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Artificial Photosynthesis
The image to the left shows the
detailed molecular structures of the two light-harvesting proteins, LH1 and
LH2, and the reaction center (RC) from a specific species of purple
photosynthetic bacteria. The view is looking down onto the plane of the
membrane in which these proteins reside. Green plant photosynthesis uses a
larger number of proteins, as well as greater numbers of energy and
electron transfer cofactors. The bacterial system is illustrative because
the issues and questions concerning how energy and electrons flow within
and between proteins are similar for all photosynthetic organisms. The
purpose of LH1 and LH2 is to increase the number of solar photons captured
and to funnel them into the RC. The closely spaced bacteriochlorophyll
molecules shown in green (above) transfer energy within LH1 and LH2 very
rapidly, as indicated; this transfer is followed by somewhat slower
transfer to the RC. Rapid energy transfer results in efficient utilization
of the photon energy. (image courtesy of K. Schulten, UIUC).
The image to the left shows a side-on
view of the RC in the photosynthetic membrane. Only the cofactors
responsible for photo-induced charge separation across the membrane are
shown. Excitation of the bacteriochlorophyll dimer (BChl a2) results in
rapid electron transfer to an adjacent BChl a acceptor followed by thermal
electron transfer to a bacteriopheophytin acceptor (a magnesium-free BChl a
that is a better electron acceptor than BChl a). Two more thermal electron
transfer events to quinone molecules, QA and QB, continue to move the
electron further from the hole that remains on BChl a2. The result is
separation of a single electron-hole pair across a 40-Å membrane with
nearly 100% quantum efficiency.
The high quantum efficiency of
photosynthetic charge separation within the RC results principally from two
important features of the structures of the protein and the electron
donor-acceptor cofactors. First, the energetics for each electron transfer
step are optimized to give the fastest forward rate and the slowest back
reaction rate. Second, the electron and hole are moved further away from
one another with each electron transfer step, resulting in progressively
weaker interactions between them. These factors combine to yield a
very long-lived charge separation.
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