Model studies of the effect of orthophospho-L-serine on biological mineralization

The effect of phospho-L-serine in supersaturated solutions of calcium phosp hate was investigated at plethostatic conditions. Phospho-L-serine inhibite d the crystal growth of hydroxyapatite {Ca5OH(PO4)(3), HAP} by blocking the crystal growth centers through adsorption. Complete inhibition was attaine d with a phospho-L-serine concentration of 4 x 10(-4) M. Analysis of the ki netics and adsorption isotherm results showed a Langmuir-type adsorption. T he amount of phospho-L-serine adsorbed on MAP decreased as pH increased fro m 7.4 to 10.4. The adsorbed species of phospho-L-serine, which in the pH ra nge 7.4-10.4 are mainly the monoprotonated species (HL2-), are located in t he inner Helmholtz plane (IHP) of the double layer at the HAP/electrolyte i nterface, interacting with the surface of HAP with both electrostatic and c hemical interactions. The location of the negatively charged HL2- species a t the IHP caused a shift of the zeta -potential to more negative values. Fi nally, a model was established for the adsorption of phospho-L-serine onto the HAP surface at pH 7.4, according to which an ion pair forms between the protonated amino group of one HL2- species and one negative site of the su rface of HAP, i.e., the phosphate or hydroxyl group. Due to electrostatic r epulsion, the negatively charged deprotonated carboxyl and phosphate groups of the adsorbed HL2- species are oriented in a manner ensuring the maximum possible distance from the negatively charged surface of MAP.