  We calculate potentials of the mean force for twenty amino acids in the
vicinity of the (111) surface of gold, for several dipeptides, and for some
analogs of the side chains, using molecular dynamics simulations and the
umbrella sampling method. We compare results obtained within three different
force fields: one hydrophobic (for a contaminated surface) and two hydrophilic.
All of these fields lead to good binding with very different specificities and
different patterns in the density and polarization of water. The covalent bond
with the sulfur atom on cysteine is modeled by the Morse potential. We
demonstrate that binding energies of dipeptides are different than the combined
binding energies of their amino-acidic components. For the hydrophobic gold,
adsorption events of a small protein are driven by attraction to the strongest
binding amino acids. This is not so in the hydrophilic cases - a result of
smaller specificities combined with the difficulty for proteins, but not for
single amino acids, to penetrate the first layer of water. The properties of
water near the surface sensitively depend on the force field.
