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Issue 20, 2004
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The S1YZ˙ metalloradical intermediate in photosystem II: an X- and W-band EPR study

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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 Sa = 0 and 1. The major features of the spectra are attributed to the interaction of the Sa = 1 state with the spin Sb = 1/2 of the tyrosyl radical. Potential contributions from the Sa = 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).

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Article information

17 May 2004
29 Jul 2004
First published
16 Aug 2004

Phys. Chem. Chem. Phys., 2004,6, 4859-4863
Article type

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

D. Koulougliotis, C. Teutloff, Y. Sanakis, W. Lubitz and V. Petrouleas, Phys. Chem. Chem. Phys., 2004, 6, 4859
DOI: 10.1039/B407355J

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