Spectral resolution of the split EPR signals induced by illumination at 5 K from the S1, S3, and S0 states in photosystem II

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 Y-D(center dot) radical in the g similar to 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 Y-Z(center dot). The three split EPR signals that were investigated, Split S-1, Split S-3, and Split S-0, all exhibit spectral features at g similar to 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 similar to 2.0 part of the Split S-1 EPR spectrum contains a contribution from magnetically uncoupled Y-Z(center dot) 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 Y-Z 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.