Simulations of oligopeptide vibrational CD: Effects of isotopic labeling

Simulated ir absorption and vibrational CD (VCD) spectra of four alanine-ba sed octapeptides, each having its main chain constrained to a different sec ondary structure conformation, were analyzed and compared with experimental results for several different peptides. The octapeptide simulations were b ased on transfer of property tensors from a sei ies of ab initio calculatio ns for a short L-alanine based segment containing 3 peptide bonds with rela tive phi, psi angles fixed to those appropriate for alpha-helix, 3(10)-heli x, Proll-like helix, and beta-sheet-like strand. The tripeptide force field (FF) and atomic polar tensors were obtained with density functional theory techniques at the BPW91/6-31G** level and the atomic axial tensor at the,n ix ed BPM/91/6-31G**/HF/d-31G level. Allowing for frequency correction due to the FF limitations, the octapeptide results obtained are qualitatively c onsistent with experimental observations for ir and VCD spectra of polypept ides and oligopeptides in established conformations. In all cases, the corr ect VCD sign patterns for the amide I and II bands were predicted, but rite intensities did have some variation front the experimental patterns. Predi cted VCD changes upon deuteration of either the peptide or side-chains as w ell as for C-13 isotopic labeling of the amide C=O at specific sites in rit e peptide chain were computed Sol analysis of experimental observations. A combination of theoretical modeling with experimental data Sol labeled comp ounds leads both to enhanced resolution of component transitions and added conformational applicability of the VCD spectra. (C) 2000 John Wiley & Sons , Inc.