UV RESONANCE RAMAN EXCITATION PROFILES AND DEPOLARIZATION RATIOS AS PROBES OF ELECTRONIC TRANSITIONS IN PEPTIDES

Abstract

UV resonance Raman (UVRR) spectroscopy is well established as a technique for probing secondary structure of peptides and proteins. Excitation between 180 to 215 nm, within the π→π* electronic transitions of the peptide backbone, results in resonance enhancement of those amide vibrations that distort the peptide bond ground state geometry towards that of the excited state. Peptide bond amide π→π* electronic transitions show no emission and appear to be homogeneously broadened. Their broad absorption spectra provide little information about underlying excited states. UVRR spectroscopy is unique in its ability to provide insight into electronic excited state geometry and localization of electronic transitions. We use UVRR excitation profiles and Raman depolarization ratio measurements to examine underlying peptide bond electronic transitions.We measured UVRR excitation profiles and Raman depolarization ratios of peptides in different conformations to elucidate the nature of these electronic transitions. We have examined short Ala peptides which adopt β-type conformations, such as: polyproline II, β-turn, and 2.51 helix, and found additional electronic transitions that underlie the NV1 π→π* transition. For a longer, 21-residue predominantly alanine peptide (AP), AAAAAAAARAAAARAAAARAA, we identified the excitation maxima for the &#945-helix and poly-proline II conformations, as well as exciton splitting in both conformations. We have also examined both the excitation profiles for the arginine (arg) residues in AP, as well as the excitation profiles for individual arg amino acid residues, to determine how peptide conformation affects the individual residue chromophores and to determine the electronic interactions between the arg and the peptide bond NV1 π→π* transition.We show that UVRR excitation profiles and Raman depolarization ratios can be used together to uncover electronic transitions that underlie broad peptide absorption bands. We utilized the UVRR excitation profiles and Raman depolarization ratios to discover exciton splitting of the π→π* electronic transition in AP, charge transfer transitions in short Ala peptides, and interactions between the electronic transitions of the AP peptide backbone and the individual arg side chains in AP. We find evidence of underlying transitions and unusual excitonic interactions that have not been predicted by theory