Model studies on the interaction of amino acids with biominerals: The effect of L-serine at the hydroxyapatite-water interface

The effect of L-serine in supersaturated solutions of calcium phosphate was investigated under plethostatic conditions. The rates of crystal growth me asured in the presence of L-serine at relatively high concentrations and in the range between 2 x 10(-3) and 1 x 10(-2) mol dm(-3) were appreciably re duced. The inhibitory effect of L-serine was found to be due to blocking of a portion of the active growth sites by adsorption. Kinetics measurements in the presence of L-serine as well as adsorption isotherm analysis suggest ed Langmuir-type adsorption of L-serine on the surface of hydroxyapatite (H AP) with a relatively low affinity for the substrate. Adsorption experiment s showed that at pH 7.4 considerable adsorption of L-serine onto HAP takes place, whereas at pH 10.0 the adsorption was negligible, suggesting that el ectrostatic interactions are dominant, Attraction between the positively ch arged protonated amino group of the L-serine molecule and the negatively ch arged HAP surface contributed largely to the adsorption. This was corrobora ted by the fact that, in the presence of L-serine in the solution, a signif icant shift of zeta -potential of the HAP particles to less negative values was found at pH values close to 7.4. At pH values higher than 10.0 essenti ally no shift: of zeta -potential takes place. On the basis of the experime ntal results, a model was proposed according to which L-serine absorbs on t he surface of HAP through electrostatic attractions exerted between one neg ative site of the HAP surface, i.e., phosphate or hydroxyl ion, and the pos itively charged protonated amino group of one L-serine molecule, forming a surface ion pair, (C) 2001 Academic Press.