Adsorption configurations and energies of amino acids on anatase and rutile surfaces

Stable adsorption configurations of several amino acid monomers on anatase (101) and (001) and rutile (110) as well as (100) were found in Car-Parrinello simulations of aqueous solutions. Adsorption energies were calculated by averaging over trajectories of the adsorbed and desorbed configuration, taking into account thermal fluctuations of the potential energy. The small adsorption energy of the cysteine on the stoichiometric (110) rutile surface is largely enhanced by inserting the S into an oxygen vacancy. Values for glutamic acid and lysine were significantly higher in the previously identified hydroxyl contact points (160 and 110 kJ/mol, respectively) than on the stoichiometric rutile surfaces (70 and 40 kJ/mol). Adsorption of histidine and glutamic acid on anatase largely depended on the surface orientation. Glutamic acid binds strongly to (101), whereas histidine on (001) was so stably bound that no molecular desorption was achieved. These results coincide with recent experiments on the crystallization of anatase in amino acid solutions [Durupthy, O.; Bill, J.; Aldinger, F. Cryst. Growth Des. 2007, 7, 2696].