title: Fluorescence and absorption spectroscopy of the weakly fluorescent chlorophyll a in cytochrome b6f of Synechocystis PCC6803.
abstract: A spectroscopic characterization of the chlorophyll a (Chl) molecule in the monomeric cytochrome b6f complex (Cytb6f) isolated from the cyanobacterium Synechocystis PCC6803 is presented. The fluorescence lifetime and quantum yield have been determined, and it is shown that Chl in Cytb6f has an excited-state lifetime that is 20 times smaller than that of Chl in methanol. This shortening of the Chl excited state lifetime is not caused by an increased rate of intersystem crossing. Most probably it is due to quenching by a nearby amino acid. It is suggested that this quenching is a mechanism for preventing the formation of Chl triplets, which can lead to the formation of harmful singlet oxygen. Using site-selected fluorescence spectroscopy, detailed information on vibrational frequencies in both the ground and Qy excited states has been obtained. The vibrational frequencies indicate that the Chl molecule has one axial ligand bound to its central magnesium and accepts a hydrogen bond to its 13(1)-keto carbonyl. The results show that the Chl binds to a well-defined pocket of the protein and experiences several close contacts with nearby amino acids. From the site-selected fluorescence spectra, it is further concluded that the electron-phonon coupling is moderately strong. Simulations of both the site-selected fluorescence spectra and the temperature dependence of absorption and fluorescence spectra are presented. These simulations indicate that the Huang-Rhys factor characterizing the electron-phonon coupling strength is between 0.6 and 0.9. The width of the Gaussian inhomogeneous distribution function is 210 +/- 10 cm-1.
authors.affiliation: Department of Physics and Astronomy and Institute for Molecular Biological Sciences, Vrije Universiteit, 1081 HV Amsterdam, the Netherlands.
authors.forename: E J
authors.initials: EJ
authors.lastname: Peterman
authors.forename: S O
authors.initials: SO
authors.lastname: Wenk
authors.forename: T
authors.initials: T
authors.lastname: Pullerits
authors.forename: L O
authors.initials: LO
authors.lastname: Pâlsson
authors.forename: R
authors.initials: R
authors.lastname: van Grondelle
authors.forename: J P
authors.initials: JP
authors.lastname: Dekker
authors.forename: M
authors.initials: M
authors.lastname: Rögner
authors.forename: H
authors.initials: H
authors.lastname: van Amerongen
mesh_terms: D001665:Binding Sites; D055592:Biophysical Phenomena; D001703:Biophysics; D002734:Chlorophyll / Q000737:chemistry*; D000077194:Chlorophyll A; D000458:Cyanobacteria / Q000737:chemistry; D003573:Cytochrome b Group / Q000737:chemistry*; D045346:Cytochrome b6f Complex; D005454:Fluorescence Polarization; D008956:Models, Chemical; D015394:Molecular Structure; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry; D011789:Quantum Theory; D013050:Spectrometry, Fluorescence; D013053:Spectrophotometry
chemical_list: D003573:Cytochrome b Group; D045322:Photosynthetic Reaction Center Complex Proteins; D002734:Chlorophyll; D045346:Cytochrome b6f Complex; D000077194:Chlorophyll A