Reduktive Photoalkylierung des Flavinkerns; Struktur und Reaktivität der Photoprodukte

“…Solutions were shown to undergo photoaddition to the flavin nucleus . deoxygenated by purging with 'white spot' nitrogen for 1 h (Knappe and Hemmerich, 1976). several authors have and all experiments described in this work were carried out under such anaerobic conditions.…”

Section: Methodsmentioning

confidence: 99%

The Photoreduction of Flavins by Amino Acids and Edta. A Continuous and Flash Photolysis Study

Heelis¹,

Parsons²,

Phillips³

et al. 1979

Photochem & Photobiology

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Abstract— Primary and secondary photochemical processes in oxygen‐free aqueous solution have been characterised for FMN alone and in the presence of EDTA and four amino acids using nanosecond and microsecond flash photolysis and continuous photolysis techniques. The relative contributions of oneelectron and two‐electron (group or hydride transfer) reactions to the deactivation of the triplet has been determined by comparing the radical concentration (560 nm) with the bleaching of the ground state (446 nm). It was concluded that one‐electron reactions (hydrogen atom or electron abstraction) are the major mode of reactivity of the flavin triplet state with all the suhstrates studied. The nature of the reactions of the flavin semiquinone radical have been studied quantitatively by microsecond flash photolysis. These secondary reactions consist of either a ‘back reaction’ between the flavin and substrate radicals (tryptophan or glycyl‐tyrosine) or the transfer of a second electron (or hydrogen atom) from the substrate radical to the flavin radical (EDTA, methionine and possibly cysteine) to form reduced flavin and oxidised substrate. From a comparison of the quantum yields of formation of reduced flavin using ‘flash’ and continuous irradiation, an additional pathway for the decay of the flavin radical is suggested to occur at low light intensities in the presence of glycyl‐tyrosine or histidine.

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“…In Fig. 1 the decay of the excited flavoquinone (10) triplet and the formation and decay of the flavosemiquinone, which runs through a maximum at about 30 ps, is presented in the absence and in the presence of sodium cyanoborohydride. The figure demonstrates the shortening of the 3Fl$x lifetime and the increase of HFl concentration upon addition of BH3CN-which will be explained further below.…”

Section: Flush Photolysis In the Presence Of'bh3cnmentioning

confidence: 99%

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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Reduktive Photoalkylierung des Flavinkerns; Struktur und Reaktivität der Photoprodukte

Cited by 28 publications

References 52 publications

The effect of pH and complexation on redox reactions between RS• radicals and flavins

The effect of pH and complexation on redox reactions between RS• radicals and flavins

The Photoreduction of Flavins by Amino Acids and Edta. A Continuous and Flash Photolysis Study

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

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