EPR studies

In the previous chapters experiments have been discussed in which one frequency is applied to excite and detect an EPR transition. In multiple resonance experiments two or more radiation fields are used to induce different transitions simultaneously [19, 20, 21, 22 and 23], These experiments represent elaborations of standard CW and pulsed EPR spectroscopy, and are often carried out to complement conventional EPR studies, or to refine the infonnation which can in principle be obtained from them. 

Levstein P R and van Willigen H 1991 Photoinduced electron transfer from porphyrins to quinones in micellar systems an FT-EPR study Chem. Phys. Lett. 187 415-22... 

Stasko A, Brezova V, Biskupie S, Dinse K-P, Sehweitzer P and Baumgarten M 1995 EPR study of fullerene radieals generated in photosensitized TiOg suspensions J. Phys. Chem. 99 8782-9... 

Stasko A, Brezova V, Rapta P, Biskupie D, Dinse K-P and Guugel A 1997 Anion radioal of ggfullerenes. An EPR study Res. Chem. Intermed. 23 453-78... 

A teehnique that is a convenient source of radieals for study by EPR involves photolysis of a mixture of di-t-butyl peroxide, triethylsilane, and the alkyl bromide corresponding to the radieal to be studied. Photolysis of the peroxide gives t-butoxy radieals, whieh selectively abstract hydrogen from the silane. This reactive silicon radieal in turn abstracts bromine, generating the alkyl radieal at a steady-state eoncentration suitable for EPR study. 

EPR studies and other physieal methods have provided the basis for some insight into the detailed geometiy of radieal species.Deduetions about strueture ean also be drawn from the study of the stereoehemistiy of reactions involving radical intermediates. Several structural possibilities must be considered. If discussion is limited to alkyl radicals, the possibilities include a rigid pyramidal structure, rapidly inverting pyramidal structures, or a planar structure. 

The radicals that are formed from the enolate in this process are rapidly destroyed so that only the stable semidione species remains detectable for EPR study. Semidiones can also be generated oxidatively from ketones by reaction with oxygen in the presence of base. The diketone is presumably generated oxidatively and then reduced to the semidione via reduction by the enolate derived from the original ketone. 

EPR studies of S-N radicals were reviewed in 1990. Many radicals containing the S-N linkage are persistent for more than several hours in solution at room temperature. Perhaps the best known example is the nitrosodisulfonate dianion [0N(S03)] , named as Fremy s salt. In the solid state this radical dianion dimerizes through weak N 0 interactions, but it forms a paramagnetic blue-violet monomer in solution. Although most chalcogen-nitrogen radicals dimerize in the solid state, a few heterocyclic C-S-N systems can be isolated as monomers (Section 11.3). 

The combination of electrochemical and EPR studies can provide valuable information about unstable S-N radical species. A classic early experiment involved the electrochemical reduction of S4N4 to the anion radical [S4N4] , which was characterized by a nine-line EPR spectrum. The decay of the radical anion was shown by a combination of EPR and... 

FIGURE 19.29 A mechanism for the pyruvate kinase reaction, based on NMR and EPR studies by Albert Mildvan and colleagues. Phosphoryl transfer from phosphoenolpyrnvate (PEP) to ADP occurs in four steps (a) a water on the Mg ion coordinated to ADP is replaced by the phosphoryl group of PEP (b) Mg dissociates from the -P of ADP (c) the phosphoryl group is transferred and (d) the enolate of pyruvate is protonated. (Adapted from Mildvan, A., 1979. Advances in Eiizymology 49 103-126.)... 

Poly(isoprene) can also he prepared by radical polymerization/ 17 Although the ratio of l,4- l,2- 4,3- units is stated to be ca 90 5 5 irrespective of the polymerization temperature (range -20-50 °C), the proportion of cis-1,4-addition increases from 0 at -20 °C to 17.6% at 50 °C. EPR studies indicate that radicals add preferentially to the 1-position.87... 

Developments in the synthesis and characterization of stable silylenes (RiSi ) open a new route for the generation of silyl radicals. For example, dialkylsilylene 2 is monomeric and stable at 0 °C, whereas N-heterocyclic silylene 3 is stable at room temperature under anaerobic conditions. The reactions of silylene 3 with a variety of free radicals have been studied by product characterization, EPR spectroscopy, and DFT calculations (Reaction 3). EPR studies have shown the formation of several radical adducts 4, which represent a new type of neutral silyl radicals stabilized by delocalization. The products obtained by addition of 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) to silylenes 2 and 3 has been studied in some detail. ... 

An EPR study 89) on two-dimensionally ordered membranes has shown (a) a strong association of HiPIP to the photosynthetic membrane, and (b) that part of this membrane associated HiPIP forms a... 

D. gigas formate dehydrogenase seems to be quite different in terms of subunit composition. It does not contain a y subunit and no heme c was detected (225). Also, two MGD were identified, but surprisingly, the enzyme contains tungsten instead of molybdenum. Mossbauer and EPR studies confirmed the presence of two [4Fe-4S] + + clusters with similar properties to the ones found in D. desulfuricans FDH (247). 

Despite its weakness, the anisotropy of the g tensor of iron-sulfur centers can be used to determine the orientation of these centers or that of the accommodating polypeptide in relation to a more complex system such as a membrane-bound complex. For this purpose, the EPR study has to be carried out on either partially or fully oriented systems (oriented membranes or monocrystals, respectively). Lastly, the sensitivity of the EPR spectra of iron-sulfur centers to structural changes can be utilized to monitor the conformational changes induced in the protein by different factors, such as the pH and the ionic strength of the solvent or the binding of substrates and inhibitors. We return to the latter point in Section IV. 

EPR studies on electron transfer systems where neighboring centers are coupled by spin-spin interactions can yield useful data for analyzing the electron transfer kinetics. In the framework of the Condon approximation, the electron transfer rate constant predicted by electron transfer theories can be expressed as the product of an electronic factor Tab by a nuclear factor that depends explicitly on temperature (258). On the one hand, since iron-sulfur clusters are spatially extended redox centers, the electronic factor strongly depends on how the various sites of the cluster are affected by the variation in the electronic structure between the oxidized and reduced forms. Theoret-... 

Although the presence of iron-sulfur centers in systems the function of which is obviously not to transfer electrons has by now been clearly established, detailed information about the role of these centers is only just beginning to emerge. Since this new field in iron-sulfur research has been reviewed (105, 260), we focus here on those systems on which important information has been obtained by performing EPR studies. 

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