UV Resonance Raman Elucidation of the Terminal and Internal Peptide Bond Conformations of Crystalline and Solution Oligoglycines

Spectroscopic investigations of macromolecules generally attempt to interpret the measured spectra in terms of the summed contributions of the different molecular fragments. This is the basis of the local-mode approximation in vibrational spectroscopy. In the case of resonance Raman spectrosdopy, independent contributions of molecular fragments require both a local-mode-like behavior and the uncoupled electronic transitions. Here, we show that the deep-UV resonance Raman spectra of aqueous solution-phase oligoglycines show independent peptide bond molecular fragment contributions, indicating that peptide bond electronic transitions and vibrational modes are uncoupled. We utilize this result to separately determine the conformational distributions of the internal and penultimate peptide bonds of oligoglycines. Our data indicate that in aqueous solution, the oligoglycine terminal residues populate conformations similar to those found in crystals (3(1)-helices and beta-strands) but with a broader distribution, while the internal peptide bond conformation's are centered around the 3(1)-helix Ramachandran angles,