Energetics and kinetics of photosynthetic water oxidation studied by photothermal beam deflection (PBD) experiments

摘要

Determination of thermodynamic parameters of water oxidation at the photosystem II (PSII) manganese complex is a major challenge. Photothermal beam deflection (PBD) spectroscopy determines enthalpy changes (H) and apparent volume changes which are coupled with electron transfer in the S-state cycle (Krivanek R, Dau H, Haumann M (2008) Biophys J 94: 1890-1903). Recent PBD results on formation of the Q A /Y Z O radical pair suggest a value of H similar to the free energy change, G, of 540 pl 40 meV previously determined by the analysis of recombination fluorescence, but presently the uncertainty range of H values determined by PBD is still high (pl250 meV). In the oxygen-evolving transition, S S, the enthalpy change may be close to zero. A prominent non-thermal signal is associated with both Q A /Y Z O formation (<1 os) and the S S transition (~1 ms). The observed (apparent) volume expansion (V of about +40 cpd per PSII unit) in the S S transition seems to revert, at least partially, the contractions on lower S-transitions and may also comprise contributions from O and proton release. The observed volume changes show that the S S transition is accompanied by significant nuclear movements, which likely are of importance with respect to energetics and mechanism of photosynthetic water oxidation. Detailed PBD studies on all S-transitions will contribute to the progress in PSII research by providing insights not accessible by other spectroscopic methods.