Self-Assembly of Cysteine Dimers at the Gold Surface: A Computational Study of Competing Interactions

摘要

The only proteinogenic acid with a mercapto group, cysteine is the main participant in the binding of proteins and peptides to the surfaces of noble metals. A chiral molecule, it becomes a major player in surface patterning for chiral amplification, biosensing, and chiral catalysis. Here, we examine the interplay of molecule-surface and molecule-molecule interactions in the self-assembly process of monomers, dimers, and trimers of L-cysteine on a (1 X 2)-reconstructed Au(110) surface, and the implications for chiral recognition. Multiple adsorbed configurations of L-cysteine and L-cysteinate in neutral and zwitterionic forms were generated using molecular dynamics simulations, serving as starting points for further density functional theory (DFT)-based optimizations. We found that binding for both monomers and dimers was stronger at kink sites formed on the surface during the chemisorption process, and was unlikely to occur along the highly coordinated trough sites. In this, DFT calculations disagreed with MD simulations using centrosymmetric potentials, which tended to maximize coordination of the adsorbate groups, and ignore differences in reactivity of the various Au sites, unless specifically included in the force field. Kink-site bound homochiral L-cysteine dimers were particularly stable relative to both heterochiral and trimer structures, while molecules more weakly bound at more stable surface locations did not exhibit chiral recognition. If barriers to the diffusion of Au atoms along the surface can be overcome, the four-atom vacancy structures proposed by Kuhnle et al. (Nature 2002, 415, 891) provide reactive kink sites, ideally spaced for binding homochiral cysteinate dimers, with highly stable COOH-based hydrogen bonding.