Flavin Chromophores

Flavin molecule by its redox properties plays an important role in energy providing reactions. Flavin occurs as riboflavin or as a nucleotide in flavin mononucleotide (FMN) and combined to adenine nucleotide in flavin adenine dinucleotide (FAD) Very recently it was shown by Spiro et al. that free fluorescence SERRS spectra from flavoproteins adsorbed at silver colloids (average size of 7.5 nm) can be obtained 

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Macheroux, P., et al. (1987). Purification of the yellow fluorescent protein from Vibrio fischeri and identity of the flavin chromophore. Biochem. Biophys. Res. Commun. 146 101-106. 

Fig. 6. Difference spectra between xanthine oxidase inactivated with various pyra-zolo [3, 4-d] pyrimidines and the native enzyme. The spectra are believed to represent the increase in absorption occurring when Mo(VI) of native enzyme is converted to Mo(IV) complexed with the inhibitors. Spectra were obtained by treating the enzyme with inhibitors in the presence of xanthine, then admitting air, so as to re-oxidize the iron and flavin chromophores. The extinction coefficients, de, are expressed per mole of enzyme flavin. Since some inactivated enzyme was present, extinction coefficients per atom of molybdenum of active enzyme will be about 30% higher than these values. (Reproduced from Ref. 33, with the permission of Dr. V. Massey.)...

Fig. 21. Top The general Jablonski diagram for the flavin chromophore. The given wavelengths for absorption and luminescence represent crude average values derived from the actual spectra shown below. Due to the Franck-Condon principle the maxima of the peak positions generally do not represent so-called 0 — 0 transitions, but transitions between vibrational sublevels of the different electronically excited states (drawn schematically). Bottom Synopsis of spectra representing the different electronic transitions of the flavin nucleus. Differently substituted flavins show slightly modified spectra. Absorption (So- - S2, 345 nm S0 -> Si,450nm 1561) fluorescence (Sj — S0) 530 nm 156)) phosphorescence (Ty Sq, 605 nm 1051) triplet absorption (Tj ->Tn,...

Trivially, photo-excitation will drastically enhance the oxidation potential of the flavin chromophore and might give rise to a great variety of reversible chemical reactions, depending on the structure of the environment and/or the pathway of potential e - as well as H -conductivity. It must be emphasized, that the oxidative action of the flavin triplet Tj is by no means confined to 1 e -uptake from suitable aromatic... 

Flavin Coenzymes.—5-Deazaflavin-adenine dinucleotide (2) can be prepared from the 5-deazaFMN,21 using a FAD pyrophosphorylase from rat liver.22 When the apoprotein of D-amino-acid oxidase from pig kidney is reconstituted with (2), no oxidation of D-alanine is observed, although the flavin chromophore in the reconstituted enzyme is reduced on the addition of DL-amino-acids.22 This has been interpreted as indicating that hydrogen transfer from the amino-acid to (2) can still... 

Aprotic solvents mimic the hydrophobic protein interior. However, a functional artificial receptor for flavin binding under physiological conditions must be able to interact with the guest even in competitive solvents. As found by spectroscopic measurements with phenothiazene-labeled cyclene, the coordinative bond between flavin and Lewis-acidic macrocyclic zinc in methanol was strong enough for this function. Stiochiometry of the complex was proved by Job s plot analysis. Redox properties of the assemblies in methanol were studied by cyclic voltammetry which showed that the binding motif allowed interception of the ECE reduction mechanism and stabilisation of a flavosemiquinone radical anion in a polar solvent. As a consequence, the flavin chromophore switched from a two-electron-one-step process to a two-step-one-electron-each by coordination. 

Figure 23.2. Reaction mechanism of PD-DNA photolyase. A photon of blue light is absorbed by the MTHF chromophore that acts as a photoantenna. The excited energy is transferred to the flavin chromophore (FADFF). The excited flavin (FADFI ) acts as a photocatalyst and transfers an electron to a CPD in DNA. The thymines are restored to their native state and the electron is transferred back to the flavin. (Reproduced with permission from Sancar, A. Structure and function of DNA photolyase cryptochrome blue-light photoreceptors. Chem. Rev. 103, 2203-2237, 2003.)...

The isolated N-terminal BLUF domain exhibits a photocycle identical to that observed with full-length AppA [48]. Photoexcitation of AppA involving a singlet excited state in the flavin chromophore leads to the formation of... 

Lumichrome [7,8-dimethylalloxazine, (23)], a flavin tautomer, has two fluorescence emissions with maxima at 440 and 540 nm in pyridine-dioxan mixtures. Nanosecond time-resolved fluorescence shows fast growth of the latter due to proton transfer from N-1 of the excited lumichrome (23 ) to N-10 during the lifetime of lumichrome singlet, and emission occurs from the excited flavinic chromophore (24 ). 

Eker, A. P. M., Hessels, J. K. C., and Van de Velde, J. (1988). Photoreactivating enzyme from the green alga Scendesmus acutus. Evidence for the presence of two different flavin chromophores. Biochemistry 27, 1758-1765. 

Analysis of P. furiosus NROR by gel filtration (Stqierdex 200) and SDS-PAGE indicate that it is a monomeric protein of approximately 45 kDa. The presence of flavin in the enzyme is indicated by its UV-visible absorption spectra (Fig. 1). The air-oxidized enzyme exhibits peaks near 375 and 450 nm, and these decrease in intensity on the addition of a reductant such as sodium dithionite or NADPH, indicating the presence of flavin chromophore. Assuming that only flavin contributes to the absorption at 450 nm on reduction, and assuming a molar absorbance of 11,500 A/ cm at 450 nm, the change in absorbance on reduction... 

Mataga, N., Chosrowjan, H., Shibata, Y., Tanaka, F., Nishina, Y., et al. Dynamics and mechanisms of ultrafast fluorescence quenching reactions of flavin chromophores in protein nanospace. J. Phys. Chem. B 104, 10667-10677 (2000)... 

Finally, the useful subscripts ox and red must be clearly defined as to their correlation either with true redox states or with flavin chromophores. For example, the addition of water to an alkyl-flavo-quinonium ion 5-RFltx 181) leads to a dihydroflavin chromophore, though the redox state did not change The true redox state of 5-RFl-4a-OH formed in this fashion is indicated by the oxygen atom, though it stems from hydroxyl ion. The picture becomes indeterminate in the case of... 

Scheme 9. Oxidized and reduced flavin chromophores. Note that the term antiaromatic applies only to the entirely flat (vibrationally excited) state of the bent flavohydroquinone...

Structure, stability and essentiality of the intermediates R-FlredH, depend very much on the nature of the residue R (see below). Since H2Fired undergoes fast autoxidation (722), photodehydrogenations are in general reversible with respect to the flavin chromophore, while photoaddition and photodealkylation are not. Furthermore, photoadditions are necessarily nucleophilic, whereas the character of photodehydro-... 

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