| Popis: |
Photosystem II (PS II) is a membrane-bound pigment-protein complex that uses light energy to drive the transfer of electrons from water to plastoquinone. When the electron transfer to the quinone acceptors is blocked, the primary radical pair, P680+Pheo-, can decay by three pathways: (i) by charge recombination to the singlet ground state, (ii) by an activated back reaction to the excited singlet state of P680 and (iii) after singlet-triplet mixing in the radical pair by recombination to the lowest excited triplet state of P680, 3P680. In PS II centers, in which the first quinone acceptor, QA, is removed or doubly reduced (QAH2), the formation of 3P680 can be monitored by the characteristic spin-polarized triplet EPR spectrum. The lifetime of 3P680 is similar to that of the chlorophyll a triplet state in solvents (τ ≈ 1.4 ms (70%)/7 ms (30%) at 5 K and τ ≈ 1.4 ms at T > 20 K under anaerobic conditions). In the presence of singly reduced QA, however, the P680 triplet state with ms lifetime could not be detected, neither by transient absorption spectroscopy nor by cw-EPR [1]. In more recent work, transient absorption spectroscopy at low temperature (T = 25 K) revealed that the primary radical pair decays to an intermediate with μs lifetime (τ ≈ 2 μs (50%)/20 μs (50%) at 25 K). From its spectral features in the QY-region, this intermediate has been identified with the triplet state of P680 [2,3]. |
