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![[Figure 1]](hi2039fig1mag.jpg)
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Figure 1
Scheme of the light-induced PSII reactions. Depicted is the arrangement of the relevant redox factors and the electron-transport paths (in a), the direction of the thylakoid membrane normal (in a) and the S-state cycling of the oxygen-evolving complex (OEC; in b). Following the absorption of a photon by the PSII pigments and ultra-fast excitation-energy transfer to a special set of chlorophylls called P680, electron transfer from P680 to a special pheophytin molecule is initiated. The primary electron donor P680 is re-reduced by a tyrosine residue (Tyr-161 of the D1 protein), and the resulting tyrosine radical delivers one oxidizing equivalent to the OEC. Driven by the one-electron oxidation that results from the absorption of a single photon, the OEC advances from the Sn state to the Sn+1 state, where the subscripts give the number of oxidizing equivalents accumulated by the OEC. The most oxidized semi-stable state of the OEC is S3; a further one-electron oxidation results in the formation of S4, a hypothetical transition-state-like intermediate, which spontaneously relaxes to the S0 state concurrently with the release of dioxygen. By application of saturating laser flashes of nanosecond duration (single-turnover flash), the OEC advances synchronously in the cycle by one S state of the cycle. Ideally, after four flashes the initial S state is reached a second time. However, on each flash a minority of PSII does not advance in the catalytic cycle (∼10% `misses'). Therefore, for applying a sequence of flashes, all signals related to the S-state cycle will exhibit a damped oscillation with a period of four flashes. |
![[Figure 1]](hi2039fig1.jpg)
 | JOURNAL OF SYNCHROTRON RADIATION |
ISSN: 1600-5775
