title: Observation of multiple radical pair states in photosystem 2 reaction centers.
abstract: Charge recombination of the primary radical pair in D1/D2 reaction centers from photosystem 2 has been studied by time-resolved fluorescence and absorption spectroscopy. The kinetics of the primary radical pair are multiexponential and exhibit at least two lifetimes of 20 and 52 ns. In addition, a third lifetime of approximately 500 ps also appears to be present. These multiexponential charge-recombination kinetics reflect either different conformational states of D1/D2 reaction centers, with the different conformers exhibiting different radical pair lifetimes, or relaxations in the free energy of the radical pair state. Whichever model is invoked, the free energies of formation of the different radical pair states exhibit a linear temperature dependence from 100 to 220 K, indicating that they are dominated by entropy with negligible enthalpy contributions. These results are in agreement with previous determinations of the thermodynamics that govern primary charge separation in both D1/D2 reaction centers [Booth, P.J., Crystall, B., Giorgi, L. B., Barber, J., Klug, D.R., & Porter, G. (1990) Biochim. Biophys. Acta 1016, 141-152] and reaction centers of purple bacteria [Woodbury, N.W.T., & Parson, W.W. (1984) Biochim. Biophys. Acta 767, 345-361]. It is possible that these observations reflect structural changes that accompanying primary charge separation and assist in stabilization of the radical pair state thus optimizing the efficiency of primary electron transfer.
authors.affiliation: Photochemistry Research Group, Department of Biology, Imperial College, London, U.K.
authors.forename: P J
authors.initials: PJ
authors.lastname: Booth
authors.forename: B
authors.initials: B
authors.lastname: Crystall
authors.forename: I
authors.initials: I
authors.lastname: Ahmad
authors.forename: J
authors.initials: J
authors.lastname: Barber
authors.forename: G
authors.initials: G
authors.lastname: Porter
authors.forename: D R
authors.initials: DR
authors.lastname: Klug
mesh_terms: D003573:Cytochrome b Group / Q000737:chemistry*; D004579:Electron Transport; D007887:Fabaceae; D005453:Fluorescence; D005609:Free Radicals; D007700:Kinetics; D008433:Mathematics; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D045332:Photosystem II Protein Complex; D010946:Plants, Medicinal; D011487:Protein Conformation; D013053:Spectrophotometry; D013696:Temperature; D013816:Thermodynamics
chemical_list: D003573:Cytochrome b Group; D005609:Free Radicals; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; C019313:cytochrome b559