Deazaflavins have been found to act as potent catalysts in the photoreduction of flavoproteins in the presence of EDTA and other "photosubstrates". In distinction to the catalysis brought about by normal flavins which involves dark reaction of the photoreduced flavin catalyst, the mechanism of the catalysis by deazaflavins has been shown to involve unstable, strongly reducing radicals which are generated by photolysis of a preformed covalent dimer. By this new method it is possible to reduce not only flavoproteins but a variety of other redox proteins, including heme proteins and iron-sulfur proteins. By virtue of its great catalytic efficiency, it is possible to employ concentrations of deazaflavin sufficiently low as not to interfere with the spectral evaluation of the reduced proteins obtained.

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Evidence is given for the fact that the excited flavin triplet (3F10*x) exhibits competitive le-and 2e-transfer chemistry, depending on the nature of the photosubstrate. As an 'external' photoreductant, the 2e-donor borohydride has been investigated. Borohydride is found to compete effectively with the 'internal' 1 e-donors, namely excess starting flavin in the ground state (Flax) and, as primary product, (alky1)dihydroflavin (RFl,,dH). It will be shown that some of the flavin radicals, observed by earlier authors in the reaction of 3Fl& with CH or C -COO-substrates or in the autophotolytic side-chain cleavage of riboflavin, are due to the dye-dye reaction (I) :(1) H i In contrast flavin photoreduction by borohydride or hydrocarbon substrates need not involve radicals, but may in fact be a hydride or 'carbanion-plus-proton' addition towards the highly polar and considerably basic (pK = 4.4) acceptor triplet (cf. reaction 11) RCOO-The products are much more photoreactive than the starting substrates, which leads to the secondary photocomproportionation (111) : 3Fl,*, + R-Fl,,dH + HFl + RFl, (R = alkyl or H). (111) This latter reaction is the second source of radicals in the system. This (cf. 11) 'photohydrogenation' of flavin is mechanistically related to the biological reduction of flavin by CH substrates.The photochemistry of flavins is in fact older than the corresponding ground-state chemistry and even biochemistry, since the side-chain photolysis of vitamin Bz was the first flavin reaction to be discovered [l]. The very complex course of this reaction, yielding lumiflavin at alkaline pH and lumichrome at neutrality, is still not completely resolved [2] (and references therein). This is largely due to a common mistake of insisting that such reactions must involve radical Dedicated to Professor Hellmut Bredereck, Universitat Stuttgart, on his 75th birthday.Abbreviations. Fl,,, flavoquinone, flavin; 3Fl,*,, excited flavin triplet; HFI Fl-, acidic and basic flavosemiquinone; Fl', oxidized flavin radical; HZFlred, flavohydroquinone, 1,5-dihydroflavin.

Ausgehend von der vielfachen Ahnlichkeit im chemischen Verhalten von lichtangeregtem Flavin und Protein-gebundenem Flavin haben wir die Flavin-abhangige Photodehydrierung systematisch untersucht: 3-Benzyllumiflavin (1) reagiert photochernisch mit Carbonsauren wie 3-Indolessigsaure, Phenoxyessigsaure oder (tert-Buty1thio)essigsaure unter Decarboxylierung und Bildung der 4a-alkylierten 4a,S-Dihydroflavinderivate 3a-c. In Gegenwart von Thiodiessigsaure oder Dithiodiessigsaure entsteht das 5-Carboxyrnethyl-1,5-dihydroflavinderivat 4 a bzw. das 4a-(Carboxymethy1thio)derivat 3d. - ReductivePhotoalkylation of theFlavin Nucleus; Structure andReactivity of the Photoproductsl.2)O n the basis of the manifold similarities between flavin photochemistry and flavin biocatalysis, we have made an effort toward systematic investigation of flavin-dependent photodehydrogenation: 3-benzyllumiflavin (1) reacts photochemically with carboxylic acids, e. g. 3-indoleacetic acid, phenoxyacetic acid and (tert-buty1thio)acetic acid, to give 4a-alkylated 4a,5-dihydroflavin derivatives 3 a -c and COz. In the presence of thiodiacetic acid and dithiodiacetic acid the 5-carboxymethyl-l,5-dihydroflavin derivative 4 a and the 4a-carboxymethylthio derivative 3d, respectively, are formed. -Alkyl sulfides are attacked in the cr-position yielding 5-substituted 1,5-dihydroflavin derivatives, which undergo trans-elimination if P-CHz-groups are present; in the case of the thiolane the adduct 4d is found to be stable because of sterical hindrance. With benzaldehyde the 5-benzoyl-l,5-dihydro derivative 4 e is formed. -Alkenes are attacked in the allylic position yielding 4a-alkylated 4a,5-dihydro derivatives, 3e -g. I n the presence of 1,4-cyclohexadiene the 1,5-dihydroflavin 4 c is formed, as also in the case of *) Korrespondenz bitte an diesen Autor richten.