Hydroxyapatite crystallization in the presence of amino acids with uncharged polar side groups: Glycine, cysteine, cystine, and glutamine

The crystal growth of hydroxyapatite (HAP), the most stable calcium phospha te phase at 37 degreesC, pH 7.40, and ionic strength 0.15 M, in the presenc e of four amino acids with uncharged polar side groups (glycine, cysteine, cystine, and glutamine) has been investigated by the constant composition t echnique: Crystallization took place exclusively on well-characterized HAP crystals in supersaturated solutions only with respect to this calcium phos phate salt. The kinetic results indicated a surface diffusion mechanism. Th e presence of the amino acids inhibited the crystal growth of HAP possibly through adsorption onto the active growth sites of the HAP crystal surface. Cysteine in particular caused a reduction in the crystal growth of HAP up to 81%. The kinetic results favored a Langmuir type adsorption model, and t he value of the affinity constant for each amino acid was calculated. Each value was related to the nature of the amino acid side group.