Time-resolved EPR of the radical pair P 865+.Q A−. in bacterial reaction centers. Observations of transient nutations, quantum beats and envelope modulation effects

Abstract

The time evolution of the spin-polarized transient EPR signals of the radical pair state P 865 +.Q A −. is studied in perdeuterated reaction centers of Rhodobacter sphaeroides R-26 in which the non-heme iron has been replaced by zinc. The transients show modulations of the signal amplitude on several time scales. Fast quantum beat oscillations with frequencies around 15 MHz occur within the first 100 ns after the light excitation because the radical pair is generated in a coherent superposition of its eigenstates. Transient nutations also occur as a result of the precession of the magnetization about the applied microwave field B 1. At the maximum B 1 field available (0.125 mT) a nutation frequency of ≈ 3.5 MHz is obtained. Small variations in the nutation frequency throughout the spectrum occur as a result of the dipolar coupling in the radical pair. Additional oscillations in the frequency ranges 1.5 to 2.0 MHz and 2.5 to 3.0 MHz are also observed which are attributed to the interaction between the nuclear spins and the unpaired electron spins. Analogies and differences to well-known nuclear modulation phenomena observed in pulsed EPR experiments are discussed.