Excitation Energy Transfer in Dimeric Light Harvesting Complex I:  A Combined Streak-Camera/Fluorescence Upconversion Study

The excitation dynamics in isolated dimers of light harvesting complex I, the peripheral light harvesting complex associated with photosystem I in green plants, was studied by time-resolved fluorescence spectroscopy. A unique combination of two techniques, fluorescence upconversion and synchroscan streak-camera measurements, revealed the energy transfer and decay of excitations over a time range from a hundred femtoseconds up to several nanoseconds, over a spectral range from 570 to 780 nm. Energy transfer from initially excited carotenoid S 2 states to the chlorophylls was found to occur within 0.15 ps. Energy transfer from chlorophyll b (Chl b )to chlorophyll a (Chl a ) occurred with two distinctly different lifetimes of 0.5 and 2−3 ps. The 0.5 ps component mainly reflects transfer to bulk Chl a, whereas the 2−3 ps component may also account for direct transfer to the special red-shifted Chl a forms. Equilibration between the bulk Chl a 's and these red-shifted forms occurs with lifetimes of 4−8 and ∼20 ps, which are assigned to intra- and intermonomer equilibration, respectively. After completion of the energy transfer processes, the fluorescence decays biexponentially. The largest fraction of excitations (75−80%) decays with a 3 ns time constant, which is attributed to both the Lhca1/Lhca4 heterodimer and a homodimer of either Lhca2 or Lhca3, whereas the remaining fraction, which decays in 0.6 ns, is assigned to the remaining homodimer. A comparison is made between the kinetics of LHCI and the more well studied CP29 and LHCII light harvesting complexes of photosystem II, which belong to the same family of Lhca/b light harvesting proteins, but do not feature the unique red-shifted Chl a forms which are found in LHCI.