Photosynthetic reaction centers convert sunlight into a transmembrane electrochemical potential difference, providing chemical energy to almost all life on earth. Light energy is efficiently transferred through chromophore cofactors to the sites, where charge separation occurs. We applied two-dimensional electronic spectroscopy to assess the role of coherences in the photoresponse of the bacterial reaction center of Rhodobacter sphaeroides. By controlling the polarization of the laser beams, we were able to assign unambiguously the oscillatory dynamics to electronic (intermolecular) coherences. The data show that these coherences are sustained for more than 1 ps, indicating that the protein coherently retains some excitation energy on this time scale. Our finding provides a mechanism for effective delocalization of the excitations on the picosecond time scale by electronic coherence, setting the stage for efficient charge separation.

Observation of coherence shifts reveals hot energy transfer and excess energy dissipation mechanisms in photosynthesis.

Light control of cell development is revealed by phytochrome structures of Myxobacteria.

Phytochromes are red light-sensing photoreceptors. Here we review the current understanding of phytochrome photoactivation, with a special focus on bacteriophytochromes and their crystallographic studies.

Phytochrome proteins translate light into biochemical signals in plants, fungi and microorganisms. Light cues are absorbed by a bilin chromophore, leading to an isomerization and a rotation of the D-ring. This relays the signal to the protein matrix. A set of amino acids, which is conserved across the phytochrome superfamily, holds the chromophore in the binding pocket. However, the functional role of many of these amino acids is not yet understood. Here, we investigate the hydrogen bonding network which surrounds the D-ring of the chromophore in the resting (Pr) state. We use UV/vis spectroscopy, infrared absorption spectroscopy and X-ray crystallography to compare the photosensory domains from Deinococcus radiodurans, the phytochrome 1 from Stigmatella aurantiaca, and a D. radiodurans H290T mutant. In the latter two, an otherwise conserved histidine next to the D-ring is replaced by a threonine. Our infrared absorption data indicate that the carbonyl of the D-ring is more strongly coordinated by hydrogen bonds when the histidine is missing. This is in apparent contrast with the crystal structure of the PAS-GAF domain of phytochrome 1 from S. aurantiaca (pdb code 4RPW), which did not resolve any obvious binding partners for the D-ring carbonyl. We present a new crystal structure of the H290T mutant of the PAS-GAF from D. radiodurans phytochrome. The 1.4 Å-resolution structure reveals additional water molecules, which fill the void created by the mutation. Two of the waters are significantly disordered, suggesting that flexibility might be important for the photoconversion. Finally, we report a spectral analysis which quantitatively explains why the histidine-less phytochromes do not reach equal Pfr-type absorption in the photoequilibrium compared to the Deinococcus radiodurans wild-type protein. The study highlights the importance of water molecules and the hydrogen bonding network around the chromophore for controlling the isomerization reaction and spectral properties of phytochromes.