Temperature dependence of antennae chlorophyll fluorescence kinetics in photosystem I reaction centre protein

Abstract

CPl, the isolated reaction centre (RC) chlorophyll(chl)-protein of plant photosystem I(PSI) containing P700 and ca. 40 antenna Chl has been isolated using sodium dodecyl sulphate and gel electrophoresis. It retained the triplet e.s.r. polarisation pattern characteristic of active charge separation and recombination. Low-temperature and time-resolved fluorescence emission spectra showed that at least tow discrete antenna chl forms were present, and excitation energy transfer between them and P700 was studied by measuring chl sub-nanosecond fluorescence decay kinetics over a range of temperatures and emission wavelengths, using ca. 100 ps Ar-ion laser excitation pulses and single-photon detection, resulting in ca. 10 ps time resolution. The two forms are F720, emitting at 720 nm (low-energy sites within the antenna) and F690, emitting at 690–695 nm. The latter form was only observed at short times ( < 200 ps) and at low temperatures. Decay kinetics were fitted to the sum of three exponentials. The two longer ( > 1 ns) components were of small amplitude and have no significance for energy transfer. The lifetime of the shortest resolved component varied in a complex way with temperature between 30 and 150 ps, also dependent on emission wavelength. At T > 200 K the lifetime was 40 ± 10 ps, independent of wavelength, dependence with a distinct minimum at 690–695 nm. A model is presented for energy transfer between the discrete chl antenna forms which accounts for the change of the observed lifetimes with temperature. In this model F690 forms a core antenna close to the RC and can transfer energy to P700 even at 10 K. Endothermic energy transfer out of F720, which is inhibited by low temperatures, gives rise to the observed temperature dependence of the F690 and F720 fluorescence lifetimes.