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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].

An attempt is made to examine whether it is possible to trap an [S.sub.2] to [S.sub.3] state intermediate in oxygen-evolving photosystem II preparations. These experiments show that an intermediate of the [S.sub.2] to [S.sub.3] transition is formed at temperatures much lower than the threshold for the [S.sub.2] to [S.sub.3] transition.

Research has been conducted on the UV-A sunlight targeting component of the photosystem II complex. The analysis of the UV-A-induced damage in the photosystem II membrane particles carried out via the use of EPR spectroscopy and chlorophyll fluorescence measurements is described.

Research has been conducted on the photosystem II water-oxidising complex's S (sub)3 state. The study of this complex has been carried out via the EPR spectroscopy and the characteristic EPR signals and the equilibria among the excited S (sub)3 state transients have been observed and discussed.

Research has been conducted on the S (sub)3 oxidation state signals. The signal formation correlated with the decay of the S (sub)3-state signals and the comparison of the long-term incubation with the near-IR excitation at liquid-helium temperatures are discussed.

The S state-dependent chlorophyll fluorescence in the nitric oxide-treated photosystem II exhibits a three-flash downshift, whereas in the electron paramagnetic resonance, the maximum S(sup)-2 signal manifests after the fourth flash. Results suggest that manganese cluster is in a redox state after treatment of photosystem II with nitric oxide.

Research describes the electron paramagnetic resonance spectra of the oxygen-evolving complex of photosystem II carrying manganese in a reduced state. Results show that the Mn-(mu-oxo)-Mn creates a small angle at approximately 20 degrees with the membrane plane and bridging oxygens lie parallel to the plane.

A study was conducted relating to the investigation of tetra nuclear Mn-centered EPR signals for electron containing oxidation-state transitions in photosystem II preparations incubated with NO. Results of the experiment indicate that the S0 and S-1 oxidation state transitions are intermediates of a reductive process leading to the unique Mn(II)-Mn(III) signal evolution. The latter is tentatively assigned to an (iso)S-2 state.

A study was conducted to characterize of the signal resulting from the interaction of NO with the S1 state of the water-oxidizing complex of photosystem II (PSII). The findings indicate that an EPR signal from the Mn cluster of PSII can be assigned to a magnetically isolated Mn(II)-Mn(III) dimer. Moreover, the signal arises from a state lower than S1 even though it is not assigned to a particular S state.

Researchers have used NO treatment at subzero temperatures to resolve and trap the intermediates of the interaction of the spin-1/2-carrying NO molecules with both the S1 and S2 states of the water oxidizing complex. It was established that the NO modified S1 state can operate as a low-temperature electron donor yielding an EPR silent state when illuminated at 200 K. There seem to be three main modes of binding of NO. There is a significant difference in the kinetics and probably the affinity of the NO binding in the S1 and S2 states.

The reversible rapid interaction of NO with the TyrY(sub D)* of photosystem II (PSII) was found to be the first step to a much slower reaction, which is irreversible. This reaction leads to the formation of a Tyr-NO species which can function as an electron donor to PSII. Upon light-induced oxidation of this species, an iminoxyl radical is created. This is the first example of a chemical modification of one of the tyrosines of PSII to create a photochemically active species.

Ultraviolet B (UV-B) radiation damages protein structures and the electron transfer system in the photosystem II (PSII) reaction center. This results in the inhibition of the electron transport system in PSII. The UV-B rays damage the water-oxidizing system first. Damage takes place at the electron transfer step between the Mn cluster and oxidized Tyr-Z. This leads to the modification and/or inactivation of tyrosine-D, cytochrome b-559, and the acceptor complex comprising the primary electron acceptor and ferredoxin.

Four new ferrous bis-chelates of bimH2, (Fe2(bim-H2)2(CH3OH)2)OAc2, (Fe2(bimH2)3)CO3, (Fe2(bimH)2)(sub n), and (Fe2(bim))(sub n) were synthesized and characterized using X-ray techniques. Precipitation of the unsolvated species Fe2(bimH2)2(CH3CO2)2 reactions was faster in warm solutions. Results suggest that metatheses with HCO3- yields ferrous compounds containing CO3- irrespective of the synthetic process. Structural, Mossbauer and magnetic properties of the ferrous compounds are discussed.

The photoinhibition of photosystem II (PSII) affects the first electron acceptor (QA) before it affects the iron molecule. Low temperature EPR and Mossbauer spectroscopy were used to measure the effect of photoinhibition on spinach PSII. Photoinhibition reduced the EPR signal from the interaction of the two molecules to 30% in 90 minutes. But non-heme iron was still detected in 70% of the PSII centers.

EPR spectroscopic studies of the influence of cyanide on the iron-quinone complex QaFe2+ at high pH values show that g=1. 98 signal due to QA-Fe2+ state slowly increases to g=2.0045, indicating the uncoupling of QA- from iron. Mossbauer spectroscopic studies show that at high pH and higher cyanide concentrations cyanide converts Fe2+ from high-spin state to a low-spin state. The reversible formation of QA-Fe2+ at low pH reveals that iron is not dissociated during pH-cyanide treatment. Increase of value at higher CN concentrations shows the binding of two or three cyanides on the iron.

A rapid or slow EPR scans were used to study the protonation of Tyr [Z.sub..] transient upon oxidation of the Mn cluster, in the temperature range 77-240 K. The results showed that the Mn cluster is oxidized by [Yz.sub..]/YzH couple during the [S.sub.1]-[S.sub.2] and [S.sub.2]-[S.sub.3] transitions.

The intermediates of the water splitting mechanism of photosystem II are discussed. The spectrum of the tyrosine [Z.sup..] is also described by observing the collapse of tyrosine [Z.sup..]-Mn spin-spin interaction above approximately 100 K.

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 changes accompanying the near-infrared (NIR) excitation of the S(sub 3) state are examined. It is shown that the NIR excitation at low temperatures, by producing tyr Z(super +) as an intermediate, speeds up charge recombination, and converts S(sub 3) to a modified S(sub 2) (s = 7/2 ground-state).

Both X- and Q-band electron paramagnetic resonance (EPR) spectroscopy were utilized in the study of the effects of fluoride and nitrogen oxide (NO) on the iron-quinone complex of the acceptor side of Photosystem II. Changes in the EPR signal of the nitrosyl complex, which occurred with the addition of the fluoride ions, were determined to be the result of superhyperfine interactions between the electronic spin and the fluorine nuclear spin. The results indicate that both NO and fluoride are bound to the same nonheme iron.