Adsorption processes of Gly and Glu amino acids on hydroxyapatite surfaces at the atomic level

The regulation mechanism of organic additives on the crystallization of inorganic crystal is fundamentally important in biomineralization. Experimentally, it was found that the amino acids glycine (Gly) and glutamic acid (Glu) could lead to the formation of rod- and plate-like hydroxyapatite (HAP) crystallites, respectively. The detailed adsorption behavior of Gly and Glu on HAP crystal faces was studied by molecular dynamics (MD) simulation. The specific adsorption sites and patterns of Gly and Glu on the (100) and (001) faces of HAP crystals were revealed at the atomic level. The amino acids adsorbed on the HAP (00 1) and (100) faces with their positive amino groups occupied vacant calcium sites, and their negative carboxylate groups occupied vacant P or OH sites precisely and formed an ordered adsorption layer. The atomic force microscopy pulling simulation and free energy calculation showed that Glu was much more difficult to depart from the HAP (001) face than that from the (100) face. This result indicated that Glu preferred to adsorb strongly onto the HAP (001) face, which resulted in the formation of plate-like HAP. However, Gly did not show any significantly preferential adsorption between these two HAP faces. Thus, the habits of HAP, rod-like crystallites, were not altered during the HAP crystallization in the presence of Gly. Combined with experimental results, our study demonstrated that the MD simulation of interfacial structures could improve our understanding of biological regulation in mineralization processes at the atomic level.