Flavin models

Biological oxidation of sulfides involves cytochromes P-450 or flavin-dependent oxygenases. A chiral flavin model was prepared by Shinkai etal. and used as the catalyst in the oxidation of aryl methyl sulfides [87]. Flavinophane 30 (Scheme 6C.10) is a compound with planar chirality. It catalyzes the oxidation of sulfides with 35% H202 in aqueous methanol at -20°C in the dark. 

Sawyer and co-workers have described the electrochemical characterization of 1-hydroxyphenazine and its 5-methylated derivative (pyocyanine) as flavin model systems and also have measured formation constants for the... 

By following the strong IR bands due to the bound oxalate, it was shown that [Rh(ox)(CO)2]-decomposes with release of C02 on oxidation, whereas the dimer [Rh(ox)(CO)2]- is stable to oxidation.169 The amide region of the spectrum was monitored in the reduction of [Cp IrClL]+ (L = flavin model, 3-dimethylalloxazine). Both the metal-coordinated and uncoordinated CO bands shift by similar amounts, indicating that the reduction is ligand-based.37... 

GOX. " Accordingly, the structure shows very few protein interactions available for substrate binding. By replacing the flavin with 8-hydroxy-5-carba-5-deaza FAD and determining the stereochemistry of the reaction, it was shown that glucose reacts on the re face of the flavin. " Modeling the substrate in the active site with the... 

Aromatic stacking interactions in flavin model systems 13ACR1000. Artificial allosteric receptors 13CEJ6162. 

He synthesized the following flavin model compound and submitted it to the action of NADH in H20 for three days in the dark, under an argon atmosphere. 

In 1979, Sayer et al (291) used l,3-dimethyl-5-(p-nitrophenylimino)-barbi-turic acid as a flavin model and were able to isolate a covalent intermediate in the reduction of this highly activated imine substrate by a thiol (methyl-thioglycolate or mercaptoethanol). 

J. M. Sayer, P. Conlon, J. Hupp, J. Fancher, R. Belanger, and E. J. White (1979), Reduction of l,3-dimethyl-5-(p-nitrophenylimino) barbituric acid by thiols. A high-velocity flavin model reaction with an isolable intermediate. J. Amer. Chem. Soc. 101,1890-1893. 

To summarize, good flavin models and protein-bound flavin must differ from free flavocoenzymes and vitamins by their lack of self-stacking, self-pairing, (photolytic) self-deleting and pronounced hydrophilic properties. 

Eckstein, J. W., and Ghisla, S. (1991). On the mechanism of bacterial luciferase. 4a,5-Dihydroflavins as model compounds for reaction intermediates. In Flavins Flavoproteins, Proc. Int. Symp., 10th, 1990, 269-272. 

Mager, H. I. X., etal. (1990). Electrochemical superoxidation of flavins generation of active precursors in luminescent model systems. Photochem. Photobiol. 52 1049-1056. 

Tu, S.-C. (1991). Oxygenated flavin intermediates of bacterial luciferase and flavoprotein aromatic hydroxylases enzymology and chemical models. Adv. Oxygenated Processes 3 115-140. 

In order to investigate the single electron donation process from a reduced flavin to a pyrimidine dimer or oxetane lesion, the photolyase model compounds 1-4 depicted in Scheme 4 were prepared [41, 42]. The first model compounds 1 and 2 contain a cyclobutane uracil (1) or thymine (2) dimer covalently connected to a flavin, which is the active electron donating subunit in photolyases. These model systems were dissolved in various solvents... 

Scheme 4 The four pyrimidine dimer and oxetane model compounds 1-4 with either a reduced and deprotonated flavin, or a pyrene as the electron donor. Depiction of the detected reaction products 5-7...

These experiments proved that a light-excited, reduced flavin is indeed able to photoreduce cyclobutane pyrimidine dimers and that these dimers undergo a spontaneous cycloreversion. The quantum yield of about 0=5% clarified that the overall dimer splitting process is highly efficient, even in these simple model systems ((]) photolyase 70%). 

Whatever the reason may be behind the strict necessity to deprotonate the flavin donor, the reduced and deprotonated flavin was established in these model studies to be an efficient electron donor, able to reduce nucle-obases and oxetanes. In the model compounds 1 and 2 the pyrimidine dimer translates the electron transfer step into a rapidly detectable chemical cycloreversion reaction [47, 48], Incorporation of a flavin and of a cyclobutane pyrimidine dimer into DNA double strands was consequently performed in order to analyse the reductive electron transfer properties of DNA. 

Flavin-cyclobutane pyrimidine dimer and flavin-oxetane model compounds like 1-3 showed for the first time that a reduced and deprotonated flavin is a strong photo-reductant even outside a protein environment, able to transfer an extra electron to cyclobutane pyrimidine dimers and oxetanes. There then spontaneously perform either a [2n+2n cycloreversion or a retro-Paternd-Buchi reaction. In this sense, the model compounds mimic the electron transfer driven DNA repair process of CPD- and (6-4)-photolyases. 

The possible mechanisms of inhibition of flavin by (—)-deprenyl, as an irreversible acetylenic inhibitor, were studied by ab initio methods with the 6-31G basis set using simplified model compounds, 3-formyl-2-imino-l-hydroxypyrazine, and propargylamine. The formation of two energetically stable cyclic adducts, the 0,N adduct 286 and a C,N adduct, was shown <1999THA147>. 

A second approach with respect to anisotropic flavin (photo-)chemistry has been described by Trissl 18°) and Frehland and Trissl61). These authors anchored flavins in artificial lipid bilayers by means of C18-hydrocarbon chains at various positions of the chromophore. From fluorescence polarization analysis and model calculations they conclude, that the rotational relaxation time of the chromophore within the membrane is small compared to the fluorescence lifetime (about 2 ns74)). They further obtain the surprising result that the chromophore is localized within the water/lipid interface, with a tilt angle of about 30° (long axis of the chromophore against the normal of the membrane), irrespective of the position where the hydrocarbon chain is bound to the flavin nucleus. They estimate an upper limit of the microviscosity of the membrane of 1 Poise. 

Besides these shortcomings the bluelight receptor and sensory transduction problem is recently being attacked on the basis of completely artificial flavin/membrane systems. These appear to provide well-defined model systems to study anisotropic flavin (photo-) chemistry. This, in turn, is an essential prerequisite which allows the primary photo-events of physiological bluelight reception to be imitated and elucidated. 

Bruice, T.C. Models and flavin catalysis. In Progress in bioorganic chemistry, Vol. 4, p. 1, E.T. Kaiser, F. J. Kezdy, (eds.), New York-London-Sidney-Toronto Wiley 1976... 

The report of Basran et al. (entry 5 of Table 2) contains two studies involving hydride transfer with nicotinamide cofactors. In morphinone-reductase catalyzed reduction by NADH of the flavin cofactor FMN (schematic mechanism in Fig. 5), the primary isotope effects are modest (around 4 for H/D), but exhibit a small value of Ajj/Aq (0.13) and an exalted isotopic difference in energies of activation (8.2kJ/mol) that alone would have generated an isotope effect around 30. The enthalpies of activation are in the range of 35-45 kJ/mol. This is behavior typical of Bell tunneling as discussed above. It can also be reproduced by more complex models, as will be discussed in later parts of this review. 

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