Counter-ion dissociation of chondroitin sulfate adsorbed on hydroxyapatite in water.

Abstract

The intrinsic viscosity and the amount of adsorption of sodium and calcium chondroitin 6-sulfate (Na2Chs and CaChs) on hydroxylapatite (HAP) were measured. The values depended on the degree of counter-ion dissociation of chondroitin-6-sulfate (Chs). Ion activity, a, and apparent degree of dissociation of counter-ions (Na+ and Ca2+), .alpha., for Chs were determined by means of ion-selective electrodes in the presence or absence of HAP. In the case of salt-added systems (NaCl or CaCl2; 5-20 mM), .alpha., obtained from the slope of the relationship between a and [Chs], was almost independent of the concentration of HAP, [HAP] (0-5 g/dl), although a decreased with increase in [HAP] due to counter-ion adsorption by HAP. In the case of salt-free systems, both a and .alpha. apparently decreased with increase in the amount of HAP added, because a portion of the counter-ions dissociated from Chs is consumed by adsorption at the surface of HAP. The apparent degree of ionization (i.e., apparent charge density along the polymer chain) of Chs adsorbed by HAP is evidently almost the same as that for Chs free from HAP in the bulk solution, irrespective of the apparent value of .alpha. obtained from the relationship between a and [Chs]. [Chs is found in cartilage, bone and tooth as a protein complex, which plays an important role in the calcification of biological hard tissues.].