Flavins, photolysis

Based on the known chemistry of flavin photolysis reactions, it appears unlikely that thymine dimer cleavage occurs via a direct energy transfer mechanism (160). One proposal suggests that in the model reaction with 1-deazariboflavin, the thymine dimer radical anion is formed via electron donation from the excited sensitizer (164). Alternatively, electron abstraction by the excited flavin could occur, resulting in the thymine dimer radical cation (159, 160), although it is unlikely that reduced flavin would act as an electron acceptor. A schematic for this mechanism is illustrated in Scheme 33, where the initial formation of a sensitizer-dimer complex is consistent with the observed saturation kinetics. The complex is activated by excitation of the ionized sensitizer (pH > 7), and electron donation to the dimer forms the dimer radical anion and the zwitterionic, neutral 1-deazariboflavin radical (162). Thymine dimer radical would spontane-... 

The first goal was, therefore, a SD-flavin preparation, which would yield direct evidence for 8a-substitution. This was first achieved by drastic acid hydrolysis of a flavin peptide preparation and subsequent direct inspection of the radical cation obtained by reduction of the hydrolysate with TiCb in 6 N HCl. At pH < 1 flavin photolysis is negligible because of dynamic proton quenching of the photoexcited flavo-quinone cation 107) and the semiquinoid state is highly stable. This allows hyperfine analysis of SD-flavin even in the presence of large amounts of non-flavin peptides and of mixtures of flavin peptides if the attachment of the peptides to the flavin nucleus is the same. The ESR spectra obtained pointed conclusively to 8a as the position of substitution 67). 

However, flavin photolysis, i.e. photo-induced selfdestruction of any type, need not be confined to intramolecular modes of reaction, although such reactions have attracted most attention owing to their high velocity and relevance to vitamin-B2 photo-instability. But quite generally, any aliphatic CH-center attached to a flavin nucleus is susceptible to intermolecular photodehydrogenation by a second flavin in the excited triplet state. Schollnhammer et al. 156) have recently studied the photolysis of lumiflavin and find that C(7)-CH3 can be the preferred center of attack (see above). 

Lu, C. Y. Lui, Y.Y. (2002). Electron transfer oxidation of tryptophan and tyrosine by triplet states and oxidized radicals of flavin sensitizers a laser flash photolysis study. Biochimica et Biophysica Acta (BBA) - General Subjects, Vol. 1571, No.l, (May 2002), pp. 71-76, ISSN 0304-4165... 

Flash photolysis has proven to be a useful technique for investigating the reactivity of flavin radicals. This approach takes advantage of the fact that illumination of flavins in the presence of suitable reducing agents (e.g. EDTA, phenols, indoles) leads to a one-electron reduction of the flavin triplet state Through the... 

Flash-photolysis studies on the heme-flavin electron transfer rate of Chromatium... 

Ultraviolet light damages proteins as well as DNA. Residues of Trp, Tyr, His, Cys, and Met are especially susceptible to photolysis, or photooxidation by 02, or by singlet oxygen. Also damaged are unsaturated lipids, porphyrins, flavins, etc. Kynurenic acid (Fig. 25-11) and urocanic acid (Eq. 14-44), an important ultraviolet filter in skin,196 are also decomposed by light. 

Ulanski P, von Sonntag C (2000) Stability constants and decay of aqua-copper(lll) - a study by pulse radiolysis with conductometric detection. Eur J Inorg Chem 1211-1217 Vaish SP, Tollin G (1971) Flash photolysis of flavins. V. Oxidation and disproportionation of flavin radicals. J Bioenergetics 2 61-72... 

Flavins (8), 8-azaflavin (9) and their analogs are characteristic oxidants toward thiols under anaerobic and other specific conditions (equation 12). S-Arylidene-l,3-dimethylbarbituric acid derivatives (10) also work as the oxidant in dioxane at 12(X-1S0 C, and the method was applied to synthesis of unsymmetrical disulfides. Electrochemical oxidation and photolysis of thiols to disulfidqs are also known. 

The rate of intramolecular electron transfer from the flavin to the heme in PCMH has been measured by laser flash photolysis using 5-deazariboflavin semiquinone radical as a 1-electron reductant (Bhat-... 

Bhattacharyya, A., Tollin, G., Mclntire, W. S., and Singer, T. P., 1985, Laser-flash-photolysis studies of p-cresol methylhydroxylase. Electron-transfer properties of the flavin and haem components, Biochem. J. 228 337n345. 

Laser flash photolysis studies using the deazariboflavin system [80, 81] have shown that pyruvate binding also exerts a strong influence on intramolecular ET. In the one-electron reduced enzyme, ET from the FMN semiquinone to the oxidized heme can be observed only in the presence of pyruvate for this reaction, k j = 500 s. When the enzyme is completely reduced by stoichiometric addition of lactate prior to laser photolysis with dRf alone, pyruvate binding inhibits ET from fully reduced flavin to oxidized heme. This reaction has an observed /cet = 2000 s in the absence of pyruvate [81]. Similar values for these two rate constants have been obtained by temperature-jump measurements [82]. 

In this chapter results of the picosecond laser photolysis and transient spectral studies on the photoinduced electron transfer between tryptophan or tyrosine and flavins and the relaxation of the produced ion pair state in some flavoproteins are discussed. Moreover, the dynamics of quenching of tryptophan fluorescence in proteins is discussed on the basis of the equations derived by the present authors talcing into account the internal rotation of excited tryptophan which is undergoing the charge transfer interaction with a nearby quencher or energy transfer to an acceptor in proteins. The results of such studies could also help to understand primary processes of the biological photosynthetic reactions and photoreceptors, since both the photoinduced electron transfer and energy transfer phenomena between chromophores of proteins play essential roles in these systems. 

Reduction of the radical first in reaction (5) is consistent with the proposed electron-transfer pathway between the hemes in the CCP-yeast cyt c complex (Fig. 16). However, flash photolysis experiments involving rapid bimolecular reduction of native cyts c by free flavin semiquinones (FH ) gave different results (111). The reactions monitored were the following ... 

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