title: 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.
authors.affiliation: Freie Universität Berlin, FB Physik, Arnimallee 14, D-14195 Berlin, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Grabolle
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
mesh_terms: D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008027:Light; D010084:Oxidation-Reduction / Q000528:radiation effects; D045332:Photosystem II Protein Complex / Q000378:metabolism / Q000502:physiology*; D011522:Protons; D013696:Temperature; D014867:Water / Q000737:chemistry / Q000378:metabolism*
chemical_list: D045332:Photosystem II Protein Complex; D011522:Protons; D014867:Water