble complex formations of gelatin and chondroitin 6-sulfate (Chs-C) were studied by measuring their viscosities and transmittances; the effects of complex formations on H+ ion binding were studied by pH titrations. Soluble complexes were formed electrostatically with low ionic strengths; the compositions of the complexes varied with pH from those of solutions. The weight fraction of Chs-C of the saturated soluble complex was found to be 0.294 at pH=5.1. The composition of the insoluble complexes increased in the fraction of Chs-C with decreasing pH. The compositions were found to agree with the theoretical predictions obtained from the condition of electroneutrality. Furthermore, from pH titrations of mixed solutions comprising gelatin and Chs-C, the apparent dissociation constants, pKa of the carboxylic groups of both polymers were obtained. They were found to increase due to soluble complex formation; soluble complex formation depresses the dissociation of H+ ions due to a shielding of the positively charged amino groups of gelatin.