Protein Dynamics Tunes Excited State Positions in Light-Harvesting Complex II

Light harvesting and excitation energy transfer in photosynthesiS are relatively well underStood at cryogenic temperatures Up to similar to 100 K, where crystal structures of several photosynthetic complexes including the major antenna complex Of green plants (LIIC II) are available at nearly atomic resolution. The situation is much more complex at higher or even physiological temperatures, because the spectroscopie properties of antenna complexes typically undergo drastic changes above similar to 100 K. We have addressed this problem using a combination of quasielastic neutron Scattering (UM). and optical spectroscopy on native LI-IC II and mutant samples lacking the Chl 2/CM a 612 pigment molecule. Absorption difference spectra of the CM 2/Chl a 612 mutant of LHC II reveal pronounced ohanges of spectral position and their widths above temperatures as low as similar to 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 temperaturel 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 CM 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.