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

Photosystem I particles from a eukaryotic organism, the green alga Chlamydomonas reinhardtii CC 2696, were studied by transient hole-burning spectroscopy at room temperature. Global analysis of the spectra recorded after excitation 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 ascribed to energy equilibration within the core antenna, whereas the 20-23 ps component is ascribed to energy trapping by the reaction center. Energy equilibration components describe both uphill and downhill energy transfer depending of the excitation wavelength. The initial transient absorbance bands after direct excitation of the red tail of the Q_y transition band of chlorophyll a (at 700, 705, and 710 nm) are 25 nm wide and structured, revealing strongly coupled excited states among a group of molecules, most likely 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 chlorophyll molecules. The results are compared to the excitation dynamics of Photosystem I from the cyanobacterium Synechocystis sp. PCC 6803. The most significant difference is that the 2-3 ps phase describes internal excitation dynamics within higher-energy antenna chlorophyll molecules in the algal PS I system rather than between bulk and red chlorophylls, as observed in cyanobacterial PS I. No indications of core antenna red pigments absorbing above 700 nm were found in the preparation from Chlamydomonas. Independent of excitation wavelength, after at most a few picoseconds, all excitons are distributed over the same pool of chlorophyll molecules centered at ∼682 nm.