The Mn(II)Mn(III) multiline EPR signal from the water oxidising complex of photosystem II is attributed to a S-2 state.

Abstract

A new multiline EPR signal has been shown to arise from the interaction of nitric oxide with Photosystem II at ¿30oC[1]. The spectrum was later assigned to the ground state of a magnetically isolated Mn(II)Mn(III) dimer [2], a fact that implied that NO acts as a reductant. The reduction procedure was studied in [3], indicating a state below S-1, but no particular S-state was assigned to the signal. Extending those studies, we sought to establish, by using flash oxymetry, fluorescence and EPR spectroscopy, whether the signal could be assigned to a specific, reduced S-state. On a Joliot-type electrode, the first maximum of oxygen evolution was observed on the 6th flash, before a dampened period-four oscillation pattern was established. One to three saturating preflashes, before the oxygen measurement, shifted the number of the maximum peak down to the third flash. A similar result was obtained by measuring fluorescence levels at each S-state. Using the EPR, a considerable amount of S2 multiline was observed on the fourth flash. These results are consistent with a Mn-cluster poised on the S-2 state in a large fraction of centres. It also appears that NO may play a very specific role in the reduction mechanism of the Mn-cluster: the S-2 state that results from the reduction of the Mn-cluster by hydrazine sulfate results in no multiline EPR signal.