Quantum efficiency distributions of photo-induced side-pathway donor oxidation at cryogenic temperature in photosystem II

We monitored illuminated-minus-dark absorption difference spectra in the range of 450-1100 nm induced by continuous illumination at 8 K of photosystem II (PSII) core complexes from Thermosynechococcus elongatus. The photo-induced oxidation of the side-path donors Cytb₅₅₉, β-carotene and chlorophyll Z, as well as the concomitant stable (t _1/2 > 1 s) reduction of the first plastoquinone electron acceptor, Q_A (monitored by the well-known 'C550' shift), were quantified as a function of the absorbed photons per PSII. The Q_A photo-induced reduction data can be described by three distinct quantum efficiency distributions: (i) a very high efficiency of ∼0.5-1, (ii) a middle efficiency with a very large range of ∼0.014-0.2, and (iii) a low efficiency of ∼0.002. Each of the observed side-path donors exhibited similar quantum efficiency distributions, which supports a branched pathway model for side-path oxidation where β-carotene is the immediate electron donor to the photo-oxidized chlorophyll (P680 ⁺). The yields of the observed side-path donors account quantitatively for the wide middle efficiency range of photo-induced Q _A reduction, but not for the PSII fractions that exhibit the highest and lowest efficiencies. The high-efficiency component may be due to Tyr _Z oxidation. A donor that does not exhibit an identified absorption in the visible-near-IR region is mainly responsible for the lowest efficiency component.