Polarization-Induced Atomic Reconfigurations of Glycine on a Metal Surface

The atomic reconfigurations of the simplest amino acid, glycine, at a realistic metal surface including a step and real surface charge are calculated. A boundary-element method for determining the polarization-induced binding of molecules to surfaces in the presence of a dielectric is coupled to quantum-chemical calculations of the gas-phase molecule. Enthalpies and potentially derived fractional charges are determined using 6-31G** basis functions and MP2 level of theory for 216 different configurations of glycine. A parameterized model captures the quantum results as a function of bond lengths, angles, and dihedrals (average deviation of <1 kcal/mol). The molecule interacts with a discretized substrate, a face-centered cubic metal slab. The slab is treated as an explicit region of lattice atoms which may include real charge, requiring the boundary conditions and hence the formalism of R. J. Zauhar and R. S. Morgan [J. Mol. Biol. 186, 815 (1985)] to be modified. The results indicate that anionic glycine is bound to a Cu <100> surface by approximately 1.6 eV in the presence of a step of atomic dimensions in a geometric configuration in agreement with the experiments of K. Uvdal, P. Bodo, A. Ihs, B. Liedberg, and W. R. Salaneck [J. Colloid Interface Sci. 140, 207 (1990)]. Neutral glycine and zwitterionic glycine are found to be oriented with their N ends toward a neutral surface in agreement with experiment. These molecules are furthermore found to undergo a critical phenomenon: Their molecular orientation "flips" by 180° with the introduction of a small, critical magnitude of real surface charge.