Fluorescence decay and spectral evolution in intact photosystem I of higher plants

A photosystem I preparation from maize, containing its full antenna complem ent (PSI-200) and in which detergent effects on chlorophyll coupling are al most completely absent, has been studied by time-resolved fluorescence tech niques with similar to 5 ps resolution at 280 and 170 K in the wavelength i nterval of 690-780 nm. The data have been analyzed in terms of both the dec ay-associated spectra (DAS) and the time-resolved emission spectra (TRES). As in a previous room temperature study [Turconi, S., Weber, N., Schweitzer , D., Strotmann, H., and Holzwarth, A. R. (1994) Biochim. Biophys. Acta 118 7, 324-334], the 280 K decay is well described by three DAS components in t he 11-130 ps time range, the fastest of which displays both positive and ne gative amplitudes characteristic of excitation transfer from the bulk to th e red antenna forms. Both the 57 and 130 ps components have all positive am plitudes and describe complex decay and equilibration processes involving t he red forms. At 170 K, four major components in the 10-715 ps time range a re required to describe the decay. The fastest represents bulk to red form transfer processes, while the 55, 216, and 715 ps decays, with all positive amplitudes, have maxima near 720, 730, and 740 nm, respectively, in accord with previous steady-state fluorescence measurements. The width and asymme try of these DAS indicate that they are spectrally complex and represent de cay and equilibration processes involving the red forms. Spectral evolution during the fluorescence decay process was analyzed in terms of the TRES. T he red shifting of the TRES was analyzed in terms of the first central spec tral moment (mean spectral energy) which is biexponential at both temperatu res. The slower component, which describes equilibration between the red fo rms, leads to spectral red shifting during the entire fluorescence decay pr ocess, and the mean lifetimes of the spectral moments at 280 and 170 K (86 and 291 ps, respectively) are similar to the mean lifetimes of the fluoresc ence decays (119 and 384 ps, respectively). Thus, both spectral evolution a nd the trapping-associated fluorescence decay occur on a similar time scale , and both processes display a very similar temperature sensitivity. On the basis of these data, it is concluded that trapping in PSI-200 is to a larg e extent rate-limited by excitation diffusion in the antenna and in particu lar by the slow "uphill" transfer from the low-energy forms to the bulk and /or inner core chlorophyll molecules.