The geometry of the secondary radical pair, P(700)(+)A(1)(-), in photosystem I (PSI) from the deuterated and N-15-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 bents observed in the transverse magnetization of P(700)(+)A(1)(-) 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 P(700)(+)A(1)(-) with respect to a magnetic reference frame. The orientation of the cofactors of the primary donor in the g-tensor system of P-700(+) is then evaluated 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 spectra, observed fora magnetically aligned sample. Thus, the orientation of the g-tensor of P-700(+) with respect to a chlorophyll based reference system could be determined. The angle between the g(1)(Z) axis and the chlorophyll plane normal is found to be 29 +/- 7 degrees, while the g(1)(Y) axis lies in the chlorophyll plane. In addition, a complete structural model for the reduced quinone acceptor, A;, is evaluated. In this model, the quinone plane of Ar is found to be inclined by 68 +/- 7 degrees relative to the membrane plane, while the P-700(+)-A(1)(-) axis makes an angle of 35 +/- 6 degrees with the membrane normal. All of these values refer to the charge separated state, P(700)(+)A(1)(-), observed at low temperatures, where forward electron transfer to the iron-sulfur centers is partially blocked. Preliminary room temperature studies of P(700)(+)A(1)(-), employing X-band quantum beat oscillations, indicate a different orientation of A(-)(1); 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.