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

Abstract

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 (ΔH) 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) radical pair suggest a value of ΔH similar to the free energy change, ΔG, of -540±40 meV previously determined by the analysis of recombination fluorescence, but presently the uncertainty range of ΔH values determined by PBD is still high (±250 meV). In the oxygen-evolving transition, S₃−−>S₀, the enthalpy change may be close to zero. A prominent non-thermal signal is associated with both Q⁻(A)/Y(•+)(Z) formation (<1 μs) and the S₃−−>S₀ transition (~1 ms). The observed (apparent) volume expansion (ΔV of about +40 ų per PSII unit) in the S₃−−>S₀ transition seems to revert, at least partially, the contractions on lower S-transitions and may also comprise contributions from O₂ and proton release. The observed volume changes show that the S₃−−>S₀ 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.

Alternating electron and proton transfer steps in photosynthetic water oxidation

“…The PBD experiments were carried out and analyzed as described previously (37,38). In the PBD experiments presented herein, 100 mL of PSII suspension were kept in a dark reservoir on ice (gently stirred) and pumped first through a thermostated laboratory-built heat exchanger and then into the sample compartment of the thermostated flow-through cuvette (3-mm optical path).…”

“…To detect proton removal from the Mn complex, we employ a photothermal beam deflection (PBD) experiment exploiting the high sensitivity of the PBD signal to density changes (34)(35)(36)(37)(38), which in the following are discussed in terms of apparent volume changes of the protein. Expansion and contraction of the PSII complex were monitored with microsecond resolution at a precision of about 2 Å 3 per PSII.…”

Seven Steps of Alternating Electron and Proton Transfer in Photosystem II Water Oxidation Traced by Time-Resolved Photothermal Beam Deflection at Improved Sensitivity

“…In earlier publications we have determined the value of an S-state independent miss parameter from the amplitude pattern in a flash series of the prominent millisecond phase accompanying the oxygen evolution of PSII. [1][2][3] This could have also been done here (see Fig. 1B, 1600 µs phase), but much of the information from the high quality 12flash data set would thereby not have been available for the analysis.…”

“…Various time-resolved investigations have supported the existence of the S 3 n intermediate as formed by proton release from the WOC. ,,− The energetic tuning of the WOC for effective redox and water oxidation chemistry by the alternating ET and PT events has been reviewed elsewhere. , Initially, however, several of the proposed intermediates of the extended reaction cycle have been hypothetical. We have recently tracked two intermediates, namely S 2 n and S 1 + , using a kinetically competent technique, which has rarely been used in PSII research and is denoted photothermal beam deflection (PBD). − …”

Oxygenic photosynthesis: history, status and perspective

“…The exact sequence of these events is subject to ongoing research. Michael Haumann and Holger Dau ventured into time-resolved calorimetry using photo-thermal beam deflection (Krivanek et al ., 2008; Klauss et al ., 2009). They obtained a wealth of signal transients that were interpreted to show the release of one proton first, oxygen release and water intake second and the release of another proton third (see Fig.…”