Probing Enantioselectivity with X-Ray Photoelectron Spectroscopy and Density Functional Theory

The enantioselectivity of gold is investigated by x-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Cysteine molecules on a chiral $\mathrm{Au}(17119{)}^S$ surface show enantiospecific core level binding energies in the amino and in the thiol group. The sign and order of magnitude of the XPS core level shifts is reproduced by DFT. Identical preparations of $D$- and $L$-cysteine layers lead to $D$-cysteine molecules in the pure ${\mathrm{NH}}_2$ form, while a small portion of $L$-cysteine molecules maintains a hydrogen rich amino group (${\mathrm{NH}}_3$). This implies enantiospecific adsorption reaction pathways and is consistent with DFT that indicates an activated hydrogen abstraction reaction from the amino group, which is downhill for $D$-cysteine.

Figure 1

Structural model of cysteine on $\mathrm{Au}(17119{)}^S$ from 5. The unit cell of the $(17119)$ surface is marked with dashed lines. In the case of $L$-cysteine the amino group binds to a step atom, while for $D$-cysteine it binds to the kink atom. The anticlockwise sequence of $\{110\}$, $\{111\}$, $\{100\}$ facets defines a $S$ kink. The color code of the atoms is given on the left.

Figure 2

X-ray photoelectron emission spectra for $D$- (blue) and $L$-cysteine (red) on $\mathrm{Au}(17119{)}^S$. (a) N $1s$ region. The constant background and two or one Gaussians to fit the $L$- or $D$-cysteine spectra, respectively, were indicated with lines. The ${\mathrm{NH}}_2$ components dominate, where in the case of $L$-cysteine also a ${\mathrm{NH}}_3$ component is visible as a high binding-energy shoulder. (b) S $2p$ region. The spectra are fitted with a doublet of Gaussians with a fixed spin-orbit splitting of 1.2 eV and an intensity ratio of $1:2$. The fits are solid black lines.

Figure 3

Systematic alteration of the core level spectra with x-ray irradiation time. (a) Decrease of the $L$-cysteine ${\mathrm{NH}}_3$ spectral weight. (b) N $1s$ binding energies for $L$-cysteine (red circles) and $D$-cysteine (blue circles). (c) S $2p$ binding energies for $L$-cysteine (red) and $D$-cysteine (blue circles). The fits (solid lines) in (b) and (c) adopt for $D$-cysteine a constant and for $L$-cysteine an exponential with the decay constant found from the $L$-cysteine ${\mathrm{NH}}_3$ spectral weight shown in (a).

Figure 4

Schematic diagram of the total energy of cysteine on the $\mathrm{Au}(17119{)}^S$ surface for the transformation of $\mathrm{CY}\mathrm{\text{−}}{\mathrm{NH}}_3$ to the $\mathrm{CY}\mathrm{\text{−}}{\mathrm{NH}}_2$ and six reaction snapshots as predicted by DFT. (a) Reaction for $L$-cysteine with a higher activation energy across a transition state (TS) with no net energy gain. (b) Reaction for $D$-cysteine with a lower activation energy and a more favorable $D\mathrm{CY}\mathrm{\text{−}}{\mathrm{NH}}_2$ total energy. The snapshots show that hydrogen abstraction from $D$-cysteine is accompanied by a directed motion from one kink to the other, while the reaction of $L$-cysteine happens at the same kink. Energies are indicated in eV; the zero line is referred to the $L\mathrm{CY}\mathrm{\text{−}}{\mathrm{NH}}_2/\mathrm{Au}(17119)$ total energy. Color code as in Fig. 1.