Magnetic characterization of the primary state of bacterial photosynthesis

Abstract

The results of reaction yield-detected magnetic resonance (RYDMR) experiments carried out on modified bacterial photosynthetic reaction centers (RC) are interpreted in terms of a model that assigns the initial charge-separated radical ion-pair state, PF, as the carrier of the spectrum. The radical pair theory, which was invoked to explain magnetic field effects in RC, was significantly expanded to take into consideration the electron dipole-dipole interaction. This is the largest interaction between the components of the radical ion pair. Quantum statistical calculations are described stimulating the RYDMR spectra and low-field effects in quinone-depleted RC. The experimental data on which the simulations are based are the magnitude of the field effect at 3000 G, the field at which 0.5 of the maximal field effect is observed, the PF population as a function of time at zero magnetic field, the RYDMR line-width for low microwave field strength, the RYDMR intensity and width as a function of microwave field, and the maximum RYDMR intensity at H1 .simeq. 2 J . It was possible to characterize PF in terms of 4 parameters, 2 containing structural information and 2 with kinetic implications. These are the dipole-dipole interaction, D = -47 .+-. 10 .times. 10-4 cm-1; the exchange interaction, J = -7.5 .+-. 1.9 .times. 10-4 cm-1; and the inverse rate constants of the decay of the radical pair states with singlet and triplet spin functions, respectively, Ks-1 = 15 .+-. 4 nsec and kT-1 = 1.8 .+-. 0.2 nsec. The structural and dynamic implications of these parameters are discussed.