title: Femtosecond transient spectroscopy and excitonic interactions in Photosystem I.
abstract: 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 Chls 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-angstroms crystal structure of cyanobacterial PSI are discussed.
authors.affiliation: Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe 85287-1604, USA.
authors.forename: A N
authors.initials: AN
authors.lastname: Melkozernov
authors.forename: S
authors.initials: S
authors.lastname: Lin
authors.forename: R E
authors.initials: RE
authors.lastname: Blankenship
mesh_terms: D002734:Chlorophyll / Q000737:chemistry* / Q000528:radiation effects; D000077194:Chlorophyll A; D000458:Cyanobacteria / Q000737:chemistry* / Q000528:radiation effects; D019281:Dimerization; D004583:Electrons; D004735:Energy Transfer; D007700:Kinetics; D008027:Light; D010777:Photochemistry; D010788:Photosynthesis / Q000528:radiation effects; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000528:radiation effects; D010860:Pigments, Biological / Q000737:chemistry / Q000528:radiation effects; D013054:Spectrophotometry, Atomic
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D010860:Pigments, Biological; D002734:Chlorophyll; D000077194:Chlorophyll A