Electron transfer mechanisms biomolecules

At present, new developments challenge previous ideas concerning the role of nitric oxide in oxidative processes. The capacity of nitric oxide to oxidize substrates by a one-electron transfer mechanism was supported by the suggestion that its reduction potential is positive and relatively high. However, recent determinations based on the combination of quantum mechanical calculations, cyclic voltammetry, and chemical experiments suggest that °(NO/ NO-) = —0.8 0.2 V [56]. This new value of the NO reduction potential apparently denies the possibility for NO to react as a one-electron oxidant with biomolecules. However, it should be noted that such reactions are described in several studies. Thus, Sharpe and Cooper [57] showed that nitric oxide oxidized ferrocytochrome c to ferricytochrome c to form nitroxyl anion. These authors also proposed that the nitroxyl anion formed subsequently reacted with dioxygen, yielding peroxynitrite. If it is true, then Reactions (24) and (25) may represent a new pathway of peroxynitrite formation in mitochondria without the participation of superoxide. 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

Simplification is necessary for understanding most real processes. In this case the description begins with material on the adsorption of biomolecules on metals then we discuss the active field that has developed in a study of electron transfer from modified metal electrodes to proteins dissolved in solution finally we describe the as-yet less well developed study of charge transfer from proteins to simple redox ions in solution. The real field of course is the kinetics and mechanism of electron transfer from proteins to biomolecules, but this area of experimental research is as yet a bridge too far. 

G. Del Re, A. Peluso, and C. Minichino H-Bridges and Electron Transfer in Biomolecules. Study of a Possible Mechanism on a Model Charge-Recombination System. Can. J. Chem. 63, 1850-1856 (1985). 

Another mechanism, possibly the dominant one, is that bacteria transfer electrons directly from the cell surface to the mineral after a regulated search and attachment process. A variety of biomolecules (including cytochromes, qui-nones, and dehydrogenases) have been identified as part of this electron-transfer pathway (Schroder et al., 2003). Several of these biomolecules are located on the outer membrane of the cell and presumably make contact with the mineral directly (Lower et al., 2001). Given that the initial rate and long-team extent of electron transfer is correlated with their surface area and the concentration of reactive sites, this seems like a reasonable explanation (Zachara et al., 1998). 

Many compounds sensitize biomolecules to damage by UVA (320-380 nm) and visible light. Two general mechanisms of sensitization are encountered. The Type I mechanism involves electron or hydrogen transfer from the target molecule to the photosensitizer in its triplet state. If 02 is present, this can be reduced to 02 by the reduced sensitizer. In the Type II mechanism, the excited sensitizer is quenched by 02, which is excited to the singlet state (typically A"g) and attacks the target molecule. Photosensitization is exploited in photodynamic therapy (PDT) for the destruction of cancerous or other unwanted cells. 

---