EPR SATURATION AND TEMPERATURE-DEPENDENCE STUDIES ON SIGNALS FROM THE OXYGEN-EVOLVING CENTER OF PHOTOSYSTEM-II

Microwave power saturation studies have been performed over the range 4-20 K on EPR signals photogenerated in PS II particles by low-temperature illumination (180-240 K). In the presence of 3% methanol (+MeOH), with no g 4.1 signal present, the multiline signal intensity (extrapolated to zero power) shows strict Curie law behaviour over the 4-20 K range. With no MeOH present in the suspension buffer (-MeOH), both the multiline and 4.1 signals show complementary deviations from Curie law behaviour. These are consistent with the signals arising respectively from the ground, S = 1/2, and first excited S = 3/2, states of a total spin = 7/2 multiplet, such as could occur in an MnIV-MnIII antiferromagnetically coupled pair. The deduced height of the 3/2 state above the 1/2 state is 9.0 K. An inferential estimate, from relaxation data, of this height for the +MeOH case is about 40 K. A broad, featureless component around g = 2 appears to underlie the multiline pattern in the presence of the 4.1 signal, and has a similar temperature behaviour to the latter. A possible exchange coupling model, involving four Mn centres, is presented to accommodate these and other findings on the S2 state signals.