title: Protein dynamics tunes excited state positions in light-harvesting complex II.
abstract: Light harvesting and excitation energy transfer in photosynthesis are relatively well understood at cryogenic temperatures up to ∼100 K, where crystal structures of several photosynthetic complexes including the major antenna complex of green plants (LHC II) are available at nearly atomic resolution. The situation is much more complex at higher or even physiological temperatures, because the spectroscopic properties of antenna complexes typically undergo drastic changes above ∼100 K. We have addressed this problem using a combination of quasielastic neutron scattering (QENS) and optical spectroscopy on native LHC II and mutant samples lacking the Chl 2/Chl a 612 pigment molecule. Absorption difference spectra of the Chl 2/Chl a 612 mutant of LHC II reveal pronounced changes of spectral position and their widths above temperatures as low as ∼80 K. The complementary QENS data indicate an onset of conformational protein motions at about the same temperature. This finding suggests that excited state positions in LHC II are affected by protein dynamics on the picosecond time scale. In more detail, this means that at cryogenic temperatures the antenna complex is trapped in certain protein conformations. At higher temperature, however, a variety of conformational substates with different spectral position may be thermally accessible. At the same time, an analysis of the widths of the absorption difference spectra of Chl 2/Chl a 612 reveals three different reorganization energies or Huang-Rhys factors in different temperature ranges, respectively. These findings imply that (dynamic) pigment-protein interactions fine-tune electronic energy levels and electron-phonon coupling of LHC II for efficient excitation energy transfer at physiological temperatures.
authors: Institute of Physics, University of Tartu , Ravila 14C, 50411 Tartu, Estonia.
authors: Kamarniso
authors: K
authors: Vrandecic
authors: Margus
authors: M
authors: Rätsep
authors: Laura
authors: L
authors: Wilk
authors: Leonid
authors: L
authors: Rusevich
authors: Maksym
authors: M
authors: Golub
authors: Mike
authors: M
authors: Reppert
authors: Klaus-Dieter
authors: KD
authors: Irrgang
authors: Werner
authors: W
authors: Kühlbrandt
authors: Jörg
authors: J
authors: Pieper
mesh_terms: D002734:Chlorophyll / Q000737:chemistry; D004735:Energy Transfer; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D016296:Mutagenesis; D017434:Protein Structure, Tertiary; D013050:Spectrometry, Fluorescence; D013696:Temperature; D013816:Thermodynamics
chemical_list: D045342:Light-Harvesting Protein Complexes; D002734:Chlorophyll