Adsorption of Amino Acids on Gold: Assessing the Accuracy of the GolP-CHARMM Force Field 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 while
the force field underestimates adsorption distances, it does reproduce
the binding energy rather well (MRUE wrt revPBE-vdW = 11.3%) with
the MRUE decreasing in the order Cys, Met > amines > aliphatic
> carboxylic
> aromatic. We also present a parametrization 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.