Previous publications (14) have drawn attention to the fact that a broad absorption band emerges between 500 and 650 rnp when one of the acyl dehydrogenasesl is reduced by substrate. It was found more recently (6) that a band of the same int.ensity and charact.erist.ics appears during t,he few seconds in which reduction of t.hese flavoproteins by dit,hionite proceeds or during subsequent oxidation by oxygen. The same phenomenon could be observed with the free flavin compounds FAD2 and FMN (7) .

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... Evidence adduced in a study of the spectra of FAD and FMN during oxidation-reduction (7) and also work by previous investigators (8-13) suggest that the new absorption band may be ascribed to a semiquinoid form of flavin. This paper describes the techniques which were used for recording rapid spectral changes and t,he spectra obtained during oxidation-reduction of three flavoproteins. In addition to one of the acyl dehydrogenases, n-amino acid oxidase of snake venom and t.he old yellow enzyme of yeast were investigated. The old yellow enzyme is of particular interest, as Haas (14) has described a red intermediate of this enzyme which appeared upon partial reduction and was thought to be a semiquinone.