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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.

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 S2 state of the oxygen-evolving complex (OEC) of photosystem II is heterogeneous, exhibiting two main EPR spectral forms, the multiline and the g = 4.1 signal. It is not clearly established whether this heterogeneity develops during the to S1 to S2 transition or is already present in the precursor states. We have compared the spectra of the S1YZ• intermediate, obtained by visible light excitation (induction of charge separation) of the S1 state at liquid He temperatures, (S1YZ•)vis, or by near-infrared (NIR) light excitation of the S2 state (utilization of the unusual property of the Mn cluster to act as an oxidant of Ys when excited by NIR), (S1YZ•)NIR. The decay kinetics of the (S1YZ•)vis spectrum at II K was also studied by the application of rapid-scan EPR. The two spectra share in common a signal with a characteristic feature at g = 2.035, but the (S1YZ•)vis spectrum contains in addition a fast decaying component 26 G wide. The analysis of the surface of the rapid-scan spectra yielded 270 ± 35 and 90 ± 15 s for the respective half-times of the two components of the (S1YZ•)vis spectrum at 11 K. (S1YZ•)vis advances efficiently to S2 when annealed at 200 K; notably the g = 2.035 signal advances to the multiline while the 26 G component advances to the g = 4.1 conformation. The "26 G" component is absent or very small, respectively, in thermophilic cyanobacteria or glycerol-containing spinach samples, in correlation to vanishing or very small amounts of the g = 4.1 component in the S2 spectrum. The results validate the assignment of S1YZ• to a true S1 to S2 intermediate and imply that the heterogeneity observed in S2 is already present in S1. Tentative valences are assigned to the individual Mn ions of the OEC in the two heterogeneous conformations of S1. [ABSTRACT FROM AUTHOR]

Near-IR (NIR) excitation at liquid He temperatures of photosystem II (PSII) membranes from the cyanobacterium Synechococcus vulcanus or from spinach poised in the S[sub 2] state results in the production of a g = 2.035 EPR resonance, reminiscent of metalloradical signals. The signal is smaller in the spinach preparations, but it is significantly enhanced by the addition of exogenous quinones. Ethanol (2-3%, v/v) eliminates the ability to trap the signal. The g = 2.035 signal is identical to the one recently obtained by Nugent et al. by visible-light illumination of the S l state, and preferably assigned to S[sub 1]Y[sub Z, sup •] [Nugent, J. H. A., Muhiuddin, I. P., and Evans, M. C. W. (2002) Biochemistry 41, 4117-4126]. The production of the g = 2.035 signal by liquid He temperature NIR excitation of the S[sub 2] state is paralleled by a significant reduction (typically 40-45% in S. vulcanus) of the S[sub 2] state multiline signal. This is in part due to the conversion of the Mn cluster to higher spin states, an effect documented by Boussac et al. [Boussac, A., Un, S., Homer, O., and Rutherford, A. W. (1998) Biochemistry 37, 4001-4007], and in part due to the conversion to the g = 2.035 configuration. Following the decay of the g = 2.035 signal at liquid helium temperatures (decay halftimes in the time range of a few to tens of minutes depending on the preparation), annealing at elevated temperatures (-80 °C) results in only partial restoration of the S2 state multiline signal. The full Size of the signal can be restored by visible-light illumination at -80 °C, implying that during the near-IR excitation and subsequent storage at liquid helium temperatures recombination with Q[sub A, sup -] (and therefore decay of the S[sub 2] state to the S[sub 1] state) occurred in a fraction of centers. In support of this conclusion, the g = 2.035 signal remains stable for several hours (at 11 K) in centers poised in the S[sub 2]···Q[sub A]... [ABSTRACT FROM AUTHOR]