Radicals, reduction spectroscopic analysis

Cuprous cyclopropylacetylide, prepared from Cul and cyclopropylacetylene in ammo niacal solution couples with 4-iodonitrobenzene in pyridine, yielding 4-nitrophenyl cyclopropylacetylene (equation 154)234. Reaction of the acetylide with tropylium tetra-fluoroborate in acetonitrile, in the presence of LiBr, affords 7-(cyclopropylethynyl)cyclo-heptatriene. The anion radicals obtained by reduction of these compounds were utilized for ESR spectroscopic analysis of the cyclopropyl P hyperfine splittings. 

Aprotic solvents mimic the hydrophobic protein interior. However, a functional artificial receptor for flavin binding under physiological conditions must be able to interact with the guest even in competitive solvents. As found by spectroscopic measurements with phenothiazene-labeled cyclene, the coordinative bond between flavin and Lewis-acidic macrocyclic zinc in methanol was strong enough for this function. Stiochiometry of the complex was proved by Job s plot analysis. Redox properties of the assemblies in methanol were studied by cyclic voltammetry which showed that the binding motif allowed interception of the ECE reduction mechanism and stabilisation of a flavosemiquinone radical anion in a polar solvent. As a consequence, the flavin chromophore switched from a two-electron-one-step process to a two-step-one-electron-each by coordination. 

The EPR and ENDOR spectroscopy was used for studies of catalytic intermediates in native and mutant cytochrome P450cam in cryogenic temperatures (6 and 77K) (Davydov et al., 2001). The ternary complex of camphor, dioxygen, and ferrous-enzyme was irradiated with y-rays to inject the second electron. This process showed that the primary product upon reduction of the complex is the end -on intermediate. This species converts even at cryogenic temperatures to the hydroperoxo-ferriheme form and after brief annealing at a temperature around 200 K, causes camphor to convert to the product. In spite of conclusions derived from x-ray analysis (Schlichtich et al., 2000) no spectroscopic evidence for the buildup of a high-valance oxyferryl/porphyrin rc-cation radical intermediate during the entire catalytic circle has been obtained. 

The reduced form of RNR reacts with dioxygen to generate the p-oxo diferric core (crystallographically defined (3)) and a tyrosyl radical necessary for the production of a reactive species responsible for the reduction of ribonucleotides. The intimate details of this dioxygen-based chemistry and the structure of the reduced enzyme are still unknown. Despite intensive spectroscopic characterization of the active site of MMO and the recent X-ray structural analysis of the hydroxylase component (4), even less is known concerning its mechanistic pathways responsible for the conversion of methane and other alkanes to their corresponding oxygenated products. 

As seen in Figure 2, the reduction step caused little, if any, change in the free radical population of the coal. Although the presence of spin-paired dianions cannot be excluded, analysis of the spectroscopic g value does not suggest formation of large amounts of aromatic radical anions either. 

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