Different proton transfer channels for the transformation of zwitterionic alanine-(H₂O)(n=2-4) to nonzwitterionic alanine-(H₂O)(n=2-4): a density functional theory study

Theoretical study of the vibrational properties of L-alanine and its zwitterionic form in the gas phase and in solution

Theoretical calculations have been performed to study the structures of L-alanine and its zwitterionic form in the gas phase and in solution. The energies and the vibrational frequencies of the L-alanine in both neutral and zwitterionic form have been calculated using HF, MP2, CCSD and density functional theory (DFT) B3LYP hybrid exchange correlation functional with different basis sets. The conductor-like solvation model of the polarizable continuum model (CPCM) has been used to compute energies, geometric structures and harmonic frequencies in solution for the L-alanine and its zwitterionic form. We have shown how the choice of the basis sets is important to improve the accuracy of the model theory. We have also observed significant changes in frequencies when L-alanine and its zwitterionic form are in solution. Classical Molecular dynamic simulation has also been conducted to get an idea about the organization of water molecules around alanine's hydrophilic sites. The results indicated that water molecules seems to prefer keeping a structure formed by hydrogen bonded water-water molecules around the COO^- site rather than forming N+-H…Ow hydrogen bond.Communicated by Ramaswamy H. Sarma.