Radical Transfer pathway

Abbreviations AdoCbl, deoxyadenosylcobalamin AdoMet, S-adenosyl methionine dopa, 3,4-dihydroxyphenylalanine ENDOR, electron nuclear double resonance EPR, electron paramagnetic resonance NMR, nuclear magnetic resonance RNR, ribonucleotide reductase RTF, radical transfer pathway. 

CLASS I RIBONUCLEOTIDE REDUCTASEoTHE RADICAL TRANSFER PATHWAY... 

Is the Tyrosyl Radical Hydrogen-bonded to the Radical Transfer Pathway ... 

Non-Native Radicals and Secondary Radical Transfer Pathways Observed in Mutant R2 Proteins... 

Rova, U., Adrait, A., PTsch, S., Gr%oslund, A., and Thelander, L., 1999, Evidence by mutagenesis that Tyr(370) of the mouse ribonucleotide reductase R2 protein is the connecting link in the intersubunit radical transfer pathway. J. Biol. Chem. 274 23746n23751. 

Schmidt, P. P., Rova, U., Katterle, B., Thelander, L., and Gr%oslund, A., 1998, Kinetic evidence that a radical transfer pathway in protein R2 of mouse ribonucleotide reductase is involved in generation of die tyrosyl free radical. J. Biol. Chem. 273 21463n21472. 

Synthesis ofvarious S—S hnked symmetric bisazaheterocycles 13MR0268. Synthetic appHcations of arylboronic acid via an aryl radical transfer pathway 130BC7999. 

Graslund A. 2002. Ribonucleotide reductase Kinetic methods for demonstrating radical transfer pathway in protein R2 of mouse enzyme in generation of tyrosyl free radical. In Enzyme kinetics and mechanism, Pt F detection and characterization of enzyme reaction intermediates, pp. 399 14. New York Academic Press. 

Figure 9.30. Electron transfer pathways in bis(hydrazine) radical cations (adapted from reference 3).

O.G. Poluektov, L.M. Utschig, A.A. Dubinskij and M. Thurnauer, ENDOR of spin-correlated radical pairs in photosynthesis at high magnetic field A tool for mapping electron transfer pathways, J. Am. Chem. Soc., 2004, 126, 1644. 

Since the latter conditions pertain to aromatic nitration solely via the homolytic annihilation of the cation radical in Scheme 16, it follows from the isomeric distributions in (81) that the electrophilic nitrations of the less reactive aromatic donors (toluene, mesitylene, anisole, etc.) also proceed via Scheme 19. If so, why do the electrophilic and charge-transfer pathways diverge when the less reactive aromatic donors are treated with other /V-nitropyridinium reagents, particularly those derived from the electron-rich MeOPy and MePy The conundrum is cleanly resolved in Fig. 17, which shows the rate of homolytic annihilation of aromatic cation radicals by NO, (k2) to be singularly insensitive to cation-radical stability, as evaluated by x. By contrast, the rate of nucleophilic annihilation of ArH+- by pyridine (k2) shows a distinctive downward trend decreasing monotonically from toluene cation radical to anthracene cation radical. Indeed, the... 

The radical anion pathway (e-c-P-d-p Scheme 2) requires a rate-determining protonation after cyclization, i.e., a slow protonation of a hard oxyanion. However, such proton transfer rates are usually diffusion controlled, so this seems unlikely [32,33], On the other hand, the carbanion closure (e-P-d-c-p) portrayed in Scheme 4 requires a very reasonable suggestion that the ratedetermining step corresponds to protonation of the soft, weakly basic radical anion 42, prior to cyclization [32-35] this is the preferred mechanism. One must use caution, however, realizing that these conclusions are drawn for the particular set of substrates which were examined. In some cases, radical anion cyclization remains a viable option. 

The Patterno-Buchi coupling of various stilbenes (S) with chloroanil (Q) to yield fran -oxetanes is achieved by the specific charge-transfer photo-activation of the electron donor-acceptor complexes (SQ). Time-resolved spectroscopy revealed the (singlet) ion-radical pair[S+% Q" ] to be the primary reaction intermediate and established the electron-transfer pathway for this Patterno-Buchi transformation. Carbonyl quinone activation leads to the same oxetane products with identical isomer ratios. Thus, an analogous mechanism is applied which includes an initial transfer quenching of the photo-activated (triplet) quinone acceptor by the stilbene donors resulting in triplet ion-radical pairs. ... 

Electron-transfer chains in plants differ in several striking aspects from their mammalian counterparts. Plant mitochondria are well known to contain alternative oxidase that couples oxidation of hydroquinones (e.g., ubiquinol) directly to reduction of oxygen. Semiquinones (anion-radicals) and superoxide ions are formed in such reactions. The alternative oxidase thus provides a bypass to the conventional cytochrome electron-transfer pathway and allows plants to respire in the presence of compounds such as cyanides and carbon monoxide. There are a number of studies on this problem (e.g., see Affourtit et al. 2000, references therein). 

As assumed, the small and positive valne of H/D kinetic isotope effect may be used as a criterion for an electron-transfer pathway. For example, anion-radicals of a-benzoyl-co-haloalkanes can react in two routes (Kimura and Takamnkn 1994). The first ronte is the common one—an electron is transferred from the oxygen anion of the carbonyl gronp to a terminal halogen. The transfer provokes fission of the carbon-halogen bond. The second ronte is the S 2 reaction, leading to a cyclic product as shown in Scheme 2.37. 

One should be aware, however, that none of the Grignard reactions of benzophenone proceeds through a completely free coupling process of benzophenone anion-radicals with alkyl radicals. For example, the portion of electron-transfer pathway in the Grignard reactions of benzophenone with isomeric C4H5MgCl was estimated to be 65, 61, and 26% for (CH3)3C—, CH3CH2CH(CH3)—, and CH3CH2CH2CH2-, respectively (Lund et al. 1999). 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

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