Resonance Raman study of flavins and the flavoprotein fatty acyl coenzyme A dehydrogenase

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Abstract
The resonance Raman (RR) spectra of FMN, FAD, FAD in D2O and 7,8-dimethyl-1,10-ethyleneisoalloxazinium perchlorate were obtained by employing KI as a collisional fluorescence-quenching agent. The spectra are very similar to those obtained recently by using the CARS technique to eliminate fluorescence. Spectra were also obtained for several species in which flavin is known to fluoresce only weakly. RR spectra of protonated FMN, FMN semiquinone cation, the general fatty acyl-CoA dehydrogenase and 2 charge-transfer complexes of fatty acyl-CoA dehydrogenase are reported. Tentative assignment of several vibrational bands can be made on the basis of the flavin spectra. RR spectra of fatty acyl-CoA and its complexes are consistent with the previous hypothesis that visible spectral shifts observed during formation of acetoacetyl-CoA and crotonyl-CoA complexes of fatty acyl-CoA dehydrogenase result from charge-transfer interactions where ground state is essentially nonbinding as opposed to interactions in which complete electron transfer occurs to form FAD semiquinone. The only significant change in the RR spectrum of FAD on binding to enzyme occurs in the 1250-cm-1 region of the spectrum, a region associated with .delta.N-H of N-3. The position of this band in fatty acyl-CoA dehydrogenase and the other flavoproteins studied to date is discussed in terms of hydrogen bonding between flavin and protein.