Abstract Submission






Invited Speakers


Prof. Barry Bruce


University of Tennessee, Knoxville


Tetrameric Photosystem I in Cyanobacteria: Structural

Functional, and Evolutionary Implications

Photosystem I (PSI) is one of two photosystems involved in oxygenic photosynthesis. PSI of cyanobacteria exists in monomeric, trimeric, and tetrameric forms, which is in contrast to the strictly monomeric form of PSI in plants and algae. The tetrameric organization raises questions about its structural, physiological, and evolutional significance. Here we report the ~3.9 Å resolution cryo-EM structure of tetrameric PSI from the thermophilic, unicellular cyanobacterium Chroococcidiopsis sp. TS-821. The structure resolves all 44 subunits and 448 cofactor molecules. We conclude that the tetramer is arranged via two different interfaces resulting from a dimer-of-dimers organization. The localization of chlorophyll molecules permits an excitation energy pathway within and between adjacent monomers. Bioinformatics analysis reveals conserved regions in PsaL subunit that correlate with the oligomeric state. Tetrameric PSI may function as a key evolutionary step between the trimeric and monomeric forms of PSI organization in photosynthetic organisms.



Prof. Greg Engel


University of Chicago


Understanding the Design Principles of Photosynthetic Light Harvesting: Controlling quantum beats in 2D electronic spectra of the FMO complex using redox chemistry

Photosynthetic organisms harvest energy from the sun and direct this energy toward the reaction center with enviable precision and efficiency.  Over the years, we have often measured and modelled how (and how fast) this process occurs, but the microscopic details of the process and how to control it or engineer similar schemes has eluded us.  One (controversial) approach to this problem has been to explore coherent energy transfer using quantum coherences. Quantum coherences, or beats, observed in ultrafast optical experiments arise when light-matter interactions from a coherent source drive a system out of thermal equilibrium. The dynamics of these coherences can, in theory, provide insights into the Hamiltonian underlying the energy transfer processes of photosynthetic pigment-protein complexes. However, a great deal of controversy has arisen based on the assignments of these signals.  Are they really  reporting on the excited state dynamics?  Are they merely ground state vibrations?  Are they vibrations in the excited state (and are they relevant)?  Is the truth somewhere in between where vibrational and electronic states mix? Recent work has shown that redox conditions affect the ultrafast energy transfer dynamics in the Fenna-Matthews-Olson pigment-protein complex in green sulfur bacteria. Here we present ultrafast two-dimensional electronic spectroscopy measurements on the Fenna-Matthews-Olson antenna complex under both oxidizing and reducing conditions. We observe many excited state coherences present in reducing conditions, which more closely mimic the natural conditions of the complex, are absent or attenuated in oxidizing conditions. Further, the presence of these coherences can be assigned to the excited state and is correlated with increased vibronic coupling in the system and faster, more efficient energy transfer through the complex.