The ion-binding capacity of highly purified reconstituted calf-skin collagen, and the effects of these ions on the precipitation and solubility of the collagen, were studied with a variety of salt solutions at ionic strength 0.16 and pH7.4. Only a small percentage of the total theoretically available anionic and cationic groups was available for ion-binding. In view of this, it appears that most of the ionizable groups of collagen are involved in intramolecular or intermolecular linkages, or both. Nevertheless, marked differences in the binding of the various ions by collagen were observed. Bivalent cations were bound in extremely small but remarkably similar quantities. In contrast, sodium was bound both in much higher and more variable quantities. Of the anions, pyrophosphate and sulphate were bound in the largest quantities, followed by phosphate, fluoride and chloride, in that order. Despite the minimal uptake by collagen of bivalent cations, they prevented the aggregation of tropocollagen into fibrils, and disaggregated fibrillar collagen. In the presence of multivalent anions, tropocollagen aggregated readily and its fibrillar stability was maintained. On the basis of the imbalance in the binding of ion pairs by the sodium pyrophosphate- and sodium phosphate-treated collagens, it was apparent that a reduced number of side-chain carboxyl groups were dissociated in the presence of these salts.
