Transient W-Band EPR Study of Sequential Electron Transfer in Photosynthetic Bacterial Reaction Centers

Electron-spin polarized (ESP) EPR spectra at W-band (95 GHz) were obtained for deuterated Fe-removed/Zn-substituted photosynthetic bacterial reaction centers (RCs) to investigate the influence of the rate of charge separation on the observed P+QA - charge separated state. Temperature dependent ESP EPR spectra for kinetically characterized Zn-substituted RCs from Rb. sphaeroides R-26 having different rates (k Q) of the electron transfer from the bacteriopheophytin to the quinone acceptor were obtained. The Zn−RCs exhibited either the native “fast” (200 ps)-1 k Q or a “slow” (3−6 ns)-1 k Q at 298 K as determined from transient optical measurements. Sequential electron-transfer polarization modeling of the polarized W-band EPR spectra obtained with these samples was used to address the reason for the differences in the electron-transfer rates. Herein, we report the k Q rate constant, the temperature dependence of k Q, and the reorganization energy for the P+H-QA and P+HQA - electron-transfer step determined from SETP modeling of the experimental spectra. The reorganization energy for the electron-transfer process between P+H-QA and P+HQA -, and not structural changes in the donor or acceptor, was found to be the dominant factor that is altered during Fe-removal procedures.