Structure of the P-700(+) A(1)(-) radical pair intermediate in photosystemI by high time resolution multifrequency electron paramagnetic resonance: Analysis of quantum beat oscillations

The geometry of the secondary radical pair, P(700)(+)A(1)(-), in photosyste m I (PSI) from the deuterated and N-15-substituted cyanobacterium Synechoco ccus lividus has been determined by high time resolution electron paramagne tic resonance (EPR), performed at three different microwave frequencies. St ructural information is extracted from light-induced quantum bents observed in the transverse magnetization of P(700)(+)A(1)(-) at early times after l aser excitation. A computer analysis of the two-dimensional Q-band experime nt provides the orientation of the various magnetic tensors of P(700)(+)A(1 )(-) with respect to a magnetic reference frame. The orientation of the cof actors of the primary donor in the g-tensor system of P-700(+) is then eval uated by analyzing time-dependent X-band EPR spectra, extracted from a two- dimensional data set. Finally, the cofactor arrangement of P(700)(+)A(1)(-) in the photosynthetic membrane is deduced from angular-dependent W-band sp ectra, observed fora magnetically aligned sample. Thus, the orientation of the g-tensor of P-700(+) with respect to a chlorophyll based reference syst em could be determined. The angle between the g(1)(Z) axis and the chloroph yll plane normal is found to be 29 +/- 7 degrees, while the g(1)(Y) axis li es in the chlorophyll plane. In addition, a complete structural model for t he reduced quinone acceptor, A;, is evaluated. In this model, the quinone p lane of Ar is found to be inclined by 68 +/- 7 degrees relative to the memb rane plane, while the P-700(+)-A(1)(-) axis makes an angle of 35 +/- 6 degr ees with the membrane normal. All of these values refer to the charge separ ated state, P(700)(+)A(1)(-), observed at low temperatures, where forward e lectron transfer to the iron-sulfur centers is partially blocked. Prelimina ry room temperature studies of P(700)(+)A(1)(-), employing X-band quantum b eat oscillations, indicate a different orientation of A(-)(1); in its bindi ng pocket. A comparison with crystallographic data provides information on the electron-transfer pathway in PSI. It appears that quantum beats represe nt excellent structural probes for the short-lived intermediates in the pri mary energy conversion steps of photosynthesis.