Magnetic field effects on radical pair intermediates in bacterial photosynthesis

We have investigated the effects of magnetic fields on the formation and decay of excited states in the photochemical reaction centers of Rhodopseudomonas sphaeroides. In chemically reduced reaction centers, a magnetic field decreases the fraction of the transient state P^F that decays by way of the bacteriochlorophyll triplet state P^R. At room temperature, a 2-kG field decreases the quantum yield of P^R by about 40%. In carotenoid-containing reaction centers, the yield of the carotenoid triplet state which forms via P^R is reduced similarly. The effect of the field depends monotonically on field-strength, saturating at about 1 kG. The effect decreases at lower temperatures, when the yield of P^R is higher. Magnetic fields do not significantly affect the formation of the triplet state of bacteriochlorophyll in vitro, the photooxidation of P-870 in reaction centers at moderate redox potential, or the decay kinetics of states P^F and P^R.The effects of magnetic fields support the view that state P^F is a radical pair which is born in a singlet state but undergoes a rapid transformation into a mixture of singlet and triplet states. A simple kinetic model can account for the effects of the field and relate them to the temperature dependence of the yield of P^R.