Magnetic Field Effect on Triplets and Radical Ions in Reaction Centers of Photosynthetic Bacteria

The primary step currently assumed in bacterial photosynthesis involves fast electron transfer between the electronically excited bacteriochlorophyll ‘special pair’ [l], ’ (BChl):, and a bacteriopheophytin, BPh, with a rate kR > (10 ps)-’ [2-41. In blocked reaction centers, the radical ions thus formed live for G 10 ns at 300 K [5] and decay by the back transfer of the electron. If this occurs in an overall singlet state of the radical ion pair, either the electronic ground states, (BChl)* and BPh, or, energetically uphill, the excited singlet state [6], ’ (BChl):, are populated with the rate constants k, and ki, respectively. As revealed by its unusual spin polarization [7,8] in the ESR and its magnetic field-dependent yield [9,11], the triplet, 3(BChl)z, is formed with the rate kT via another recombination channel opening when hyperline interaction induces singlet-to-triplet transitions in the pair state (see kinetic scheme in fig.1). Such singlet-to-triplet transitions are partially supressed in an external magnetic field of some 100 Oe. From the magnetic field effect on the triplet yield the splitting of the triplet and singlet states of the radical pair, i.e., the spin exchange integral J has been estimated to be very small, <l 0m3 cm-‘. This value for J is at least one order of magnitude too small [ 121 to allow for the fast experimental electron transfer rate kR [2-41 of the forward reaction. Such a discrepancy casts some doubt [ 121 on the view that the hyperfine interaction indeed develops at the site of the initially formed radicals and demonstrates that information on the forward electron transfer can be derived from the magnetic field effect on the back reaction. For the determination of the recombination rates and the electronic exchange