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

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 (Delta 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) (aEuro cent+) radical pair suggest a value of Delta H similar to the free energy change, Delta G, of -540 +/- A 40 meV previously determined by the analysis of recombination fluorescence, but presently the uncertainty range of Delta H values determined by PBD is still high (+/- 250 meV). In the oxygen-evolving transition, S-3 -> S-0, the enthalpy change may be close to zero. A prominent non-thermal signal is associated with both Q (A) (-) /Y (Z) (aEuro cent+) formation (< 1 mu s) and the S-3 -> S-0 transition (similar to 1 ms). The observed (apparent) volume expansion (Delta V of about +40 (3) per PSII unit) in the S-3 -> S-0 transition seems to revert, at least partially, the contractions on lower S-transitions and may also comprise contributions from O-2 and proton release. The observed volume changes show that the S-3 -> S-0 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.