Fluorescence kinetics of Photosystem I core particles with different low energy chlorophyll content: A unified quantitative analysis

Abstract

All Photosystem I (PSI) core particles contain a small fraction (3-10%) of chlorophylls (Chls) that absorb at wavelengths longer than the primary electron donor P700. The number and energies of these long wavelength Chls are strongly species dependent. We present room temperature time-resolved fluorescence data of five cyanobacterial PS I core complexes containing different amounts of these low energy (LE) Chls, obtained using a synchroscan streak camera. A global analysis reveals considerable differences between equilibration components (3.4-15 ps) and trapping components (23-50 ps) in different PSI complexes. The unprecedented high quality of this spectral-temporal data allowed for a quantative (target) analysis in terms of a compartmental model describing the energy transfer between, and trapping from the different pools of Chls in PSI. With this analysis we resolved the room temperature emission spectra of the various (LE) Chl pools, as well as their energies. We conclude that the large kinetic differences between the various PSI complexes can be attributed completely to the differences in LE Chl content. The energies of the pools of LE Chls were found to be considerably higher at room temperature as compared to cryogenic temperatures. The RT emission spectra of the LE Chls are consistent with our view that these pools are considerably homogeneously and inhomogeneously broadened. From modelling based upon the PSI structure from Synechococcus elongatus we conclude that the intrinsic trapping rate from the reaction centre occurs in about 500 fs, and that the average single-site lifetime in the antenna must be about 100 fs.