Exploration of the Conformational and Reactive Dynamics of Glycine and Diglycine on TiO2: Computational Investigations in the Gas Phase and in Solution

The adsorption of glycine (Gly) on TiO2 in the gas phase and the behavior of a set of preadsorbed diglycine (Gly-Gly) molecules in solution are studied by using classical nonreactive and reactive (ReaxFF) simulations. The initial dynamic phase of Gly adsorption is rendered through the nonreactive evaporation of a droplet followed by reactive dynamics of the deposited peptide layer. Gly adsorbs strongly on the surface in a wide variety of orientations which are dominated by a carboxyl bidentate coordination to two titanium ions. The binding mode involves mainly anionic species, which are formed after transferring a proton to the surface. Gly zwitterions are present in the second layer. In the time scale explored, water activity does not perturb substantially the orientation of preadsorbed Gly-Gly molecules which remain strongly bound to the substrate through their carboxyl groups. The results of this investigation are in satisfactory agreement with previous theoretical studies and available experimental data.