Alanine: Then There Was Water

An ab initio study of the addition of successive water molecules to the amino acid l-alanine in both the nonionized (N) and zwitterionic (Z) forms are presented. The main focus is the number of waters needed to stabilize the Z form and how the solvent affects conformational preference. The solvent is modeled by ab initio electronic structure theory, the EFP (effective fragment potential) model, and the isotropic dielectric PCM (polarizable continuum method) bulk solvation techniques. The EFP discrete solvation model is used with a Monte Carlo algorithm to sample the configuration space to find the global minimum. Bridging structures are predicted to be the lowest energy Z minima after 3-5 discrete waters are included in the calculations, depending on the level of theory. Second-order perturbation theory and PCM stabilize the Z structures by approximately 3-6 and 7 kcal/mol, respectively, relative to the N global minimum through the addition of up to 8 waters. Subsequently, the contributions of each are approximately 1 kcal/mol relative to the N global minimum. The presence of 32 waters appears to be close to converging the N-Z enthalpy difference, DeltaH(N-Z).