Transient EPR spectroscopy of the charge separated state P +Q − in photosynthetic reaction centers. Comparison of Zn-substituted Rhodobacter sphaeroides R-26 and Photosystem I

Abstract

Transient EPR spectra measured at 24 GHz and room temperature are compared for: (i) bacterial reaction centers (bRC) of Rhodobacter sphaeroides R-26 in which the non-heme iron has been replaced by zinc, (ii) Photosystem I (PS I) particles from Synechocystis 6803 and (iii) PS I in perdeuterated Synechococcus lividus algae. A comparison of Zn-bRC spectra at 9 GHz and 24 GHz in liquid and frozen solution is also presented. The spectra are assigned to the charge separated state P +·Q −· (P = primary chlorophyll donor, Q = primary quinone acceptor) and show no evidence of motional narrowing thus confirming the condition of slow molecular motion of the RC complex on the microsecond time scale of the experiment. The spectra are interpreted on the basis of the correlated coupled radical pair concept and are dependent upon the relative orientation of the two radical ions as well as their magnetic interaction parameters which are available from independent experiments. For PS I, simulations using independently measured g-tensors confirm that the phylloquinone x-axis (along the C=O bonds) lies roughly parallel to the axis connecting P +· and Q −· (dipole axis). For the Zn-bRCs the spectra show that these two axes make an angle of ∼ 60° with each other in agreement with the two independent sets of atomic coordinates which are available for the ground state structure of Rhodobacter sphaeroides R-26. The two sets of coordinates differ in the orientation of Q which results in large changes in the simulated spectra, primarily because of a shift in the effective g-factor along the dipolar axis. Starting from one of the two sets of atomic coordinates, it is shown that a rotation of Q −· through 12° about its x-axis results in a change in the sign of the polarization of the X-band spectrum. The orientation of Q −· (and P +·) in the charge separated state can thus be determined with a high degree of accuracy by transient EPR, if the magnetic interaction tensors are sufficiently well known from independent measurements.