Time-resolved W-band (95 GHz) EPR spectroscopy of Zn-substituted reaction centers of Rhodobacter sphaeroides R-26 Original Research Article

Transient EPR spectra of protonated and deuterated Zn-substituted reaction centres of Rhodobacter sphaeroides R-26, measured at a microwave frequency of 95 GHz and an external magnetic field of 3.5 T, are presented. The high-field/high-frequency spin-polarized spectrum of the correlated coupled radical pair P865+.QA−. after laser flash excitation is a very sensitive monitor of the relative orientation of the two g-matrices and the dipolar tensor with respect to each other. Therefore, detailed structural information of the RC concerning the relative orientation of the primary donor P865+. and the acceptor QA−. with respect to the axis connecting the two molecules in the RC can be derived. Together with the information obtained by high-field cw-EPR on single crystals, the enhanced resolution of the polarized high-field spectra allows an unambiguous assignment of the g-matrix of the donor P865+. to the molecular axis system. The experimental results are compared with earlier X-band (9 GHz) and K-band (24 GHz) EPR experiments.