Solvent effect on the adsorption behavior of anionic aspartic acid on the HA (110) surface by density functional theory

In this work, the solvent effect on the adsorption of anionic aspartic acid on the HA (110) surface was studied by the density functional theory (DFT). The adsorption energies, densities of states, population charges and difference charge density were analyzed. The 'monodenate' Ca-O and 'bridge bidenate' O-Ca-O interactions are formed between anionic aspartic acid (ASP) and HA (110) surfaces under vacuum and COSMO-water conditions. In the water environment (WE) condition, 'water-bridge' hydrogen bonds (H-bonds) are formed among the-COO^-, H₂O and PO₄ in HA. In the water layer (WL) condition, there are not only 'bridge bidenate' O-Ca-O interactions, but also a 'water-bridge' H-bond and a H-bond between the-NH₂ of ASP and H₂O. In addition, the adsorption energy results suggested that there is the largest interaction of ASP-HA (110)-Vacuum model, the ASP-HA (110)-WL model is the second, the ASP-HA (110)-CW model is the third, and the ASP-HA (110)-WE model is the lowest. Furthermore, the results showed that the adsorption of ASP on the HA (110) surface is lower in the COSMO-water condition than that in vacuum. When the-COO^- is surrounded by water molecules, the adsorption is the lowest due to the inhibition of the main Ca-O electrostatic interaction. However, when there is a water layer on the HA (110) surface in the ASP-HA (110)-WL model, which does not block the-COO^- in ASP completely, the-COO^- can still interact with the Ca atom. Therefore, the adsorption is the relative larger in the ASP-HA (110)-WL model compared with that in the ASP-HA (110)-WE model, however, it is weaker compared with the ASP-HA (110)-Vacuum model. The conclusions are hopeful to provide valuable theoretical guidance on the interaction of amino acids and HA in different solvents. (authors)