Vibrational analysis of various isotopomers of L‐Alanyl‐L‐Alanine in aqueous solution: Vibrational absorption, vibrational circular dichroism, Raman, and Raman optical activity spectra

Abstract

AbstractIn a recent work (Knapp‐Mohammady, M.; Jalkanen, K. J.; Nardi, F.; Wade, R. C.; Suhai, S. Chem Phys 1999, 240, 63–77) the structures of the zwitterionic species of L‐alanyl‐L‐alanine (LALA) in aqueous solution using a combination of molecular mechanics (MM) and density functional theory (DFT) have been reported. Subsequently, the vibrational absorption (VA) and vibrational circular dichroism (VCD) and the Raman and Raman Optical Activity (ROA) spectra have been reported. In this work an analysis of the aqueous solution VA, VCD, Raman, and ROA spectra for various isotopomers of LALA are reported. DFT Becke3LYP/6‐31G* theory has been used to determine the geometry, Hessian, atomic polar tensors (APT), and atomic axial tensors (AAT), and the electric dipole–electric dipole polarizability derivatives (EDEDPD), which are required for us to simulate the VA, VCD, and Raman spectra. The electric dipole–magnetic dipole polarizability derivatives (EDMDPD) and the electric dipole–electric quadrapole polarizability derivatives (EDEQPD) have been calculated at the RHF/6‐31G* level of theory. The VA, VCD, Raman, and ROA spectral simulations for the various isotopomers are compared with the experimentally measured spectra. With the DFT, explicit water molecules, and a continuum solvent model we are better able to reproduce the vibrational absorption and Raman spectra than previously reported. The AAT have been implemented at the DFT level, although not within the continuum treatment. The VCD sign pattern could be reproduced with the DFT atomic axial tensors calculated for the LALA plus explicit water molecules. The continuum treatment of the solvent for the calculation of these tensors appears to be a secondary effect. The ROA spectra are not well reproduced due to the failure to take into account electron correlation via DFT and the continuum treatment of the solvent for the EDMDPD and EDEQPD. We look forward to reporting ROA simulations utilizing more accurate tensors in the near future. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003