Starting from the eight B3LYP/6-31G* optimized N-acetyl-L-alanine N′-methylamide (AAMA) structures recently reported by Jalkanen and Suhai, 1 we studied the effect of hydration on the AAMA geometries, relative energies, and vibrational properties by applying the solvent continuum model, by adding four explicit water molecules to each of the conformers, and finally by combining the two approaches. For the four lowest energy AAMA+4H 2 O complexes, we have calculated the B3LYP/6-31G* Hessians and atomic polar tensors (APT), RHF/6-31G* atomic axial tensors (AAT), RHF/6-311+G** electric dipole-electric dipole polarizability derivatives (EDEDPD), RHF/6-31G* electric dipole-magnetic dipole polarizability derivatives (EDMDPD), and RHF/6-31G* electric dipole-electric quadrupole polarizability derivatives (EDEQPD), which gave us the required quantities to simulate the vibrational absorption (VA), Raman, vibrational circular dichroism (VCD), and Raman optical activity (ROA) spectra. The explicit water model reveals the H-bonding between the AAMA and the water molecules. The water stabilizes two AAMA structures (P II and R R ), which are not stable in the isolated state. The influence of the explicit water molecules on the AAMA vibrational spectra was also discussed. The solvent continuum model applied to the AAMA+4H 2 O complexes further modified the orientations of the water molecules and influenced the vibrational modes and intensities. By comparing the calculated and the observed Raman, VCD, and ROA spectra, we suggest that the P II structure of AAMA (φ ∼ -93°, ψ ∼ 128°) is the dominant one in aqueous solutions.

Ab initio B3LYP/6-31G* optimized geometries, vibrational frequencies, and absorption intensities have been calculated for the L-alanine zwitterion (ALAZW) structures stabilized by four neighboring water molecules. The ALAZW structures were stabilized by the addition of four neighboring 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 recommended cavity radius obtained from the solute volume calculations. Geometry optimization of the ALAZW in the presence of the explicit water molecules resulted in different optimized structures for the amino acid itself. The distributed origin gauge atomic axial tensors and the electric dipoleelectric dipole polarizability derivatives calculated at the RHF level of theory were combined with the B3LYP normal modes, frequencies, and atomic polar tensors to calculate the vibrational absorption, the vibrational circular dichroism, and polarized Raman scattering intensities for the ALAZW structures. These calculated vibrational spectra of the solute were found to be very sensitive to the relative arrangement of the neighboring water molecules.

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