A new approach to determining the geometry of weakly coupled radical pairsfrom their electron spin polarization patterns

Analytical expressions for the spin polarized EPR lineshapes of weakly coup led radical pairs (RPs) are derived as functions of the angles between the anisotropic g-tensors of the radicals and the vector describing the dipolar coupling. It is shown that with a singlet precursor the EPR signal of the RP can be written as a linear function of the dipolar coupling. Under these conditions, the calculated powder spectrum can be expressed as a linear co mbination of four powder spectra, which are independent of the geometry of the RP. To reproduce the experimental spectra the optimal set of coefficien ts can be found by least-squares fitting. The advantage of this approach is that the four powder spectra must only be calculated once. This treatment shows very clearly the restrictions placed on the information obtainable fr om such spectra. Most importantly, a unique set of angles can only be obtai ned if the absolute amplitude of the spectrum is known. In general, the cal culated spectrum is related to the experimental spectrum by an unknown, arb itrary scaling factor. In this case, sets of angles consistent with the dat a are obtained. Possible strategies for obtaining unique geometric informat ion are discussed and demonstrated with the experimental data for the state P(865)(1 .)QA(-.) in Zn-substituted bacterial reaction centres. (C) 2001 E lsevier Science B.V. All rights reserved.