Theoretical Study of Aqueous N-Acetyl-l-alanine N‘-Methylamide: Structures and Raman, VCD, and ROA Spectra

Abstract

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 α 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.