The interaction of proteins with hydroxyapatite. II. Role of acidic and basic groups

The elution behavior from hydroxyapatite columns of the modification products of seven basic and three acidic proteins has been investigated. Three classes of NH2 derivatives were prepared. These consisted of (1) replacement by a guanidyl group with no change in charge; (2) blocking with loss of charge; and (3) replacement of positive charges by negative ones. Two types of COOH derivatives were prepared: (1) blocking with loss of charge; and (2) replacement of COOH by SO3H with no change in charge. The elution behavior of the derivatives in PO4, F-, Cl-, ClO-4, and Ca2+ ion eluants showed that (1) the elution patterns are determined by the isoelectric points of the proteins, there being no symmetry between the binding or elution behavior of acidic and basic proteins; (2) the binding of basic proteins requires the presence of a high density of positively charged groups; (3) the binding of all proteins to hydroxyapatite equilibrated with phosphate buffer is enhanced by a decrease in the number of their negative charges; and (4) calcium ions affect the binding of proteins to hydroxyapatite at the level of carboxyls, since clusters of carboxyls strengthen both the interaction with Ca2+ and the binding to hydroxyapatite.