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 ∼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...