Fluorescence line-narrowing measurements at low temperature were performed on the Fenna-Matthews-Olson complex of Prosthecochloris aestuarii. Superimposed on the phonon wing, several vibronic bands could be observed. By use of these data, the temperature dependence of the lowest-energy absorption band was modeled based on the linear harmonic Franck-Condon approximation. The overall Huang-Rhys factor was estimated to be 0.45. The maximum of the phonon distribution was located at 20 cm-1. Thirty vibrational modes could be observed, and their Franck-Condon factors were estimated. The strongest modes were located at 36, 70, and ∼195 cm-1. For the full width at half-maximum of the inhomogeneous broadening, a value of 80 cm-1 was determined. We did not find any evidence for the presence of different excitonic states in the lowest-energy absorption band.

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 (Chlb)to chlorophyll a (Chla) occurred with two distinctly different lifetimes of 0.5 and 2-3 ps. The 0.5 ps component mainly reflects transfer to bulk Chla, whereas the 2-3 ps component may also account for direct transfer to the special red-shifted Chla forms. Equilibration between the bulk Chla'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 Chla forms which are found in LHCI.

In this work, the transfer of excitation energy was studied in native and cation-depletion induced, unstacked thylakoid membranes of spinach by steady-state and time-resolved fluorescence spectroscopy. Fluorescence emission spectra at 5 K show an increase in photosystem I (PSI) emission upon unstacking, which suggests an increase of its antenna size. Fluorescence excitation measurements at 77 K indicate that the increase of PSI emission upon unstacking is caused both by a direct spillover from the photosystem II (PSII) core antenna and by a functional association of light-harvesting complex II (LHCII) to PSI, which is most likely caused by the formation of LHCII-LHCI-PSI supercomplexes. Time-resolved fluorescence measurements, both at room temperature and at 77 K, reveal differences in the fluorescence decay kinetics of stacked and unstacked membranes. Energy transfer between LHCII and PSI is observed to take place within 25 ps at room temperature and within 38 ps at 77 K, consistent with the formation of LHCII-LHCI-PSI supercomplexes. At the 150-160 ps timescale, both energy transfer from LHCII to PSI as well as spillover from the core antenna of PSII to PSI is shown to occur at 77 K. At room temperature the spillover and energy transfer to PSI is less clear at the 150 ps timescale, because these processes compete with charge separation in the PSII reaction center, which also takes place at a timescale of about 150 ps.