Distinction of 2e- and le- Reduction Modes of the Flavin Chromophore as Studied by Flash Photolysis

Abstract

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.

Quenching of Excited Triplet Lumiflavin by Lumiflavin Radicals Formed in Its T‐t Reaction

“…The decay rate parameter of the radicals, ks, was determined [(4 2 0.7) x 10' M -k ' ] from the second order analysis of LFH. after the disappearance of 'LF, which is not different from the published values (6.2 x 10' M-ls-': Vaish and Tollin, 1971 and 3.9 x 10' M-'s-': Hemmerich et al, 1980). When X' is LFH' and LF?, Eq.…”

“…Lumiflavin was synthesized according to Hemmerich et al (1956) and recrystallized from formic acid. 1.4-Benzoquinone was used after recrystallization from water.…”

The Effect of Blue Light on Plants and Microorganisms

“…A model for the specific binding of flavins (coenzyme) to a protein (apoprotein) can be achieved by anchoring the flavin with a long aliphatic chain to the membrane and thus the flavin exposes its four active positions [42,67,101,102,109]. It has been shown that flavins in the artificial systems can undergo both le-and 2e-redox reactions [40,41].…”

Reactivity of Excited States of Flavin and 5‐deazaflavin in Electron Transfer Reactions

“…(a) The addition of nucleophilic substrates to yield the hydroquinonic form of the dye in the primary process (transfer of 2e--equivalents) as recently demonstrated by flash photolysis with the hydride and cyanide anion in the flavin photochemistry (Traber et a!., 1980a;Hemmerich et al, 1980;Traber et al, 1980b) and supposed with oxalate in the 5deazaflavin photochemistry (Goldberg et al, 1980) and (b) the transfer of unpaired electrons to yield the dye semiquinone anion and the substrate radical cation (ie--transfer) which is a common mechanism in dye photochemistry.…”