Femtosecond Visible Transient Absorption Spectroscopy of Chlorophyll f-Containing Photosystem I
Abstract Photosystem I (PSI) from Chroococcidiopsis thermalis PCC 7203 grown under far-red light (FRL; >725?nm) contains both chlorophyll a and a small proportion of chlorophyll f . Here, we investigated excitation energy transfer and charge separation using this FRL-grown form of PSI (FRL-PSI). We compared femtosecond transient visible absorption changes of normal, white-light (WL)-grown PSI (WL-PSI) with those of FRL-PSI using excitation at 670?nm, 700?nm, and (in the case of FRL-PSI) 740?nm. The possibility that chlorophyll f participates in energy transfer or charge separation is discussed on the basis of spectral assignments. With selective pumping of chlorophyll f at 740?nm, we observe a final ∼150?ps decay assigned to trapping by charge separation, and the amplitude of the resulting P700 +• A 1 −• charge-separated state indicates that the yield is directly comparable to that of WL-PSI. The kinetics shows a rapid 2?ps time constant for almost complete transfer to chlorophyll? f if chlorophyll a is pumped with a wavelength of 670?nm or 700?nm. Although the physical role of chlorophyll f is best supported as a low-energy radiative trap, the physical location should be close to or potentially within the charge-separating pigments to allow efficient transfer for charge separation on the 150?ps timescale. Target models can be developed that include a branching in the formation of the charge separation for either WL-PSI or FRL-PSI.
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