Energy transfer dynamics of an isolated light harvesting complex of Photosystem I from spinach: time-resolved fluorescence measurements at 295 K and 77 K

Abstract

Time-resolved fluorescence relaxation measurements have been performed at 295 K and 77 K on a preparation of the light-harvesting complex of Photosystem I (LHCI) containing about 100 chlorophylls (Chl) per complex. At both temperatures five exponential components were required to fit the data. The resulting determined lifetimes are 30 ps, 200 ps, 1.0 ns, 3.3 ns and 6.5 ns. Previous steady-state fluorescence measurements demonstrated that the emission at 735 nm increases dramatically upon cooling to 77 K. Time-resolved fluorescence measurements performed at 77 K reveal the presence of two risetimes for emission between 700 and 750 nm. These results are consistent with an efficient trap for electronic excitation existing at longer wavelengths than the absorption maximum of P700, the reaction center of Photosystem I (PS I). Excitation of Chl b results in a relative amplitude enhancement of decay components emitting between 720 and 750 nm. This confirms that Chl b is specifically associated with long wavelength emitters in Photosystem I.