TIME-RESOLVED EPR OF THE RADICAL PAIR P(865)(-) IN BACTERIAL REACTIONCENTERS - OBSERVATION OF TRANSIENT NUTATIONS, QUANTUM BEATS AND ENVELOPE MODULATION EFFECTS()Q(A)()

The time evolution of the spin-polarized transient EPR signals of the radical pair state P865+.Q(A)-. is studied in perdeuterated reaction c enters of Rhodobacter sphaeroides R-26 in which the non-heme iron has been replaced by zinc. The transients show modulations of the signal a mplitude on several time scales. Fast quantum beat oscillations with f requencies around 15 MHz occur within the first 100 ns after the light excitation because the radical pair is generated in a coherent superp osition of its eigenstates. Transient nutations also occur as a result of the precession of the magnetization about the applied microwave fi eld B1. At the maximum B1 field available (0.125 mT) a nutation freque ncy of almost-equal-to 3.5 MHz is obtained. Small variations in the nu tation frequency throughout the spectrum occur as a result of the dipo lar coupling in the radical pair. Additional oscillations in the frequ ency 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 un paired electron spins. Analogies and differences to well-known nuclear modulation phenomena observed in pulsed EPR experiments are discussed .