UV Resonance Raman Investigation of Electronic Transitions in α-Helical and Polyproline II-Like Conformations

Abstract

UV resonance Raman (UVRR) excitation profiles and Raman depolarization ratios were measured for a 21-residue predominantly alanine peptide, AAAAA(AAARA) 3 A (AP), excited between 194 and 218 nm. Excitation within the π→π* electronic transitions of the amide group results in UVRR spectra dominated by amide vibrations. The Raman cross sections and excitation profiles provide information about the nature of the electronic transitions of the α-helix and PPII-like peptide conformations. AP is known to be predominantly α-helical at low temperatures and takes on a polyproline II (PPII) helix-like conformation at high temperatures. The PPII-like and α-helix conformations show distinctly different Raman excitation profiles. The PPII-like conformation cross sections are approximately twice those of the α-helix. This is due to hypochromism that results from excitonic interactions between the NV 1 transition of one amide group with the higher energy electronic transitions of other amide groups, which decreases the α-helical NV 1 (π→π*) oscillator strengths. Excitation profiles of the α-helix and PPII-like conformations indicate that highest signalto-noise Raman spectra of α-helix and PPII-like conformations are obtained at excitation wavelengths of 194 and 198 nm, respectively. We also see evidence of at least two electronic transitions underlying the Raman excitation profiles of both the α-helical and PPII-like conformations. In addition to the well known ∼190 nm π→π* transitions, the Raman excitation profiles and Raman depolarization ratio measurements show features between 205-207 nm, which in the α-helix likely results from the parallel excitonic component. The PPII-like helix appears to also undergo excitonic splitting of its π→ π* transition which leads to a 207 nm feature.

Elucidating Peptide and Protein Structure and Dynamics: UV Resonance Raman Spectroscopy

“…This will ultimately enable UVRR to detect HB of side chains in peptides and proteins. The arg side chain shows an isolated band at 1178 cm −1 which is easily observed in AP, for example 46. Our work is also investigating the role of arg residues in α-helix stabilization.…”

“…The lowest energy n→π* transition at ~210 nm is electronically forbidden and weak. It gives rise to the strong CD signature of the α-helix conformation but gives rise to negligible resonance Raman enhancement 19,46. The ~190 nm π→π* transition is strongly allowed, with a strong absorption band that gives rise to strong UVRR intensities 47.…”

Conformation of poly-l-glutamate is independent of ionic strength

“…15 Also, the AmIII 3 region intensity slightly decreases. 1, 16 Surprisingly, the relative intensity of PPII to 2.5 1 -helix bands does not change, even though the 2.5 1 -helix conformation is stabilized by electrostatic repulsion between side chains. 14, 17 The intensity in the AmI region increases dramatically because of a surprising increase in the Raman cross section of the underlying ~1660 cm −1 water O-H bending band due to the presence of a Cl − → water charge transfer band.…”

Interaction of mercury and copper on papain and their combined inhibitive determination

“…2B. All the samples similarly displayed the dual absorbance spectra, a strong absorbance with a peak at 210 nm and a broad band centered around 280 nm, where a weak n → * electronic transition occurred at 210 nm for ␣-helix structure while a higher frequency → * electronic transition at 280 nm for Trp residues [8,14,19]. Clearly, the UV absorption intensity of papain changed with the variation of the binary ions concentration, but its shape almost kept in the same.…”