Flash photolysis of flavins. V. Oxidation and Disproportionation of flavin radicals

Vaish, Shiv P.; Tollin, Gordon

doi:10.1007/bf01648921

Cited by 94 publications

(72 citation statements)

References 15 publications

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“…This D,D reaction corresponds to the photoelectron transfer, which has been observed earlier by Vaish and Tollin [5,6] with the 'binary' system flavin/ phenol, the latter being a 'le--only' donor substrate The rate of radical decay is then determined by the unstable substrate radical, and not by the much more slower dismutation of the flavosemiquinone.…”

Section: Flash Photolysis Of Flavin In Aqueous Buifer: Fi As Donorsupporting

confidence: 52%

“…Holmstrom [4] was the first to observe such radicals in riboflavin photolysis, while Vaish and Tollin [5,6] demonstrated fast photoelectron transfer and spontaneous back donation with suitable substrates, such as phenol :…”

Section: (111)mentioning

confidence: 99%

“…2 the reformation of 3-methyllumiflavin after the flash is shown. Without NaBH3CN (curve a) a fast regeneration due to the triplet decay is observed, followed by a second-order recombination of Fl+ and HFl [Eqn (6)]. Nearly all the flavin is regenerated (total bleaching 5 %, reversible bleaching 2-3 %).…”

Section: Flush Photolysis In the Presence Of'bh3cnmentioning

confidence: 99%

“…This substrate, however, will reduce the 'oxidized radicals' obtained by either of the reactions (Sa) and (7a), as shown in Eqn [ll]: (11) thus transforming the rapid zero-bleaching process [Eqn (6)] into the slower half-bleaching process [Eqn (9)]. This is also proved by the decrease of radical decay rate.…”

Section: Flush Photolysis In the Presence Of'bh3cnmentioning

confidence: 99%

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

Hemmerich

Knappe

Kramer³

et al. 1980

Eur J Biochem

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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.

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Section: Flash Photolysis Of Flavin In Aqueous Buifer: Fi As Donorsupporting

confidence: 52%

Section: (111)mentioning

confidence: 99%

Section: Flush Photolysis In the Presence Of'bh3cnmentioning

confidence: 99%

Section: Flush Photolysis In the Presence Of'bh3cnmentioning

confidence: 99%

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

Hemmerich

Knappe

Kramer³

et al. 1980

Eur J Biochem

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“…In the literature, model reactions have been described between free radical and ferricyanide or cytochrome c leading to the conclusion that a free radical-ferricyanide reaction has a greater probability [56]. As far as the flavocytochrome b2 is concerned, this problem is not resolved in spite of a study of the flavocytochrome b2 -cytochrome c transfer [24].…”

Section: Comments On The"simp1est" Reaction Scheme Proposedmentioning

confidence: 99%

Flavocytochrome b₂: Kinetic Studies by Absorbance and Electron‐Paramagnetic‐Resonance Spectroscopy of Electron Distribution among Prosthetic Groups

Capeilléere-Blandin

Iwatsubo

Labeyrie

et al. 1975

European Journal of Biochemistry

102

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The reduction by L-lactate of the prosthetic groups of flavocytochrome b2 (L-lactate cytochrome c oxidoreductase from aerobic yeast, a tetrameric molecule containing one haem and one flavin mononucleotide per protomer) was reinvestigated. It was confirmed that the enzyme ultimately takes up 3 electrons per protomer from this 2-electron donor. Stopped-flow absorbance data at an haem isosbestic point to follow the oxidized flavin and in a haem band indicate that, under the conditions used, haem and flavin reduction time courses are indistinguishable, both being biphasic (phases I and 11). Comparison with electron paramagnetic resonance data (Fe3 + haem and flavosemiquinone signals) led to a complete description at 24 "C of the time courses of the various reduction states of the prosthetic groups. It has been previously demonstrated (Morton and Sturtevant, 1964) that, after the formation of the enzyme-substrate complex, the electron transfer to the enzyme takes place as the first and rate-limiting step of the turnover.In the present study, an initial burst of fully reduced flavin, of small amplitude, is detected at the very beginning of phase I (before 6 ms). The redox forms which accumulate thereafter till the end of phase I (30-35 ms) are the reduced haem (up to 80 %), the flavin semiquinone (up to 50 %) and the fully reduced flavin (from 25 % up to 35 %); the total of electrons distributed at the end of phase I is about 2 per protomer meaning that, in this phase, each enzyme site acts as a 2-electron and not a 3-electron acceptor. A 2-electron flow as the limiting step during phase I with the rate constant k, accounts for the steady-state electron flow during catalysis. Phase I is followed by the much slower phase 11 which corresponds to the entry of the third electron and cannot be involved in the turnover.The interpretation of the results are given as a scheme, with the proper rate constants, allowing a satisfactory fitting of experimental data by simulation. Among the elementary steps required are a rapid distribution of one electron from reduced flavin to the haem, a rapid interprotomers dismutation between couples of flavin semiquinone regenerating two oxidized flavin per tetramer. The very low reactivity of the latter for the entry of the third electron per protomer is tentatively explained by the occurrence of a slow additional step limiting the final reduction reaction.It was observed that, over phase I and the beginning of phase 11, from 15 to 200 ms, all the redox species remain apparently under equilibrium conditions. Parallel studies (titrations of flavocytochrome b2 by L-lactate) showed that the set of equilibrium parameters relative to haem and flavin species is significantly different in the "final" equilibrium (after 30 s) from that in the time interval 15 -200 ms. Such an anomaly suggests a conformation change takes place very slowly in the molecule after the acceptance of the first two electrons per protomer. Eur. J. Biochem. 54 (1975)

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Transient Phenoxyl Radicals: Formation and Properties in Aqueous Solutions

Steenken¹,

Neta

2009

Patai's Chemistry of Functional Groups

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Flash photolysis of flavins. V. Oxidation and Disproportionation of flavin radicals

Cited by 94 publications

References 15 publications

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

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

Flavocytochrome b₂: Kinetic Studies by Absorbance and Electron‐Paramagnetic‐Resonance Spectroscopy of Electron Distribution among Prosthetic Groups

Transient Phenoxyl Radicals: Formation and Properties in Aqueous Solutions

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Flash photolysis of flavins. V. Oxidation and Disproportionation of flavin radicals

Cited by 94 publications

References 15 publications

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

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

Flavocytochrome b2: Kinetic Studies by Absorbance and Electron‐Paramagnetic‐Resonance Spectroscopy of Electron Distribution among Prosthetic Groups

Transient Phenoxyl Radicals: Formation and Properties in Aqueous Solutions

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Flavocytochrome b₂: Kinetic Studies by Absorbance and Electron‐Paramagnetic‐Resonance Spectroscopy of Electron Distribution among Prosthetic Groups