Adsorption of Amino Acids on Gold: Assessing the Accuracy of the GolP-CHARMM Forcefield and Parametrization of Au-S Bonds.

The interaction of amino acids with metal electrodes plays a crucial role in bioelectrochemistry and the emerging field of bionanoelectronics. Here we present benchmark calculations of the adsorption structure and energy of all natural amino acids on Au(111) in vacuum using a van-der-Waals density functional (revPBE-vdW) that showed good performance on the S22 set of weakly bound dimers (mean relative unsigned error (MRUE) wrt CCSD(T)/CBS = 13.3%) and adsorption energies of small organic molecules on Au(111) (MRUE wrt experiment = 11.2%). The vdW-DF results are then used to assess the accuracy of a popular force field for Au-amino acid interactions, GolP-CHARMM, which explicitly describes image charge interactions via rigid-rod dipoles. We find that whilst the force field underestimates adsorption distances, it does reproduce the binding energy rather well (MRUE wrt to revPBE-vdW = 11.3%) with the MRUE decreasing in the order Cys, Met > amines > aliphatic > carboxylic > aromatic. We also present a parameterization of the bonding interaction between sulfur- containing molecules and the Au(111) surface and report force field parameters that are compatible with GolP-CHARMM. We believe the vdW-DF calculations presented herein will provide useful reference data for further force field development, and that the new Au-S bonding parameters will enable improved simulations of proteins immobilized on Au-electrodes via S-linkages.