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The interaction of EPR split signals from photosystem II (PSII) are reported from the [S.sub.0], [S.sub.1] and [S.sub.3] states. The formation spectra of split EPR signals induced in PSII enriched membranes are investigated.

The spilt EPR signals, originating from the S1, S3, and S0 states in spinach photosystem II are investigated. The three split EPR signal spectra exhibit spectral features in the g~2 region together with, for each signal, characteristic peaks and troughs.

The methanol dependence of the split EPR signals originating from the magnetic interaction between the Ca [Mn.sub.4] cluster and the [Y.sub.Z] radical in photosystem II (PSII), which are induced by illumination at 5 K, is reported. It is found that the magnitudes of the 'split [S.sub.1]' and 'split [S.sub.3]' signals in [S.sub.1] and [s.sub.3] states of PSII centers, respectively, are diminished with an increase in methanol concentration, due to an increase in the extent of magnetic coupling within the cluster.

The interaction EPR split signals from photosystem II (PSII) have been reported from the S0, S1, and S3 states. The signals are induced by illumination at cryogenic temperatures and are proposed to reflect the magnetic interaction between Yz• and the Mn4Ca cluster. We have investigated the formation spectra of these split EPR signals induced in PSII enriched membranes at 5 K using monochromatic laser light from 400 to 900 nm. We found that the formation spectra of the split S0, split S1, and split S3 EPR signals were quite similar, but not identical, between 400 and 690 nm, with maximum formation at 550 nm. The major deviations were found between 440 and 480 nm and between 580 and 680 nm. In the regions around 460 and 680 nm the amplitudes of the formation spectra were 25-50% of that at 550 nm. A similar formation spectrum was found for the S2-state multiline EPR signal induced at 0 °C. In general, the formation spectra of these signals in the visible region resemble the reciprocal of the absorption spectra of our PSII membranes. This reflects the high chlorophyll concentration necessary for the EPR measurements which mask the spectral properties of other absorbing species. No split signal forniation was found by the application of infrared laser illumination between 730 and 900 nni from PSII in the S0 and S1 states. However, when such illumination was applied to PSII membranes poised in the S3 state, formation of the split S3 EPR signal was observed with maximum formation at 740 nm. The quantum yield was much less than in the visible region, but the application of intensive illumination at 830 nm resulted in accumulation of the signal to an amplitude comparable to that obtained with illumination with visible light. The split S3 EPR signal induced by NIR light was much more stable at 5 K (no observable decay within 60 mm) than the split S3 signal induced by visible light (50% of the signal decayed within 30 mm). The split S3 signals induced by each of these light regimes showed the same EPR spectral features and microwave power saturation properties, indicating that illumination of P511 in the S3 state by visible light or by NIR light produces a similar configuration of Yz• and the Mn4Ca cluster. [ABSTRACT FROM AUTHOR]

S-State-dependent split EPR signals that are induced by illumination at cryogenic temperatures (5 K) have been measured in spinach photosystem II without interference from the YD• radical in the g ∼ 2 region. This allows us to present the first decay-associated spectra for the split signals, which originate from the CaMn4 cluster in magnetic interaction with a nearby radical, presumably YZ•. The three split EPR signals that were investigated, "Split S1", "Split S3", and Split S0", all exhibit spectral features at g ∼ 2.0 together with surrounding characteristic peaks and troughs. From microwave relaxation studies we can reach conclusions about which parts of the complex spectra belong together. Our analysis strongly indicates that the wings and the middle part of the split spectrum are parts of the same signal, since their decay kinetics in the dark at 5 K and microwave relaxation behavior are indistinguishable. In addition, our decay-associated spectra indicate that the g ∼ 2.0 part of the "Split S1" EPR spectrum contains a contribution from magnetically uncoupled YZ• as judged from the g value and 22 G line width of the EPR signal. The g value, 2.0033–2.0040, suggests that the oxidation of YZ at 5 K results in a partially protonated radical. Irrespective of the S state, a small amount of a carotenoid or chlorophyll radical was formed by the illumination. However, this had relaxation and decay characteristics that clearly distinguish this radical from the split signal spectra. In this paper, we present the "clean" spectra from the low-temperature illumination-induced split EPR signals from higher plants, which will provide the basis for further simulation studies. [ABSTRACT FROM AUTHOR]

Methanol binds to the CaMn4 cluster in photosystem II (PSII). Here we report the methanol dependence of the split EPR signals originating from the magnetic interaction between the CaMn4 cluster and the Yz• radical in PSII which are induced by illumination at 5 K. We found that the magnitudes of the ‘split S1’ and ‘split S3’ signals induced in the S1 and S3 states of PSII centers, respectively, are diminished with an increase in the methanol concentration. The methanol concentrations at which half of the respective spectral changes had occurred ([MeOH]½) were 0.12 and 0.57%, respectively. By contrast, the ‘split S0’ signal induced in the S0 state is broadened, and its amplitude is enhanced. ([MeOH]½) for this change was found to be 0.54%. We discuss these observations with respect to the location and nature of the methanol binding site. Furthermore, by comparing this behavior with methanol effects reported for other EPR signals in the different S states, we propose that the observed methanol-dependent changes in the split S1 and split S0 EPR signals are caused by an increase in the extent of magnetic coupling within the cluster. [ABSTRACT FROM AUTHOR]