Spectra correlation with electron transfer

From the point of view of the solvent influenee, there are three features of an electron spin resonance (ESR) speetrum of interest for an organic radical measured in solution the gf-factor of the radical, the isotropie hyperfine splitting (HFS) constant a of any nucleus with nonzero spin in the moleeule, and the widths of the various lines in the spectrum [2, 183-186, 390]. The g -faetor determines the magnetic field at which the unpaired electron of the free radieal will resonate at the fixed frequency of the ESR spectrometer (usually 9.5 GHz). The isotropie HFS constants are related to the distribution of the Ti-electron spin density (also ealled spin population) of r-radicals. Line-width effects are correlated with temperature-dependent dynamic processes such as internal rotations and electron-transfer reaetions. Some reviews on organic radicals in solution are given in reference [390]. 

Similar to bacterial RC there is spectral and ESR evidence that a pheophytin a molecule operates as an intermediary electron acceptor in PSII-RC. Optical absorbance changes, with a spectrum similar to that of a pheophytin a anion radical could be detected in PSII-enriched particles illuminated at low redox potentials (— 0.65 V) [57,77]. The appearance of the Ph signal could be correlated to a decrease in the extent of the rise in fluorescence of PSII of chlorophyll a observed upon illumination [78]. This apparent discrepancy (reduction of an electron acceptor is expected to cause an increase of fluorescence) is now explained by the fact that the fluorescence increase is in reality a delayed fluorescence emitted by the return to the ground state of P -682 regenerated by electron transfer from the pheophytin anion [79]. The lifetime, of this transient fluorescence rise is 2-4 ns, and that of electron transfer from Ph to P -6%2 = 4 ns, when PSII particles are poised at —0.45 V [73]. This transient fluorescence increase is, however, almost totally suppressed when A,j,(Ph) is prereduced chemically before illumination. Using this experimental criterium the midpoint potential of the Ph /Ph couple has been estimated to be -0.61 V [73,80]. 

For thorium there are only estimates of the corresponding potential. An early estimate, of -2.4 V, was based on a relation between this quantity and the frequency of the first electron transfer absorption band in the UV spectrum of an aqueous thorium perchlorate solution (9). However, the spectral measurements did not quite reach the absorption maximum, and the necessary extrapolation introduced some uncertainty. Another value, -3.6 V, was based on the RESPET treatment of J0rgensen (10,11). The adjustable parameters in the RESPET equation were fixed using experimental values for other actinide elements (12). This method yields a value of -0.69 V for U(IV)/(III). Another rather simple method correlates this potential with the number of 5/"electrons for the element and gives -3.41 V for thorium and -0.54 V for uranium (13). A more sophisticated estimate (14), using a method proposed by Nugent et al. (12) (described later), gave -3.8 V for thorium. 

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