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 andVCD spectra

Recently we have looked for spectroscopic probes for secondary structural e lements in the vibrational spectra of N-acetyl-L-alanine N'-methyl amide (N ALANMA), L-alanine (LA), N-acetyl-L-alanyl-L-alanine N'-methyl amide (NALAL ANMA) and L-alanyl-L-alanine (LALA). Our goal has been to identify spectros copic probes which can be used to identify specific secondary structural el ements in peptides, polypeptides and proteins. In this work we present our comparative analysis of the MP2, B3LYP, RHF and SCC-DFTB quantum force fiel ds to predict the vibrational absorption (VA) and vibrational circular dich roism (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 Hessia ns, atomic polar tensors (APT) and atomic axial tensors (AAT) which are req uired for simulating the VA and VCD spectra. We have also calculated the AA T 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 experime ntally measured spectra for NALANMA. (C) 1999 Elsevier Science B.V. All rig hts reserved.