High-Spin States (S ≥ 5/2) of the Photosystem II Manganese Complex

The Mn4 complex which is involved in water oxidation in photosystem II (PSII) is known to exhibit two types of EPR signals in the S2 state, one of the five redox states of the enzyme cycle: either a multiline signal (S = 1/2) or a signal at g = 4.1 (S = 3/2 or S = 5/2). The S = 1/2 state can be converted to that responsible for the g = 4.1 signal upon the absorption of near-infrared (IR) light [Boussac, A., Girerd, J.−J., and Rutherford, A. W. (1996) Biochemistry 35, 6984−6989]. It is shown here that a third state gives rise to signals at g = 10 and 6. This state is formed by IR illumination of the S = 1/2 state at 65 K, a temperature where IR illumination leads to the loss of the S = 1/2 signal but to no formation of the g = 4.1 state. On the basis of the corresponding decrease of the S = 1/2 state, the new state can be trapped in ∼40% of the PSII centers. Warming of the sample above 65 K, in the dark, leads to the loss of the g = 10 and 6 resonances with the corresponding appearance of the g = 4.1 signal. It is suggested that the IR-induced conversion of the S = 1/2 state into the g = 4.1 state at 150 K involves the transient formation of the new state. The new state is attributed to a S = 5/2 state of the Mn4 complex (although a S value > 5/2 is also a possibility). Spectral simulations indicate an E/D ratio of −0.05 with |D| ≤ 1 cm-1. The resonances at g = 10 and 6 correspond to the gz of the ±5/2 and ±3/2 transition, respectively. The temperature-dependent conversion of this S = 5/2 state into the g = 4.1 state is proposed to be due to relaxation of the ligand environment around the Mn4 cluster that leads to a change in the zero field splitting parameters, assuming an S = 5/2 value for the g = 4.1 state. The new form of the S2 state reported here may explain some earlier data where the S2 state was present and yet not detectable as either a S = 1/2 or a g = 4.1 EPR signal.