Canonical, Deprotonated, or Zwitterionic? A Computational Study on Amino Acid Interaction with the TiO2 (101) Anatase Surface

The interaction of 11 amino acids with the TiO2 (101) anatase surface was investigated by means of PBED-2* periodic simulations, both from a static and dynamic points of view. Several adsorption states, with the amino acid in its canonical, zwitterionic, or deprotonated forms, were considered. The strongest interactions correspond to dative interactions between the electron pairs of N or O atoms of the amino acids and the Ti atoms of the surface. For glycine, the most stable configuration corresponds to the deprotonated (N,O) binding mode, at variance to that observed for the adsorption of glycine on rutile (110) surface, for which the dissociative (O,O) binding mode was determined to be the preferred one. For the remaining amino acids, the following general trend was identified: those amino acids with acidic or a basic lateral chain groups, except Arg, exhibit a deprotonated (N,O) binding mode as glycine, because the additional dative interactions with the side chain overcome the destabilizing effect induced by steric hindrances. However, for the remaining amino acids, with weaker side chain interactions, the zwitterionic state is the most stable conformer because the steric hindrances between the lateral chains and the surface are minimized. Overall, the present results indicate that the most stable amino acid adsorption state on the surface arises from a delicate balance between favorable interactions and steric hindrances of the amino acids with the surface, the latter ones becoming particularly relevant when lateral chains are considered.