MONTE-CARLO SIMULATION STUDIES OF THE SOLVATION OF IONS .3. THE NON INTRAMOLECULARLY H-BONDED FORM OF GLYCINE ZWITTERION

The stability and the characteristics of the water solution of the non intramolecularly H-bonded (phi = 60-degrees) form of glycine zwitterion were studied with a Monte Carlo simulation in the N,P,T ensemble, using TIP4P potentials for water and analogous transferable potentials for glycine. The results were compared with those from previous simulations of glycine (phi = 0-degrees) and of its component ions. The analysis of the solution shows the presence of the same four types of water already found: (1) bulk waters; (2) hydrophobically bound waters near the CH2 group; waters tightly bound (3) to the polar oxygens and (4) to the polar hydrogens. The energetics of the solution process is more favourable for the phi = 60-degrees form than for the phi = 0-degrees one, as far as the enthalpic term is concerned, by 2.6 kcal/mol, due to the compensation between the presence of an additional H-bond with water, coupled with a slightly smaller disruption of the solvent, and less favourable individual H-bonds on the average. Conversely, a considerable increase in the partial molar volume is detected. The in vacuo energy difference (including Hartree-Fock and thermal energies) between the two conformers at the 6-31G* level favours the phi = 0-degrees form with respect to the phi = 60-degrees one by about 5 kcal/mol, thus the former should be the most likely to be found in solution as well.