Polarization at metal–biomolecular interfaces in solution

Metal surfaces in contact with water, surfactants and biopolymers experience attractive polarization owing to induced charges. This fundamental physical interaction complements stronger epitaxial and covalent surface interactions and remains difficult to measure experimentally. We present a first step to quantify polarization on even gold (Au) surfaces in contact with water and with aqueous solutions of peptides of different charge state (A3 and Flg-Na3) by molecular dynamics simulation in all-atomic resolution and a posteriori computation of the image potential. Attractive polarization scales with the magnitude of atomic charges and with the length of multi-poles in the aqueous phase such as the distance between cationic and anionic groups. The polarization energy per surface area is similar on aqueous Au {1 1 1} and Au {1 0 0} interfaces of approximately −50 mJ m−2 and decreases to −70 mJ m−2 in the presence of charged peptides. In molecular terms, the polarization energy corresponds to −2.3 and −0.1 kJ mol−1 for water in the first and second molecular layers on the metal surface, and to between −40 and 0 kJ mol−1 for individual amino acids in the peptides depending on the charge state, multi-pole length and proximity to the surface. The net contribution of polarization to peptide adsorption on the metal surface is determined by the balance between polarization by the peptide and loss of polarization by replaced surface-bound water. On metal surfaces with significant epitaxial attraction of peptides such as Au {1 1 1}, polarization contributes only 10–20% to total adsorption related to similar polarity of water and of amino acids. On metal surfaces with weak epitaxial attraction of peptides such as Au {1 0 0}, polarization is a major contribution to adsorption, especially for charged peptides (−80 kJ mol−1 for peptide Flg-Na3). A remaining water interlayer between the metal surface and the peptide then reduces losses in polarization energy by replaced surface-bound water. Computed polarization energies are sensitive to the precise location of the image plane (within tenths of Angstroms near the jellium edge). The computational method can be extended to complex nanometre and micrometer-size surface topologies.