title: A new approach to determining the geometry of weakly coupled radical pairs from their electron spin polarization patterns.
abstract: Analytical expressions for the spin polarized EPR lineshapes of weakly coupled 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 combination of four powder spectra, which are independent of the geometry of the RP. To reproduce the experimental spectra the optimal set of coefficients 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 from such spectra. Most importantly, a unique set of angles can only be obtained if the absolute amplitude of the spectrum is known. In general, the calculated spectrum is related to the experimental spectrum by an unknown, arbitrary scaling factor. In this case, sets of angles consistent with the data are obtained. Possible strategies for obtaining unique geometric information are discussed and demonstrated with the experimental data for the state P+*(865)Q-*(A) in Zn-substituted bacterial reaction centres.
authors.affiliation: Department of Chemistry, Brock University, St Catharines, Ontario, Canada.
authors.forename: Y
authors.initials: Y
authors.lastname: Kandrashkin
authors.forename: A
authors.initials: A
authors.lastname: van der Est
mesh_terms: D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D011487:Protein Conformation; D011506:Proteins / Q000737:chemistry*; D019097:Spin Trapping / Q000379:methods
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D011506:Proteins