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Chiral selectivity of amino acid adsorption on chiral surfaces--the case of alanine on Pt.

We study the binding pattern of the amino acid alanine on the naturally chiral Pt surfaces Pt(531), Pt(321), and Pt(643). These surfaces are all vicinal to the {111} direction but have different local environments of their kink sites and are thus a model for realistic roughened Pt surfaces. Alanine has only a single methyl group attached to its chiral center, which makes the number of possible binding conformations computationally tractable. Additionally, only the amine and carboxyl group are expected to interact strongly with the Pt substrate. On Pt(531), we study the molecule in its pristine as well as its deprotonated form and find that the deprotonated one is more stable by 0.47 eV. Therefore, we study the molecule in its deprotonated form on Pt(321) and Pt(643). As expected, the oxygen and nitrogen atoms of the deprotonated molecule provide a local binding "tripod" and the most stable adsorption configurations optimize the interaction of this "tripod" with undercoordinated surface atoms. However, the interaction of the methyl group plays an important role: it induces significant chiral selectivity of about 60 meV on all surfaces. Hereby, the L-enantiomer adsorbs preferentially to the Pt(321)(S) and Pt(643)(S) surfaces, while the D-enantiomer is more stable on Pt(531)(S). The binding energies increase with increasing surface density of kink sites, i.e., they are largest for Pt(531)(S) and smallest for Pt(643)(S).