Efficiency and role of loss processes in light‐driven water oxidation by PSII

Abstract

Its superior quantum efficiency renders PSII a model for biomimetic systems. However, also in biological water oxidation by PSII, the efficiency is restricted by recombination losses. By laser-flash illumination, the secondary radical pair, P680(+)Q(-) (A) (where P680 is the primary Chl donor in PSII and Q(A), primary quinone acceptor of PSII), was formed in close to 100% of the PSII. Investigation of the quantum efficiency (or yield) of the subsequent steps by time-resolved delayed (10 micros to 60 ms) and prompt (70 micros to 700 ms) Chl fluorescence measurements on PSII membrane particles suggests that (1) the effective rate for P680(+) Q(-) (A) recombination is approximately 5 ms(-1) with an activation energy of approximately 0.34 eV, circumstantially confirming dominating losses by reformation of the primary radical pair followed by ground-state recombination. (2) Because of compensatory influences on recombination and forward reactions, the efficiency is only weakly temperature dependent. (3) Recombination losses are several-fold enhanced at lower pH. (4) Calculation based on delayed-fluorescence data suggests that the losses depend on the state of the water-oxidizing manganese complex, being low in the S(0)-->S(1) and S(1)-->S(2) transition, clearly higher in S(2)-->S(3) and S(3)-->S(4)-->S(0). (5) For the used artificial electron acceptor, the efficiency is limited by acceptor-side processes/S-state decay at high/low photon-absorption rates resulting in optimal efficiency at surprisingly low rates of approximately 0.15-15 photons s(-1) (per PSII). The pH and S-state dependence can be rationalized by the basic model of alternate electron-proton removal proposed elsewhere. A physiological function of the recombination losses could be limitation of the lifetime of the reactive donor-side tyrosine radical (Y(.) (Z)) in the case of low-pH blockage of water oxidation.

Thermodynamic Limitations of Photosynthetic Water Oxidation at High Proton Concentrations

“…The DF decays reflect the competing charge recombination losses. Therefore, the time integral of the DF provides a relative measure of the probability for miss events by charge recombination (32).…”

“…In Phase I (S-state preparation at pH 6.4), the oxygen-evolving complex was advanced to the desired S-state by zero (S 1 ), one (S 2 ), two (S 3 ), or three (S 0 ) saturating flashes spaced by 700 ms, a period ensuring a maximal transition probability (i.e. a minimal value of the Kok miss parameter) (32). In Phase II (changed pH), the pH of the medium was changed by the addition and rapid mixing of Buffer II, facilitating a jump toward the chosen pH value (here in the pH range from 3 to 9).…”

Chlorophyll a fluorescence induction: a personal perspective of the thermal phase, the J–I–P rise

“…7 a), a major fraction of the active PSIIs are assumed to be closed, and in the S 3 state. Indeed, Grabolle and Dau (2007) had assumed that PSIIs with OEC in the S 2 and S 3 states are responsible for a higher DLE than PSIIs with OEC in S 0 and S 1 states. Consistent with this hypothesis is the suggestion that the reduction of Y Z ?…”

Solar water-splitting into H2 and O2: design principles of photosystem II and hydrogenases

“…Both factors (together with the subsequent reactions described below) significantly minimize competing recombination reactions, leading to an excellent quantum efficiency of PSII for water-splitting (over 90%), 41,42 but to a less favorable energy efficiency. 43,44 (iii) Reduction of plastoquinone Q B by Q A c À and protonation at the 'acceptor side' of PSII. Q A is a firmly bound PQ molecule that under normal circumstances can be reduced only to the semiquinone level.…”