UV Resonance Raman Investigation of Electronic Transitions in α-Helical and Polyproline II-Like Conformations
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- 作者：Bhavya Sharma ; Sergei V. Bykov ; Sanford A. Asher
- 刊名：Journal of Physical Chemistry B
- 出版年：2008
- 出版时间：September 18, 2008
- 年：2008
- 卷：112
- 期：37
- 页码：11762-11769
- 全文大小：441K
- 年卷期：v.112,no.37(September 18, 2008)
- ISSN：1520-5207
文摘
UV resonance Raman (UVRR) excitation profiles and Raman depolarization ratios were measured for a 21-residue predominantly alanine peptide, AAAAA(AAARA)3A (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 polyproline II (PPII)-like peptide conformations. AP is known to be predominantly α-helical at low temperatures and to take on a 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 NV1 transition of one amide group with higher energy electronic transitions of other amide groups, which decreases the α-helical NV1 (π→π*) oscillator strengths. Excitation profiles of the α-helix and PPII-like conformations indicate that the highest signal-to-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 the 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.