STRUCTURE AND VIBRATIONAL-SPECTRA OF THE ZWITTERION L-ALANINE IN THE PRESENCE OF EXPLICIT WATER-MOLECULES - A DENSITY-FUNCTIONAL ANALYSIS

Ab initio B3LYP/6-31G optimized geometries, vibrational frequencies, and absorption intensities have been calculated for the L-alanine zwit terion (ALAZW) structures stabilized by four neighboring water molecul es. The ALAZW structures were stabilized by the addition of four neigh boring water molecules because at the B3LYP/6-31G level of theory the ALAZW is not stable in the absence of the water molecules and will be converted to the nonionized species. The ALAZW was not stable at this level of theory within the Onsager continuum model using the recommen ded cavity radius obtained from the solute volume calculations. Geomet ry optimization of the ALAZW in the presence of the explicit water mol ecules resulted in different optimized structures for the amino acid i tself. The distributed origin gauge atomic axial tensors and the elect ric dipole-electric dipole polarizability derivatives calculated at th e RHF level of theory were combined with the B3LYP normal modes, frequ encies, and atomic polar tensors to calculate the vibrational absorpti on, the vibrational circular dichroism, and polarized Raman scattering intensities for the ALAZW structures. These calculated vibrational sp ectra of the solute were found to be very sensitive to the relative ar rangement of the neighboring water molecules.