The transient EPR spectra and spin dynamics of coupled three-spin systems in photosynthetic reaction centres

The concept of introducing an additional, stable paramagnetic species into photosynthetic reaction centres to increase the information content of thei r spin polarized transient EPR spectra is investigated theoretically. The l ight-induced electron transfer in such systems generates a series of couple d three-spin states consisting of sequential photoinduced radical pairs cou pled to the stable spin which acts as an "observer". The spin polarized tra nsient EPR spectra are investigated using the coupled three-spin system P()I(-)Q(A)(-) in pre-reduced bacterial reaction centres as a specific exampl e which has been studied experimentally. The evolution of the spin system a nd the spin polarized EPR spectra of P(+)I(-)Q(A)(-) and Q(A)(-) following recombination of the radical pair (P = primary donor, I = primary acceptor, Q(A) = quinone acceptor) are calculated numerically by solving the equatio ns of motion for the density matrix. The net polarization of the observer s pin is also calculated analytically by perturbation theory for the case of a single, short-lived, charge-separated state. The result bears a close res emblance to the chemically induced nuclear polarization (CIDNP) generated i n photolysis reactions in which a nuclear spin plays the role of the observ er interacting with the radical pair intermediates. However, because the Ze eman frequencies of the three electron spins involved are usually quite sim ilar, the polarization of the electron observer spin in strong magnetic fie lds can reflect features of the CIDNP effect in both, high and low magnetic fields. The dependence of the quinone spin polarization on the exchange co uplings in the three-spin system is investigated by numerical simulations, and it is shown that the observed emissive polarization pattern is compatib le with either sign, positive or negative, for a range of exchange coupling s, J(PI), in the primary pair. The microwave frequency and orientation depe ndence of the spectra are discussed as two of several possible criteria for determining the sign of J(PI).