ELECTRON-SPIN POLARIZATION MODEL APPLIED TO SEQUENTIAL ELECTRON-TRANSFER IN IRON-CONTAINING PHOTOSYNTHETIC BACTERIAL REACTION CENTERS WITH DIFFERENT QUINONES AS Q(A)

Electron spin polarization develops on P+[QFe(2+)](-) in iron-containi ng photosynthetic bacterial reaction centers (RC) of Rhodobacter sphae roides. The spin-polarized electron paramagnetic resonance (EPR) spect ra of the oxidized primary donor (P+) depend on tau(H)-, the lifetime of the radical pair P+H- formed prior to P+[QFe(2+)](-). The polarized EPR signal can be described by the sequential electron transfer polar ization (SETP) model in which the chemically induced dynamic electron polarization (CIDEP) developed in P+H- is projected onto the correlate d radical pair polarization (CRPP) developed in P+[QFe(2+)](-). Replac ing the native ubiquinone-10 with various anthraquinones and naphthoqu inones alters both the free energy and rate of electron transfer from H- to QFe(2+), which in turn modifies tau(H)-. At long tau(H)- the pol arized P+ EPR signal is dominated by the CIDEP component of SETP. At s hort tau(H)- the signal is dominated by the CRPP component, while at i ntermediate tau(H)-'S the signal can only be described using the full SETP model. The ranges of tau(H)- where polarization is dominated by i nteractions on the prior or observed radical pair are influenced by th e EPR microwave frequency and RC isotopic composition. Experimental sp ectra of spin-polarized P+ from a series of Rb, sphaeroides RCs having tau(H)-'s ranging from 0.33 to 25 ns are modeled with SETP. The model accounts for differences in the polarization line shape with deuterat ion of the RCs or increase in the EPR microwave frequency.