(621dn) Surface Modification for Enhancing the Enantiospecificity of Chiral Cu Surfaces

The amino acids are building blocks of proteins in the human body. They are chiral, having two enantiomers which has almost identical physical properties. However, they provide dramatically different biological effects. One enantiomer of a drug has good efficacy while the opposite enantiomer often has no effect at all. Therefore, chiral separation has been an important issue in pharmaceutical industry. To separate the enantiomers, utilizing solid chiral surfaces can be one of promising methods. High-miller index metal surfaces often define intrinsically chiral structures. To furthre enhance the enantiospecificity, step decoration, that is doping the kink site of chiral metal surface with second metal, is investigated. It may cause one enantiomer adsorbed on the surface more stable than the opposite, thereby inducing the larger enantiospecific energy difference. Here, we performed density functional theory (DFT) calculations to systemically explore the enhancement of enantiospecificity upon the step decoration. For this, we extensively examined the adsorption configurations and energetics of each enantiomer of alanine, serine, and cysteine, and their enantiospecific energy differences on pure, Pd-, Au-decorated Cu(643)^S, Cu(421)^S, and Cu(531)^S, respectively. By analyzing the relationship between the kink density of each surface and the enantiospecficity, we suggest an optimal kink density which has the largest potential to separate the two enantiomers. Our results provide useful insight of how to improve the enantiospecificity of chiral species on the intrinsically chiral metal surfaces.