Mode Recognition in UV
Resonance Raman Spectra of
Imidazole: Histidine Monitoring in Proteins
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Abstract
The imidazole side-chains of histidine residues perform
key roles
in proteins, and spectroscopic markers are of great interest. The
imidazole Raman spectrum is subject to resonance enhancement at UV
wavelengths, and a number of UVRR markers of structure have been investigated.
We report a systematic experimental and computational study of imidazole
UVRR spectra, which elucidates the band pattern, and the effects of
protonation and deprotonation, of H/D exchange, of metal complexation,
and of addition of a methyl substituent, modeling histidine itself.
A consistent assignment scheme is proposed, which permits tracking
of the bands through these chemical variations. The intensities are
dominated by normal mode contributions from stretching of the strongest
ring bonds, C<sub>2</sub>N and C<sub>4</sub>C<sub>5</sub>, consistent
with enhancement via resonance with a dominant imidazole π–π*
transition
