Water electron paramagnetic resonance

Tomkiewicz, M.A., Groen, A., and Cocivera, M. Electron paramagnetic resonance spectra of semiquinone intermediates observed during the photooxidation of phenol in water, J. Am. Chem. Soc., 93(25) 7102-7103,1971. 

A recent electron paramagnetic resonance study (13) has shown that I also degrades by a photoreduction pathway in the presence of alcohols or water, through the intermediacy of the corresponding ketyl radical. Since corresponding ketyl radicals and triplet states often have overlapping absorption spectra, it is important to emphasize that the transient observed in our optical flash photolysis experiments is the triplet state (not the ketyl radical), the key evidence being that both the yield and the lifetime of the transient are decreased by triplet quenchers. An important question... 

Oxidation rate constant k, for gas-phase second order rate constants, koH for reaction with OH radical, kND3 with N03 radical and kQ3 with 03 or as indicated, data at other temperatures see reference rate constant for the reaction with OH- k = (2.5 0.3) x H)6 M-1 s-1 was measured in 66.7% dioxane-water at 35.7°C (Hine et al. 1956 quoted, Roberts et al. 1992) koH = 1.11 x 10 13 cm3 molecule-1 s-1 with atmospheric lifetime x = 0.43 yr at 298 K, measured range 277-377 K (flash photolysis resonance fluorescence and discharge flow electron paramagnetic resonance, Orkin et al. 1997)... 

The possibility of using the electron paramagnetic resonance properties of Gd3+ to probe its environment in and interactions with biological molecules has previously received little attention in the literature (40). However, the possibility exists that Gd2+ will be a sensitive EPR probe for characterizing macromolecular biological systems such as the Ca2+-ATPase. The EPR spectra of Gd3+, which has S = 7/2. in neutral water and in two different buffers are shown in Figure 13A. The linewidths were found to be independent of pH over the usable range of these buffers and independent of temperature between 4 and 30°C, The spectrum of Gd2+ in neutral water is centered around 3248 G, with a linewidth of 530 G. As shown, Gd3+ in Pipes buffer, but not in Tes buffer, yielded a spectrum similar to that of the aqueous Gd2+ solution. On this basis, all of our Gd3+ EPR and NMR studies... 

To understand the mechanism of water oxidation, it is necessary to characterize each of the S states. A variety of spectroscopic methods have been brought to bear on this problem. Electron paramagnetic resonance (EPR) and X-ray spectroscopies have been especially useful because these techniques allow the Mn complex to be probed directly. EPR spectroscopy has the restriction that the Mn complex must be paramagnetic to be studied. The S2 state is an odd-electron state, and EPR spectroscopy has been used extensively to study the Mn complex in the S2 state. X-ray spectroscopy has the advantage that any state of the Mn complex is observable. However, the successful application of EPR and X-ray spectroscopies requires that a specific S state be prepared in high yield in highly concentrated samples. 

Titanium(III) exchanged 3A zeolite can also split water according to Eyring and coworkers (18). Illumination with visible light causes the evolution of h rogen as evidenced by mass spectrometry. As with the silver system described above, the titanium 3A zeolite process is not catalytic and loses reversibility. A detailed report concerning the electron paramagnetic resonance spectra of the titanium(III) 3A zeolite system has also been recently reported (19). 

PSII = Photosystem II WOC = Water-oxidizing complex OEC = Oxygen-evolving complex (B)RC = (Bacterial) Reaction Center Chi = Chlorophyll Bchl = Bacteriochloro-phyll XRD = X-ray diffraction EPR = Electron paramagnetic resonance EXAFS = Extended X-ray absorption fine stmctnre ENDOR = Electron-nuclear double resonance ESEEM = Electron spin echo envelope modulation (Tyr = Yz) = DlTyrl61 ATP = Adenosine Triphosphate KIE = Kinetic isotope effect UV = Ultraviolet (FT-)IR = (Fourier Transform) InfraRed. 

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