Invited Speakers
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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. |
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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. |