title: FTIR study of the photoinduced processes of plant phytochrome phyA using isotope-labeled bilins and density functional theory calculations.
abstract: Fourier transform infrared spectroscopy was used to analyze the chromophore structure in the parent states Pr and Pfr of plant phytochrome phyA and the respective photoproducts lumi-R and lumi-F. The spectra were obtained from phyA adducts assembled with either uniformly or selectively isotope-labeled phytochromobilin and phycocyanobilin. The interpretation of the experimental spectra is based on the spectra of chromophore models calculated by density functional theory. Global (13)C-labeling of the tetrapyrrole allows for the discrimination between chromophore and protein bands in the Fourier transform infrared difference spectra. All infrared difference spectra display a prominent difference band attributable to a stretching mode with large contributions from the methine bridge between the inner pyrrole rings (B-C stretching). Due to mode coupling, frequencies and isotopic shifts of this mode suggest that the Pr chromophore may adopt a distorted ZZZssa or ZZZasa geometry with a twisted A-B methine bridge. The transition to lumi-R is associated with only minor changes of the amide I bands indicating limited protein structural changes during the isomerization site of the C-D methine bridge. Major protein structural changes occur upon the transition to Pfr in which the chromophore adopts a ZZEssa or ZZEasa-like state. In addition, specific interactions with the protein alter the structure of the B-C methine bridge as concluded from the substantial downshift of the respective stretching mode. These interactions are removed during the photoreaction to lumi-F (ZZE-->ZZZ), which involves only small protein structural changes.
authors.affiliation: Sektion Biophysik, Institut für Molekulare Medizin und Zellforschung, Albert-Ludwigs-Universität, Freiburg, Germany.
authors.forename: Pascale
authors.initials: P
authors.lastname: Schwinté
authors.forename: Harald
authors.initials: H
authors.lastname: Foerstendorf
authors.forename: Zakir
authors.initials: Z
authors.lastname: Hussain
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Gärtner
authors.forename: Maria-Andrea
authors.initials: MA
authors.lastname: Mroginski
authors.forename: Peter
authors.initials: P
authors.lastname: Hildebrandt
authors.forename: Friedrich
authors.initials: F
authors.lastname: Siebert
mesh_terms: D001654:Bile Pigments / Q000737:chemistry*; D003198:Computer Simulation; D007553:Isotope Labeling; D008027:Light; D008956:Models, Chemical; D008958:Models, Molecular; D051942:Phytochrome A / Q000737:chemistry* / Q000528:radiation effects*; D010940:Plant Proteins / Q000737:chemistry* / Q000528:radiation effects*; D011829:Radiation Dosage; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods*
chemical_list: D001654:Bile Pigments; D051942:Phytochrome A; D010940:Plant Proteins