Interactions of aspartic acid, alanine and lysine with hydroxyapatite.

Abstract

Adsorption of amino acids (aspartic acid, alanine and lysine) on synthetic hydroxyapatite (HAP; Ca10(PO4)6(OH)2) was investigated. The surface ion (calcium or phosphate ion) with the same sign of the electric charge as that of the terminal group of the adsorbed amino acid was released from HAP. Aspartic acid and lysine showed high affinity to HAP in weak acidic solution and in weak alkaline solution, respectively, owing to the opposite charges of HAP and these amino acids. On the other hand, the affinity of alanine was low and almost independent of the solution pH. It was concluded that the dominant factor for the adsorption is the electrostatic interaction between the amino acid and the HAP surface. This interaction was shielded by the addition of an indifferent salf for HAP : KCl. However, when calcium or phosphate ion was added to the solution, the adsorbed amount of aspartic acid or lysine increased, respectively, due to the increase in the opposite surface charges to that of the adsorbed amino acid.