Charge Separation Kinetics in Intact Photosystem II Core Particles Is Trap-Limited. A Picosecond Fluorescence Study

Abstract

The fluorescence kinetics in intact photosystem II core particles from the cyanobacterium Thermosynechococcus elongatus have been measured with picosecond resolution at room temperature in open reaction centers. At least two new lifetime components of approximately 2 and 9 ps have been resolved in the kinetics by global analysis in addition to several known longer-lived components (from 42 ps to approximately 2 ns). Kinetic compartment modeling yields a kinetic description in full agreement with the one found recently by femtosecond transient absorption spectroscopy [Holzwarth et al. (2005) submitted to Proc. Natl. Acad. Sci. U.S.A.]. We have for the first time resolved directly the fluorescence spectrum and the kinetics of the equilibrated excited reaction center in intact photosystem II and have found two early radical pairs before the electron is transferred to the quinone Q(A). The apparent lifetime for primary charge separation is 7 ps, that is, by a factor of 8-12 faster than assumed on the basis of earlier analyses. The main component of excited-state decay is 42 ps. The effective primary charge separation rate constant is 170 ns(-)(1), and the secondary electron-transfer rate constant is 112 ns(-)(1). Both electron-transfer steps are reversible. Electron transfer from pheophytin to Q(A) occurs with an apparent overall lifetime of 350 ps. The energy equilibration between the CP43/CP47 antenna and the reaction center occurs with a main apparent lifetime of approximately 1.5 ps and a minor 10 ps lifetime component. Analysis of the overall trapping kinetics based on the theory of energy migration and trapping on lattices shows that the charge separation kinetics in photosystem II is extremely trap-limited and not diffusion-to-the-trap-limited as claimed in several recent papers. These findings support the validity of the assumptions made in deriving the earlier exciton radical pair equilibrium model [Schatz, G. H., Brock, H., and Holzwarth, A. R. (1988) Biophys. J. 54, 397-405].