An ab initio study of adsorption of alanine on the chiral calcite(2131) surface

Density functional theory calculations has been used to examine the adsorption of D- and L-alanine (Ala) on the chiral calcite(211 1) surface. We find negligible differences (< 1 kcal/mol) in adsorption energies for the most stable minima of D- and L-Ala on the calcite(21 $(3) over bar $1) surface. The strongest interaction in the adsorbed system is between the surface Ca and the O atom of the carboxyl group on Ala. The source of the weak enantiospecificity is the relative difference in dimensions of the Ala and the atoms on the calcite(21 $(3) over bar $1) surface. The surface O and Ca atoms on the calcite(21 $(3) over bar $1) surface are separated by 6 angstrom, while the Ala molecule is roughly 3-4 angstrom in size. This disparity in dimensions prevents the Ala molecule from making three strong points of contact with the surface, a prerequisite for strong enantiospecificity.