Affinity Scale for the Interaction of Amino Acids with Silica Surfaces

A systematic computational study of the adsorption of 15 different amino acids (AA) (Gly, Ala, Met, Phe. Ser, Thr, Cys, Tyr, Asn, Gin, Asp, Glu, His, Lys, and Arg) on a hydroxylated silica surface has been addressed by ab initio ONIOM2(B3LYP/6-311++G(d,p):MNDO) method within a cluster approach. A model cluster cut out from the (001) surface of an all-silica edingtonite terminated by silanol groups (2.2 OH nm(-2)) was used to simulate the surface. The adsorption process is mainly dictated by the hydrogen-bond (H-bond) interactions between both the COOH moiety and the side-chain functionalities of the considered AA and the terminal silanol groups of the surfaces. The computed adsorption energies were corrected for basis set superposition error and the role of dispersive interactions, not accounted for by the B3LYP functional, were estimated in a posterior fashion showing to be substantial for the adsorption free energies. Large AA and rich in hydrophilic functionalities in the lateral chain exhibit the most favorable adsorption energies because of the complementary role between dispersive interactions and H-bonds of medium strength with the silica surface. On the basis of the computed adsorption energies, an affinity scale of the considered AA for hydroxylated silica surface is established, which indicates that the nonpolar (Gly, Ala, Met, Phe) and the basic ones (His, Lys, Arg) are the least and the most prone to be adsorbed on the silica surface, respectively. Finally, assessments of the reliability of the structures obtained were performed by comparing the computed adsorption energies with experimental data related to the hydrophilic/hydrophobic character of the AA.