Adsorption of Amino Acids at the Gold/Aqueous Interface: Effect of an External Electric Field
Denys Biriukov, Zdeněk Futera
Abstract
Gaining accurate molecular descriptions of metal/bio interfaces is a necessary step toward numerous important applications, particularly in electrochemistry and bionanotechnology. Here, using atomistic molecular dynamics simulations and free energy calculations with the GolP-CHARMM force field, we investigate how applied static electric field disturbs the structure of an aqueous Au (111)/amino acid (AA) interface. We show that adsorption of positively charged AAs (arginine, histidine, and lysine) is more affected by the external electric field than that of negatively charged (aspartic and glutamic acids) and charge-neutral AAs (alanine, glycine, tryptophan, and asparagine). The adsorption free energies of positively charged AAs can vary within 55% when static fields of up to 0.5 V/Å are applied, in contrast to considerably weaker responses of negatively charged and charge-neutral AAs (up to 25%). The difference arises from the role of a charged side chain in the adsorption on the gold surface. Positively charged amines adsorb stronger than negatively charged carboxylates, and this fact together with the trend within positively charged AAs (arginine > histidine > lysine) is related to the affinity of their side chains for Au (111) surfaces and ability to replace water molecules in the first adsorption layer. The adsorption via a deprotonated carboxyl group is less favorable, indirect, and facilitated by hydrogen bonding with the adsorbed water, while the positive amines directly interact with surface gold atoms. This eventually leads to relatively weaker adsorption of negatively charged AAs and their smaller response to the external electric fields due to solvent-induced electrostatic screening. These fundamental results provide a useful insight into the molecular arrangement at the electrified biointerfaces with gold, which can help to interpret electrochemical phenomena and advance bioelectronic applications.