A comparative study of MP2, B3LYP, RHF and SCC-DFTB force fields in predicting the vibrational spectra of N-acetyl-L-alanine-N′-methyl amide:: VA and VCD spectra

Recently we have looked for spectroscopic probes for secondary structural elements in the vibrational spectra of N-acetyl-L-alanine N'-methyl amide (NALANMA), L-alanine (LA), N-acetyl-L-alanyl-L-alanine N'-methyl amide (NALALANMA) and L-alanyl-L-alanine (LALA). Our goal has been to identify spectroscopic probes which can be used to identify specific secondary structural elements in peptides, polypeptides and proteins. In this work we present our comparative analysis of the MP2, B3LYP, RHF and SCC-DFTB quantum force fields to predict the vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra of NALANMA. We have utilised MP2/6-31G*, B3LYP/6-31G*, RHF/6-31G* and SCC-DFTB level theory to determine the geometries and Hessians, atomic polar tensors (APT) and atomic axial tensors (AAT) which are required for simulating the VA and VCD spectra. We have also calculated the AAT at the RHF/6-31G** level and utilised the above force fields to document the difference between the RHF and B3LYP methodologies for the calculation of these tensors. The VA and VCD simulations are compared with the experimentally measured spectra for NALANMA. (C) 1999 Elsevier Science B.V. All rights reserved.