Riboflavin redox potential

AH oi complex formation reported as —6.1 kcal-mole at pH 6.9. Shifts in riboflavin redox potential in presence of interactant also noted. 

The ionisation state of molecules in the solution state appears to be an important variable in photodegradation mechanisms. A recent pubhcation on riboflavin oral liquid preparations shows that the formulation is most photostable at pHs between 5 and 6, where the non-ionised form predominates [78]. The rate of photolysis increase 80-fold at pH 10.0, owing to increased redox potential. Conversely, at pH 3.0, the increased photolysis is associated with the excited singlet state, in addition to the triplet state. 

Flavin coenzymes are usually bound tightly to proteins and cycle between reduced and oxidized states while attached to the same protein molecule. In a free unbound coenzyme the redox potential is determined by the structures of the oxidized and reduced forms of the couple. Both riboflavin and the pyridine nucleotides contain aromatic ring systems that are stabilized by resonance. Part of this resonance stabilization is lost upon reduction. The value of E° depends in part upon the varying amounts of resonance in the oxidized and reduced forms. The structures of the coenzymes have apparently evolved to provide values of E° appropriate for their biological functions. 

The oxidation of cyclohexene was systematically investigated by means of metal porphyrins which have distinct redox potentials due to different metals [134]. The best results were obtained with Mn(III)TPP/L-Cys/NaBH4 which furnished a product mixture consisting of cyclohexenone (46.4%) > cyc-lohexanol (23.8%) > cyclohexenol (19.5%) > cyclohexanone (9.0%) > epoxide (1.4%), relative yields given in parenthesis. In the presence of KOH and riboflavin as an electron transfer reagent the product distribution was similar but the total yield was considerably improved [135]. 

Flavoenzymes are widespread in nature and are involved in many different chemical reactions. Flavoenzymes contain a flavin mononucleotide (FMN) or more often a flavin adenine dinucleotide (FAD) as redox-active prosthetic group. Both cofactors are synthesized from riboflavin (vitamin B2) by microorganisms and plants. Most flavoenzymes bind the flavin cofactor in a noncovalent mode (1). In about 10% of aU flavoenzymes, the isoalloxazine ring of the flavin is covalently linked to the polypeptide chain (2, 3). Covalent binding increases the redox potential of the flavin and its oxidation power, but it may also be beneficial for protein stability, especially in flavin-deficient environments. 

Due to the much wider redox potential of the flavin enzymes, riboflavin is involved in the transfer of either one or two electrons. This is different from nicotinamides which participate in double electron transfer only. Values between +0.19V (stronger oxidizing effect than NAD ) and -0.49 V (stronger reducing effect than NADH) have been reported. 

Like FMN, both FAD and riboflavin give well-defined redox peaks on the i—E curves at approximately the same potentials as FMN, suggesting that the isoalloxazine moieties of these compounds participate in the electron transport. Since AMP itself exhibits no redox peak in the potential range investigated, it is assumed that the AMP moiety of FAD is not reduced. Thus the reaction mechanisms for the redox reactions of FMN and FAD (Elving et al., 1973 Janik and Elving, 1968) can be given as in Fig. 31. 

The results obtained for FAD, as well as for riboflavin and AMP, are similar to those for FMN, except for the absence of a redox reaction for AMP in the potential range investigated. 

By various electrochemical methods we investigated adsorption phenomena in the riboflavin—dihydroriboflavin system. The chronopotentiometric method gave relationships between the surface concentration of the redox-system components and the potential of the electrode, and these were then compared with the polarographic curves [27]. The capacity curves (chronopotentiograms) were recorded on a suspended mercury drop at such current densities that the contribution of diffusion was negligibly small. In order to determine differences in the adsorption behavior of the riboflavin — dihydroriboflavin systems under conditions with and without a... 

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