Riboflavine electron-transfer complex

Electron transfer from a macrocycle (82), based on cyclen, complex to coordinated riboflavin proceeds via an inner sphere electron transfer pathway. The riboflavin coordinates through the imide and the relevance to the interception of biological electron transfer pathways is discussed.709... 

The carbon dioxide anion-radical was used for one-electron reductions of nitrobenzene diazo-nium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik and Okhlobystin 1979). The double bonds in maleate and fumarate are reduced by CO2. The reduced products, on being protonated, give rise to succinate (Schutz and Meyerstein 2006). The carbon dioxide anion-radical reduces organic complexes of Co and Ru into appropriate complexes of the metals(II) (Morkovnik and Okhlobystin 1979). In particular, after the electron transfer from this anion radical to the pentammino-p-nitrobenzoato-cobalt(III) complex, the Co(III) complex with thep-nitrophenyl anion-radical fragment is initially formed. The intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand. 

Riboflavin (vitamin B2) has been reported to improve the exercise capacity of a patient with Complex I deficiency. After conversion to flavin monophosphate and FAD, riboflavin functions as a cofactor for electron transport in Complex I, Complex II, and electron transfer flavopro-tein. Nicotinamide has been used because Complex I accepts electrons from NADH and ultimately transfers electrons to Q10. 

Chlorophyll studies of adducts with various biological molecules are also known (bovine plasma albumin and (3-carotene [195], quinone riboflavin [196], and NADH [173]. Mitsui et al. [196] have shown that in porphyrin complexes of viologen the counterion (I-, C1-, Br-) affects the electron transfer process by reduction of the electron-accepting properties of viol-... 

Mieloszyk and colleagues [226] determined that flavins which form charge-transfer complexes with proteins exist in both the ground and excited electronic states. In the flavin-riboflavin-binding protein the trytophan is considered to be the donor. Other such complexes of flavin coenzymes and apoenzymes are known [227-229]. 

Zinc(II) cyclene was linked to phenothiazene group which served as electron-donor (Scheme 22). To the complexed zinc(II), a riboflavin tetraacetate molecule coordinated. Upon irradiation, the flavin became a strong oxidant and the transfer of electrons could be easily observed by emission quenching. 

For complexes of donors with low ionization potential or acceptors of high electron affinity (or both) the charge-transfer band may be found in the near infrared. For example, the p-phenylenediamine-chloranil complex shows a new absorption at 942 m in acetonitrile. In polar solvents the complex of )S-carotene and iodine shows a new absorption at 1000 m/z. This complex is a 1 2 complex, characterized as [(C4oH56)I ]l3". The charge-transfer absorption is attributed to the moiety (C4oH56-+I" ) in which the I is acting as a very powerful electron acceptor . A band at 900 mju has been assigned to a donor-acceptor complex of riboflavin and dihydroflavin . 

Riboflavin, also called vitamin B2, is stmcturally composed of an isoafloxazine ring with a ribityl side chain at the nitrogen at position 10. This vitamin functions metabol-icafly as the essoitial component of two flavin coenzymes, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), complexed with proteins, which act as intmnediaries in transfers of electrons in biological oxidation-reduction reactions. Both FAD and FMN function as coenzymes for flavoproteins of flavoenzymes. Flavoproteins are essoitial for the metabolism of carbohydrates, amino acids, and lipids and for pyridoxine and folate conversion to their respective coenzyme forms. 

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