Hybrid inorganic-organic materials are desirable for the use in technological and medicinal applications, in particular since organic and bio-inspired systems present a large range of functionalities, are abundant, cheap, and comparably environment friendly. Theoretical modeling of the electronic structure of these systems is challenging, because the results can be very sensitive to the choice of the exchange-correlation approximation and the high number of possible adsorbate geometries. Furthermore, predicting structures of flexible biomolecules on metal surfaces is a challenge for simulation in itself, because: (i) already at this level, predictions based on empirical force-field potentials are likely bound to fail and (ii) the structure search problem involves a high-dimensional and eventually energetically-degenerate search space.
We here present first-principles structure searches and electronic properties of flexible model peptides on metallic surfaces. For the highly-flexible peptides HisProPheH+ and ArgH+, which are known from experiment to self-assemble into dimers or hexamers on Cu(111), we tackle the structure-search problem by extending the Fafoom first-principles genetic-algorithm package, which works on internal molecular degrees of freedom, by a treatment of position and orientation of molecules with respect to the surface. By comparing two protonation states (Arg and ArgH+) of an amino acid in the gas phase, we conclude that the charge reduces the accessible the size of conformational space: while Arg presents several isoenergetic alternative conformations, ArgH+ presents a single energetically-privileged minimum structure. However, when ArgH+ gets in contact with the Cu(111) surface, the positive charge is efficiently screened by concentration of electrons from the surface close to the charged group and the conformational space is expanded with respect to the gas-phase picture, even though it still retains one well defined minimum. This efficient screening of a peptide’s charge is likely to play a role in the experimentally-observed self-assembly.