The magnetic field dependence of the electron spin polarization in consecutive spin correlated radical pairs in type I photosynthetic reaction centres

The magnetic field/microwave frequency dependence of the spin polarized EPR spectra of the sequential spin correlated radical pairs P⁺A^? ₁ and P⁺F^? _x in type I photosynthetic reaction centres is investigated. Experimental data are presented for photosystem (PS) I and reaction centres of heliobacteria at × band (9.7 GHz) and K band (24 GHz). In photosystem I at ambient temperatures the lifetime of A ^? ₁ is ~290 ns and both states are observable by transient EPR. In heliobacteria, electron transfer to F_x occurs within ~600 ps and only the state P⁺F^? _x is observed. The experimental data show a net polarization of P⁺ in the state P⁺F^? _x, which displays a clear dependence on the strength of the external field. The net polarization generated in sequential radical pairs is expected to pass through a maximum as a function of the Zeeman energy when the characteristic time of singlet-triplet mixing is comparable with the lifetime of the precursor. In PS I, the precursor lifetime (290ns) is much longer than the characteristic time of singlet-triplet mixing at × band (9 GHz, 3 kG) and K band (24 GHz, 8 kG). As a result, the observable net polarization decreases with the field strength in this region. In contrast, in heliobacteria, the precursor lifetime (600 ps) is much shorter than the characteristic time of singlet-triplet mixing, and the net polarization increases in the same range of Zeeman energies. The polarization patterns in these two systems can be described using the specific limiting cases of a short lived and long lived precursor radical pair and written as a sum of several contributions. The spectra are simulated on this basis using parameters derived entirely from independent experimental data, and good agreement between the experimental polarization patterns is obtained. The calculated polarization patterns are sensitive to spin dynamics on a timescale much shorter than the spectrometer response time, and the expected influence of a 10 ns component in the electron transfer, as observed optically in some PS I, preparations is discussed. No significant influence from such a component is found in the spin polarization patterns of PS I from the cyanobacterium Synechocystis 6803.