DOES THE FORMATION OF THE S-3-STATE IN CA2+-DEPLETED PHOTOSYSTEM-II CORRESPOND TO AN OXIDATION OF TYROSINE-Z DETECTABLE BY CW-EPR AT ROOM-TEMPERATURE

The tyrosine EPR signal(s) have been investigated at room temperature in Ca2+-depleted Photosystem II. It was found that at low, non-saturating microwave powers, the tyrosine signal present after flash illumination decayed monophasically in about 40% of the centers with a t(1/2) of 780 s. This decay is attributed to TyD degrees reduction. The radical formed as the formal S-3 state and detected as a split EPR signal at helium temperature, decayed monophasically at room temperature with a t(1/2) of 110 s. When the tyrosine EPR signals were monitored at room temperature using a high, saturating microwave power, in addition to the 780 s phase, a faster phase of 110 s was observed. This additional phase is attributed to TyrD degrees, the amplitude of which is enhanced by dipolar coupling to the fast relaxing S-3 state. Its decay corresponds to the loss of the fast relaxing S-3 species. Similar effects reported earlier and attributed to the formation of TyrZ degrees are reinterpreted as a relaxation enhancement of TyD degrees. No evidence was found from this approach to attribute the split S-3 EPR signal to a TyrZ degrees signal which is detectable at room temperature.