Studies of the manganese site of photosystem II by electron spin resonance spectroscopy
The Mn-containing catalytic site for photosynthetic water oxidation undergoes changes in oxidation states during the catalytic cycle. One of these intermediates, the S2 state, can be studied directly by e.s.r. at liquid-helium temperatures. Two distinct e.s.r. signals from the S2 state are produced when dark-adapted Photosystem II membranes are illuminated in the 130–200 K range: a g= 4.1 signal or a signal centred at g= 2.0 with many hyperfine lines, referred to as the multiline e.s.r. signal. The yields and magnetic properties of these e.s.r. signals are found to depend on the temperature at which the S2 state is formed and the choice of cryoprotectant (ethylene glycol or sucrose). The intensity of the g= 4.1 e.s.r. signal obeys the Curie law in the 4.0–20.0 K temperature range. The S2-state multiline e.s.r. signal exhibits an intensity maximum at 7.0 K which is independent of microwave powers below 2 mW, if the samples contain 30 % ethylene glycol. This non-Curie behaviour is not observed in samples containing 0.4 mol dm–3 sucrose. A model is presented in which the S2 state e.s.r. signals arise from an exchange-coupled Mn tetramer, where both ferromagnetic and antiferromagnetic exchange occur. According to our model, the multiline e.s.r. signal observed in samples suspended in 30 % ethylene glycol originates from the thermally populated first excited s= 1/2 state of the exchange-coupled Mn tetramer, whereas the g= 4.1 e.s.r. signal arises from the ground s= 3/2 state of the Mn tetramer in a configuration that makes the s= 1/2 state thermally inaccessible. The different behaviour of the S2-state multiline e.s.r. signal in samples containing sucrose can be explained by a small conformational change of the Mn complex which alters the exchange couplings. In support of our assignment of the multiline e.s.r. signal, we present spectral simulations at S-, X- and Q-bands. The fits to the experimental spectra at X- and Q-bands are improved if a small degree of anisotropy is introduced in the g tensor of the Mn complex.