Glycine adsorption at nonstoichiometric (010) hydroxyapatite surfaces: A B3LYP study

The adsorption of glycine on the Ca-rich and P-rich HA(010) nonstoichiometric surfaces has been studied at B3LYP level using a polarized triple-ζ basis set within periodic boundary conditions. Although the Ca-rich and P-rich HA(010) nonstoichiometric surfaces exhibit different terminations, giving rise to different electrostatic features in the adsorption regions, glycine preferentially adsorbs as a zwitterion on both surfaces. When adsorbed in a canonical form, the proton of the COOH group is always transferred to the HA(010) surface except for one case, which, in turn, is also the least stable one. Glycine adsorbs by favorable electrostatic interactions between COO ^-/Ca ²⁺ and NH ₃ ⁺/PO ₄ ^2- species, while dispersion interactions play a minor role. The harmonic B3LYP vibrational spectrum is in very good agreement with the experimental one and, when merged with contributions due to glycine adsorbed on stoichiometric HA(010) surfaces, allows one to explain the origin of the "squat shape" of the COO ^- stretching band centered at about 1600 cm ^-1. The comparison also highlights that the Ca-rich rather than the P-rich termination, is the most suitable to explain subtle features of the experimental spectrum.