Spectral Resolution of the Split EPR Signals Induced by Illumination at 5 K from the S1, S3, and S0States in Photosystem II

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Abstract
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.

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