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(+)Q(A)(-) charge separated state. Temperature dependent ESP EPR spectra for kinetically characterized Zn-substituted RCs from Rb. sphaeroides R-26 having different rates (kh) 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 ka rate constant, the temperature dependence of k(Q), and the reorganization energy for the P(+)H(-)Q(A) and P(+)HQ(A)(-) electron-transfer step determined from SETP modeling of the experimental spectra. The reorganization energy for the electron-transfer process between P(+)H(-)Q(A) and P(+)HQ(A)(-), and not structural changes in the donor or acceptor, was found to be the dominant factor that is altered during Fe-removal procedures.