CONVERSION OF THE SPIN-STATE OF THE MANGANESE COMPLEX IN PHOTOSYSTEM-II INDUCED BY NEAR-INFRARED LIGHT

The manganese complex (Mn-4) which is responsible For water oxidation in photosystem II is EPR detectable in the S-2 state, one of the five redox states of the enzyme cycle. The S-2 State is observable at 10 K either as a multiline signal (spin 1/2) or as a signal at g = 4.1 (spi n 3/2 or spin 5/2). It is shown here that at around 150 K the state re sponsible for the multiline signal is converted to that responsible fo r the g = 4.1 signal upon the absorption of infrared light, This conve rsion is fully reversible at 200 K. The action spectrum of this conver sion has its maximum at 820 nm (12 200 cm(-1)) and is similar to the i ntervalence charge transfer band in di-mu-oxo-((MnMnIV)-Mn-III) model systems, It is suggested that the conversion of the multiline signal t o the g = 4.1 signal results from absorption of infrared light by the Mn cluster itself, resulting in electron transfer from Mn-III to Mn-IV . The g = 4.1 signal is thus proposed to arise from a state which diff ers from that which gives rise to the multiline signal only in terms o f this change in its valence distribution. The near-infrared light eff ect was observed in the S-2 state Of Sr2+-reconstituted photosystem II and in Ca2+-depleted, EGTA (or citrate-)-treated photosystem II but n ot in ammonia-treated photosystem II. Earlier results in the literatur e which showed that the g = 4.1 state was preferentially formed by ill umination at 130 K are reinterpreted as being the result of two photoc hemical events: the first being photosynthetic charge separation resul ting in an S-2 State which gives rise to the multiline signal and the second being the conversion of this state to the g = 4.1 state due to the simultaneous and inadvertent presence of 820 nm light in the broad -band illumination given. There is therefore no reason to consider the state responsible for the g = 4.1 signal as a precursor of that which gives rise to the multiline signal.