DENSITY FUNCTIONAL THEORY MODELING OF INTERACTIONS BETWEEN AMINO ACIDS AND CHIRAL MINERAL SURFACES

Hazen and coworkers have demonstrated that right- and left-handed calcite {214} surfaces preferentially select D- or L-aspartic acid, but what is the mechanism of selection? To answer this question we have applied density functional theory (DFT), an accurate first-principles method, to model chiral interactions between amino acids and chiral surfaces of calcite and quartz. DFT has been used to examine the adsorption of amino acids on chiral and achiral metal surfaces, but the extension to mineral surfaces is non-trivial since the mineral surface is more complicated and the calculations are correspondingly more computationally expensive. At the molecular scale, chiral selective adsorption of organic molecules onto a mineral surface requires three non-colinear points of interaction. Adsorption of amino acids likely involves bonding with the amino (NH₂) group and the carboxyl (COOH) group. The orientation of these groups and the nature of the third interaction required for chiral selection are not known. In our initial work we examined the adsorption of alanine, the simplest chiral amino acid, on ideally truncated calcite {214} surfaces. We will report on our extensive calculations to map out the potential energy surface for both L and D-alanine on calcite (214). While these calculations are computationally intensive, they yield detailed quantitative information on the energetics and structural details of adsorption that are not readily accessible through experiments.