The S1YZ˙ metalloradical intermediate in photosystem II: an X- and W-band EPR study

Abstract

Visible light illumination at liquid He temperatures of photosystem II (PSII) membranes poised in the S₁-state, results in the production of a metalloradical signal with resonances at g = 2.035 and g ∼ 2.0 at X-band (J. H. A. Nugent, I. P. Muhiuddin, and M. C. W. Evans, Biochemistry, 2002, 41, 4117–4126). A similar signal has been obtained by near IR excitation of samples poised in the S₂ state (D. Koulougliotis, J.-R. Shen, N. Ioannidis, and V. Petrouleas, Biochemistry, 2003, 42, 3045–3053). The signal has been attributed to the magnetic interaction of the tyrosyl Z radical with the Mn cluster in the S₁ state. In an effort to obtain further information about the interactions of tyrosine Z with the Mn cluster, and about the integer-spin S₁ state we have employed EPR spectroscopy at two frequencies, X and W-band. The spectrum at W band is characterized by novel resonances at g = 2.019, g ∼ 2.00 and g = 1.987. For the analysis of the spectra at the two microwave frequency bands a spin Hamiltonian has been applied under the following basic assumptions: The S₁ state of the Mn cluster is characterized by two low lying spin states S_a = 0 and 1. The major features of the spectra are attributed to the interaction of the S_a = 1 state with the spin S_b = 1/2 of the tyrosyl radical. Potential contributions from the S_a = 0 state are suppressed under the present experimental conditions. A satisfactory fit reproducing all features of the spectra is achieved with the same set of fitting parameters for the signals at both bands. An anisotropic ferromagnetic exchange interaction results from the fit with the coupling value being of the same order of magnitude with the value of the zero field splitting term of the Mn cluster (S = 1).