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The spectra of the [S.sub.1][Y.sub.Z] intermediate obtained are compared by visible light excitation of the [S.sub.1] state at liquid He temperatures or by near-infrared (NIR) light excitation of the [S.sub.2] state. The results validate the assignment of [S.sub.1][Y.sub.Z] to a true [S.sub.1] to [S.sub.2] intermediate and imply that the heterogeneity observed in [S.sub.2] is already present in [S.sub.1].

Research has been conducted on the heme-contained electron-transfer protein cytochrome c (sub)5. The relaxation properties of this cytochrome have been investigated via a Model-Free analysis.

A new study investigates the local mobility of the reduced and oxidized forms of rat microsomal cytochrome b5, using rotating frame 15N relaxation NMR measurements. Detailed study of the data shows different local mobility along the backbone nitrogens for both oxidized and reduced cytochrome. Such differential is thought to be important in the control of molecular recognition process involving cytochrome b5.

The cytochrome c6 of the photosynthetic algae Monoraphidium braunii is a protein consisting of 89 amino acid involved in the electron transport chain. Unlike the cytochrome c6 of yeasts and horses, that of M. braunii is negatively charged. A combination of spectroscopic analyses were employed in the elucidation of the solution structure of the oxidized form of this cytochrome. Results reveal that the oxidized structure is very similar to that of the reduced form.

Saturation-recovery electronic paramagnetic resonance (EPR) spectroscopy is utilized to examine the magnetic attributes and location of two forms of the S2 state of photosystem II (PSII). These include the altered multiline EPR signal species in NH3-treated PSII and the multiline EPR signal species in untreated PSII. Test results support findings regarding the asymmetric location of the Mn4 cluster as regards the redox-active tyrosines found in PSII.

The use of dinucleating bis(carboxylato) ligand XDX and phenoxyl radical ligand BIDPhE enables the synthesis of the phenoxyl radical diiron complex (Fe2O(XDK)(MeOH5)(H2O))(NO3)2). The radical diiron complex is an active site in the R2 protein of ribonucleotide reductase. The diiron complex is obtained as a stable solid.

Saturation-recovery EPR spectroscopic studies on photosystem II (PSII) membranes of spinach and on PSII core complexes of cyanobacterium Synechocystis sp. PCC 6803 reveal the symmetric location of tyrosines D and Z, relative to the non-heme Fe(II) in Mn-depleted photosystem II. The spin-lattice relaxation for both the YD and YZ radicals are increased by non-heme Fe(II). Spectral data confirm the presence of a C2 symmetry axis among the chromophores of PSII.

Saturation-recovery and progressive microwave power saturation EPR spectroscopic studies of the determination of the location of chlorophyll(z) in photosystem II (PSII) reveal that chlorophyll(z) resides on the hydrophobic surface of the D1/D2 complex. This location is in close association with Cyt b559. This proximity explains the fast reduction of Chl(z+) by Cyt b(559). Chl(z) is not analogous to the 'voyeur' monomer bacteriochlorophylls of the bacterial reaction center.

Trapping of intermediates of the S-state transitions that involve the tyrosyl radical is of ultimate importance, as they can test critical models employing a role of tyrosine (Tyr Z) in proton protein transfer, and provide important clues about the mechanism of water oxidation. Details on the observations of the trapping of metalloradical intermediates of the S-state transitions, at liquid helium temperatures are discussed.

The near-infra red excitation of the S(sub)2 state at liquid helium temperatures produces metalloradical intermediates assigned to S(sub)1Y(sub)z, suggesting a proton exchange between tyrosine Z and the manganese cluster and alteration of the magnetic properties of the manganese cluster.

The pulsed electron paramagnetic resonance (EPR) method of saturation recovery was used to examine the effect of dark adaptation on the magnetic properties of the Mn site in the O2-evolving center of photosystem II.The spin-lattice relaxation of the stable tyrosine radical of ribonucleotide reductase from Escherichia coli was demonstrated to serve as a probe of the magnetic properties of the EPR-silent dinuclear Fe(III) center. Moreover, the results indicate that the Mn cluster structure is tetrameric.