UV RESONANCE RAMAN SPECTROSCOPY STUDY OF PEPTIDE CONFORMATIONAL TRANSITIONS

The conformational transition between alpha-helix-like conformations and the polyproline II conformation (now recognized by many as the conformation of unfolded peptides) is investigated here. We utilized UV resonance Raman spectroscopy together with circular dichroism and nuclear magnetic resonance spectroscopy to investigate the conformations of three polyalanine peptides and the impacts of salt bridge side chain interaction and external surfactants on the transitions between these conformations. We found that the macrodipole-terminal charge interactions typically affect the alpha-helix stability more strongly than the salt bridge side chain interactions do. The alpha-helix-turn-alpha-helix conformation can form in short peptides with ~20 residues. The arginine vibration band at ~1170 cm-1 was found to report on the guanidinium group hydration. Addition of anionic surfactants induces alpha-helix-like conformations in short cationic peptides through the formation of peptide-surfactant aggregates. The studies here highlight the crucial roles of hydrogen bonding, hydrophobic effect and electrostatic interactions in the peptide conformational transitions. In addition, the impact of sample self absorption on the observed resonance Raman intensities has also been theoretically investigated. In general, the Raman intensities increase as the excitation approaches resonance. However, narrow bandwidth impurity absorption can cause the observed Raman intensities to decrease