Polymorphism in Cysteine Adsorption on Au Surfaces: Zwitterions versus Uncharged Molecules

The adsorption of amino acids on coinage metal surfaces is of interest for a range of biological applications. Core to the advancements of these applications is understanding the structure of the adsorbed molecules and the state they are present in. Cysteine, the focus of this study, has been studied extensively, both experimentally and theoretically. Here, density functional theory (DFT) and DFT-driven molecular dynamics are used to examine the different adsorption modes of the cysteine monomer on an Au(111) surface in vacuum. Dimeric structures and their influence on the adsorption mode of the individual molecules are also considered. We find that the most stable monomeric binding mode is an unconventional zwitterion with the ammonium group formed by donation of the mercapto hydrogen. Moreover, we observe the transformation of neutral adsorbed molecules to unconventional zwitterions through direct or indirect proton transfer. Conventional zwitterions are unstable in monomeric form, either in gas phase or adsorbed structure calculations.