Comparison of resonance Raman spectra of flavin‐3,4‐dihydroxybenzoate charge‐transfer complexes in three flavoenzymes

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Abstract
Various flavoenzymes can form charge‐transfer (CT) complexes with their substrates or analogs. The CT bands are red‐shifted from the flavin absorption bands. In the present study, under the resonance Raman condition when 647.1 nm excitation was used, resonance Raman spectra were obtained for three CT complexes formed by 3,4‐dihydroxybenzoic acid (protocatechuic acid, PCA) with the flavoenzymes old yellow enzyme I (OYE I), 12‐oxophytodienoate 10,11‐reductase (LeOPR I) and dihydroorotate dehydrogenase A (DHOD A). Many flavin ring modes were intensity‐enhanced under these conditions, although to varying degrees. However, the Raman intensity enhancements of the PCA modes were highly selective. Using phenolate binding to OYE I as a model system, and deuterium substitution in PCA, the enhanced modes were identified as benzene in‐plane ring modes containing contributions from C–H bending. The intensity enhancement for these modes is up to several thousand‐fold. The mechanism for the selective intensity enhancement is not understood. Although the CT absorption bands for PCA binding to OYE I, DHOD A and LeOPR I are very similar, the positions of the Raman modes of bound PCA and their response to deuterium substitution are distinct. This demonstrates that the Raman data can detect differences in the chemistry of the CT binding sites that are not apparent in the absorption spectral data. Copyright © 2001 John Wiley & Sons, Ltd.