Exciton equilibration and Photosystem II exciton dynamics — a fluorescence study on Photosystem II membrane particles of spinach

Abstract

The Photosystem II (PS II) exciton dynamics at room temperature was investigated by steady-state and picosecond fluorescence measurements on PS II membrane particles of spinach. In the closed as well as in the open-reaction center state, emission and excitation spectra were found to be almost independent of the excitation ( λ ex) and emission wavelength ( λ em), respectively. This holds even for anti-Stokes spectra ( λ em < λ ex). The minor differences observed are suggestive of excitation energy transfer from short-wavelength forms of chlorophyll (Chl) to long-wavelength Chl. The relation between the emission spectrum and the excitation spectrum of the PS II antenna system is well described by the Stepanov relation (Sov. Phys. Dokl. 2 (1957) 81–84) originally derived for a single pigment in thermal equilibrium with its environment. The picosecond fluorescence decays measured at various emission wavelengths were described in terms of four exponential lifetime components with common time constants. For Chl b excitation at 645 nm, the amplitude of the fastest decay component is negative for λ em > 690 nm indicating excitation energy transfer with a time constant of 15 ps. A model for the PS II exciton dynamics in the open reaction center state is proposed which describes the fluorescence relaxation in terms of three phases: (I) ultra-fast exciton equilibration (time constants < 15 ps) within a peripheral pigment pool consisting of LHC pigments and within an inner pigment pool presumably consisting of PS II core pigments, (II) rapid exciton equilibration (time constant of 15 ps) between these two pigment pools and (III) excited state decay by primary charge separation according to the reversible radical pair model (time constants of 131 ps and 322 ps), but maintenance of the exciton equilibrium distribution. Because the equilibration time of 15 ps is fast in comparison to the mean exciton lifetime of 220 ps, the steady-state fluorescence properties are determined mainly by the thermal equilibrium distribution of excited states on the PS II chlorophylls (Chl a and Chl b).