Near-IR Irradiation of the S2State of the Water Oxidizing Complex of Photosystem II at Liquid Helium Temperatures Produces the Metalloradical Intermediate Attributed to S1YZ•

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Abstract
Near-IR (NIR) excitation at liquid He temperatures of photosystem II (PSII) membranes from the cyanobacterium Synechococcus vulcanus or from spinach poised in the S2 state results in the production of a g = 2.035 EPR resonance, reminiscent of metalloradical signals. The signal is smaller in the spinach preparations, but it is significantly enhanced by the addition of exogenous quinones. Ethanol (2−3%, v/v) eliminates the ability to trap the signal. The g = 2.035 signal is identical to the one recently obtained by Nugent et al. by visible-light illumination of the S1 state, and preferably assigned to S1YZ• [Nugent, J. H. A., Muhiuddin, I. P., and Evans, M. C. W. (2002) Biochemistry 41, 4117−4126]. The production of the g = 2.035 signal by liquid He temperature NIR excitation of the S2 state is paralleled by a significant reduction (typically 40−45% in S. vulcanus) of the S2 state multiline signal. This is in part due to the conversion of the Mn cluster to higher spin states, an effect documented by Boussac et al. [Boussac, A., Un, S., Horner, O., and Rutherford, A. W. (1998) Biochemistry 37, 4001−4007], and in part due to the conversion to the g = 2.035 configuration. Following the decay of the g = 2.035 signal at liquid helium temperatures (decay halftimes in the time range of a few to tens of minutes depending on the preparation), annealing at elevated temperatures (−80 °C) results in only partial restoration of the S2 state multiline signal. The full size of the signal can be restored by visible-light illumination at −80 °C, implying that during the near-IR excitation and subsequent storage at liquid helium temperatures recombination with QA- (and therefore decay of the S2 state to the S1 state) occurred in a fraction of centers. In support of this conclusion, the g = 2.035 signal remains stable for several hours (at 11 K) in centers poised in the S2···QA configuration before the NIR excitation. The extended stability of the signal under these conditions has allowed the measurement of the microwave power saturation and the temperature dependence in the temperature range of 3.8−11 K. The signal intensity follows Curie law temperature dependence, which suggests that it arises from a ground spin state, or a very low-lying excited spin state. The P1/2 (microwave power at half-saturation) value is 1.7 mW at 3.8 K and increases to 96 mW at 11 K. The large width of the g = 2.035 signal and its relatively fast relaxation support the assignment to a radical species in the proximity of the Mn cluster. The whole phenomenology of the g = 2.035 signal production is analogous to the effects of NIR excitation on the S3 state [Ioannidis, N., Nugent, J. H. A., and Petrouleas, V. (2002) Biochemistry 41, 9589−9600] producing an S2‘YZ• intermediate. In the present case, the intermediate is assigned to S1YZ•. The NIR-induced increase in the oxidative capability of the Mn cluster is discussed in relation to the photochemical properties of a Mn(III) ion that exists in both S2 and S3 states. The EPR properties of the S1YZ• intermediate cannot be reconciled easily with our current understanding of the magnetic properties of the S1 state. It is suggested that oxidation of tyr Z alters the magnetic properties of the Mn cluster via exchange of a proton.

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