Excitation dynamics in the core antenna of PSI from Chlamydomonas reinhardtii CC 2696 at room temperature

Photosystem I particles from a eukaryotic organism, the green alga Chlamydo monas reinhardtii CC 2696, were studied by transient hole-burning spectrosc opy at room temperature. Global analysis of the spectra recorded after exci tation of chlorophyll a molecules in Photosystem I at selected wavelengths between 670 and 710 nm reveals excitation dynamics with subpicosecond, 2-3 ps, and 20-23 ps components. The subpicosecond and 2-3 ps components are as cribed to energy equilibration within the core antenna, whereas the 20-23 p s component is ascribed to energy trapping by the reaction center. Energy e quilibration components describe both uphill and downhill energy transfer d epending of the excitation wavelength. The initial transient absorbance ban ds after direct excitation of the red tail of the Q(y) transition band of c hlorophyll a (at 700, 705, and 710 nm) are 25 nm wide and structured, revea ling strongly coupled excited states among a group of molecules, most likel y reaction center chlorophyll molecules. Excitation at shorter wavelengths (670, 680, and 695 nm) results in only 5-7 nm wide initial absorbance bands originating from photobleaching and stimulated emission of antenna chlorop hyll molecules. The results are compared to the excitation dynamics of Phot osystem I from the cyanobacterium Synechocystis sp. PCC 6803. The most sign ificant difference is that the 2-3 ps phase describes internal excitation d ynamics within higher-energy antenna chlorophyll molecules in the algal PS I system rather than between bulk and red chlorophylls, as observed in cyan obacterial PS I. No indications of core antenna red pigments absorbing abov e 700 nm were found in the preparation from Chlamydomonas. Independent of e xcitation wavelength, after at most a few picoseconds, all excitons are dis tributed over the same pool of chlorophyll molecules centered at similar to 682 nm.