Structure of the   Radical Pair Intermediate in Photosystem I by High Time Resolution Multifrequency Electron Paramagnetic Resonance: Analysis of Quantum Beat Oscillations

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Abstract
The geometry of the secondary radical pair, , in photosystem I (PSI) from the deuterated and 15N-substituted cyanobacterium Synechococcus lividus has been determined by high time resolution electron paramagnetic resonance (EPR), performed at three different microwave frequencies. Structural information is extracted from light-induced quantum beats observed in the transverse magnetization of at early times after laser excitation. A computer analysis of the two-dimensional Q-band experiment provides the orientation of the various magnetic tensors of with respect to a magnetic reference frame. The orientation of the cofactors of the primary donor in the g-tensor system of is then evaluated by analyzing time-dependent X-band EPR spectra, extracted from a two-dimensional data set. Finally, the cofactor arrangement of in the photosynthetic membrane is deduced from angular-dependent W-band spectra, observed for a magnetically aligned sample. Thus, the orientation of the g-tensor of with respect to a chlorophyll based reference system could be determined. The angle between the axis and the chlorophyll plane normal is found to be 29 ± 7°, while the axis lies in the chlorophyll plane. In addition, a complete structural model for the reduced quinone acceptor, , is evaluated. In this model, the quinone plane of is found to be inclined by 68 ± 7° relative to the membrane plane, while the − axis makes an angle of 35 ± 6° with the membrane normal. All of these values refer to the charge separated state, , observed at low temperatures, where forward electron transfer to the iron−sulfur centers is partially blocked. Preliminary room temperature studies of , employing X-band quantum beat oscillations, indicate a different orientation of in its binding pocket. A comparison with crystallographic data provides information on the electron-transfer pathway in PSI. It appears that quantum beats represent excellent structural probes for the short-lived intermediates in the primary energy conversion steps of photosynthesis.

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