Paper SS-WeA10
Enantioselectivity and Auto-Amplification by Adsorption

Wednesday, November 12, 2014, 5:20 pm, Room 309

Metal surfaces can be rendered chiral by cleavage along low symmetry planes of the bulk metal lattice. Their chirality results in the enantioselective adsorption of chiral molecules from racemic mixtures. This work has developed and applied a ¹³C isotopic labeling method for mass spectrometric detection and quantification of enantiospecific adsorption on chiral surfaces. Enantiomerically pure chiral compounds in which one enantiomer is available in an isotopically labelled form allow the design of experiments in which one can expose chiral or achiral surfaces to mixtures of varying enantiomeric excess in the gas phase (ee_g) and then use mass spectrometry to determine enantiomeric excess on the surface (ee_s).

Exposure of a racemic mixture of D-aspartic acid and ¹³C-L-aspartic acid to the chiral Cu(3,1,17)^R&S surfaces results in the adsorption of a non-racemic monolayer because of the selective adsorption of one enantiomer over the other. ¹³C-labeling allows mass spectrometry to distinguish the two enantiomers during desorption from the surface.

These measurements have been used to quantify the enantiospecific adsorption equilibrium constants and the enantiospecific difference in the free energies of adsorption of D- and L-aspartic acid on the chiral Cu(3,1,17)^R&S surfaces.

Not surprisingly, exposure of a racemic mixture of D- and ¹³C-L-aspartic acid to the achiral Cu(111) results in the adsorption of a racemic mixture; in other words, a gas phase mixture with ee_g = 0 results in an adsorbed mixture with ee_g = 0. However, exposure of the achiral Cu(111) surface to a gas phase mixture with ee_g = 0.2 results in an adsorbed phase with ee_s = 0.4. In spite of the fact that the surface is achiral adsorption results in auto-amplification of enantiomeric excess. Although the mechanism of auto-amplification has not been confirmed, one can show that this can be a simple consequence of adsorption of gas phase monomers in the form of homochiral clusters L_n or D_n.

In general, during adsorption on chiral surfaces, the phenomena of enantiospecific adsorption and auto-amplification must be occurring and either competing or augmenting one another.