Adsorption of proline and glycine on the TiO2(110) surface: a density functional theory study.

The optimal adsorption modes for the amino acids glycine and proline on the ideal TiO(2)(110) surface are investigated by using density functional theory (PBE) applying periodic boundary conditions. Binding modes with anionic acid moieties bridging two titanium atoms after transferring a proton to the surface are the most stable configurations for both molecules investigated-similar to previous results for carboxylic acids. In contrast to the latter compounds, amino acids can form hydrogen bonds via the amino group towards the surface-bound proton; this provides an additional stabilisation of 15-20 kJ mol(-1). Zwitterionic binding modes are less stable (by 10-20 kJ mol(-1)) and are less important for proline. Neutral modes are energetically even less favourable. Calculations of vibrational frequencies and core-level shifts complement the adsorption study and provide guidance for future experimental investigations. Control of the computational parameters is crucial for the derivation of accurate results. The layout and thickness of the slab model used are also shown to be decisive factors. Calculations with a different GGA-functional (PW91) provide very similar relative energies, although the absolute energies change by about 20 kJ mol(-1). Results derived with the hybrid functional PBE0 show an even greater stabilisation of the anionic binding modes with respect to the zwitterionic modes. A previously observed discrepancy between experimental and theoretical results for glycine could be solved, although the experimentally proposed free rotation of the C-C bond could not be reproduced.