Femtosecond Transient Spectroscopy and Excitonic Interactions in Photosystem I

Ultrafast dynamics of excitation transfer in the Photosystem I (PSI) core antenna from the cyanobacterium Synechocystis sp. PCC 6803 were detected at 77 K by using femtosecond transient absorption spectroscopy with selective excitation at 700, 695, and 710 nm. At low temperature, the efficiency of uphill energy transfer in the core antenna significantly decreases. As a result, the spectral profile of the PSI equilibrated antenna shifts to lower energies because of a change of chlorophyll (Chl) excited-state distribution. Observed on a 2-ns time scale, P₇₀₀ photooxidation spectra are largely excitation wavelength independent. In the early time spectra, excitation of P₇₀₀ induces transient photobleaching at 698 nm accompanied by a resonant photobleaching band at 683 nm decaying within 250-300 fs. Chemical oxidation of P₇₀₀ does not affect the transient band at 683 nm. This band is also present in 200-fs spectra induced by selective excitation of ChIs at 710 nm (red pigments C708), which suggests that this high-energy transition may reflect an excitonic interaction between pigments of the reaction center and closely located red pigments. Possible candidates for the interacting molecules in the 4-Å crystal structure of cyanobacterial PSI are discussed.