Density functional theory study of enantiospecific adsorption at chiral surfaces

Density functional theory calculations are carried out for the adsorption of a chiral molecule, (S)- and (R)-HSCH(2)CHNH(2)CH(2)P(CH(3))(2), on a chiral surface, Au(17 11 9)(S)(). The S-enantiomer is found to bind more strongly than the R-enantiomer by 8.8 kJ/mol, evidencing that the chiral nature of the kink sites at the Au(17 11 9) surface leads to enantiospecific binding. The adsorption of two related chiral molecules, HSCH(2)CHNH(2)COOH ("cysteine") and HSCH(2)CHNH(2)CH(2)NH(2), does not, however, lead to enantiospecific binding. The results of the density functional calculations are broken down into a local binding model in which each of the chiral molecule's three contact points with the surface provides a contribution to the overall adsorption bond strength. The enantiospecific binding is demonstrated to originate from the simultaneous optimization of these three local bonds. In the model, the deformation energy costs of both the molecule and the surface are further included. The model reveals that the molecule may undergo large deformations in the attempt to optimize the three bonds, while the surface deforms to a lesser extent. The most favorable binding configurations of each enantiomer are, however, characterized by small deformation energies only, justifying a local binding picture.