Electron spin polarization in consecutive spin-correlated radical pairs: Application to short-lived and long-lived precursors in type 1 photosynthetic reaction centres

An analytical treatment of the spin dynamics in sequential photoinduced cor related coupled radical pairs is presented and applied to the spectra of th e states P(+)A(1)(-) and P+FX- in type 1 photosynthetic reaction centres. E xpressions for the spin polarized spectra are derived for the specific limi ting cases of a very short-lived and very long-lived primary radical pair w hich correspond to the situation found in heliobacteria and photosystem I ( PSI), respectively. The inhomogeneous line-broadening due to the unresolved hyperfine couplings is taken explicitly into account. It is shown that the density matrix of the secondary pair rho(2) can be written as the sum of t wo terms corresponding to (i) the part which is independent of the spin dyn amics in the precursor, (ii) the additional spin polarization which is gene rated during the, lifetime of the precursor and transferred to the secondar y pair. The latter term contains two contributions which arise from the dif ference of the Zeeman interactions of the radicals in the primary pair and from the inhomogeneous line broadening. The predicted polarization patterns are compared to those established for chemically induced dynamic electron polarization (CIDEP) when uncoupled radicals are generated from a radical p air precursor. The expressions are then used to simulate the experimental s pectra of the consecutive pairs PCA; and P+FX- in PSI using parameters deri ved entirely from independent experimental data. Excellent agreement with t he experimental results is obtained. The spectra of P+FX- in heliobacteria at X- and K-band are also simulated and it is shown that the observed polar ization patterns can be reproduced assuming direct electron transfer from A (0) to F-X with a time constant of tau = 600 ps.