AVS 60th International Symposium and Exhibition
    Surface Science Thursday Sessions
       Session SS-ThM

Paper SS-ThM3
Enantiospecific Adsorption and Decomposition of Aspartic Acid on Cu(hkl)R&S Surfaces

Thursday, October 31, 2013, 8:40 am, Room 202 A

Session: Chirality & Enantioselectivity on Surfaces
Presenter: A.J. Gellman, Carnegie Mellon University
Authors: A.J. Gellman, Carnegie Mellon University
Y. Yun, Carnegie Mellon University
B.S. Mhatre, Carnegie Mellon University
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One of the key experimental challenges in the study of enantioselective surface chemistry is the development of tools or methods that can distinguish one enantiomer from the other. For two reasons we have identified aspartic acid ( HO2CCH(NH2)CH2CO2H) as an ideal chiral probe for studying enantioselective surface chemistry on naturally chiral Cu single crystal surfaces. Firstly, it exhibits extremely highly enantiospecific surface reaction rates because of the fact that it decomposes by an explosive mechanism with highly non-linear kinetics. Secondly, it is an amino acid and as a consequence we are able to use * L-aspartic acid-1,4-13C2 (*L-Asp) which can be distinguished from D-aspartic acid (D-Asp) using mass spectrometry. These have been studied on the achiral Cu(110) and chiral Cu(3,1,17)R&S surfaces. Decomposition of Asp on the Cu(110) and chiral Cu(3,1,17)R&S surfaces yields CO2 and CH3-CºN as primary products. Not surprisingly, 13C labeling in * L-aspartic acid-1,4-13C2 show that the 13CO2 comes only from the carboxylate end-groups while labeling of other atoms shows that the CH3-CºN product arises only form the C2 and C3 carbon atoms. Exposure of a racemic mixtures of *L-Asp and D-Asp has been used to establish enantiospecific adsorption equilibria on the Cu(3,1,17)R&S surfaces. During exposure to the racemic mixture, the enantiomer with the higher binding energy is capable of displacing the more weakly bound enantiomer to establish a non-racemic adsorbed layer. In other words, exposure of the Cu(3,1,17)R&S surfaces to the racemic mixture in the gas phase results in a separation and enrichment of one of the two enantiomers on the surface. This can be measured quantitatively to determine the ratios of the enantiospecific adsorption equilibrium constants; KSD/KRD = KRL/KSD = 2.29 ± 0.17. These translate into an enantiospecific difference in free energies of adsorption of ΔΔG = 3.15 ± 0.29 kJ/mol. This favors adsorption of L-Asp on the Cu(3,1,17)R surface and D-Asp on the Cu(3,1,17)S surface. Although the adsorption energetics of D-Asp and *L-Asp on the Cu(3,1,17)R&S surfaces are enantiospecific, the decomposition kinetics are not. However, on the Cu(643)R&S surfaces the decomposition kinetics exhibit extremely high enantioselectivity. This arises from the non-linear kinetics of the explosion decomposition mechanism.