Studies of the manganese site of photosystem II by electron spin resonance spectroscopy

Abstract

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.