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The Design and
Synthesis of Artificial Photosynthetic Membranes
Moore, Thomas1,
Moore, Ana1 and Gust, Devens1
Arizona State University, Tempe, AZ 85287-16041
Abstract-
Artificial reaction centers having varied molecular architecture have
been designed and synthesized. By modifying the structural and thermodynamic
factors controlling intramolecular energy and electron transfer in these
structures, they have undergone directed evolution so that the best
ones demonstrate energy and electron transfer efficiencies approaching
those found in natural photosynthetic membranes. In order to assemble
an energy transducing artificial photosynthetic membrane, selected artificial
reaction centers have been incorporated vectorially into liposomal membranes.
In particular, one artificial reaction center consisting of covalently
linked carotenoid, porphyrin and quinone moieties (C-P-Q triad) has
been studied extensively. Photoinduced electron transfer from the porphyrin
to the quinone moiety generates C-P.+-Q.- which
is followed by a secondary electron transfer involving the carotenoid
to yield C.+-P-Q.-. In this way the triad-based
artificial reaction centers convert light energy to vectorial redox
potential across the liposomal membrane. This redox potential drives
a quinone-based, proton-transporting redox loop that generates proton-motive
force (pmf). Potential- and pH-sensitive dyes and the carotenoid band
shift have been used to measure a total pmf of over 200 mV. The pmf
can be expressed as a  pH
of ca. 3.1 in the presence of potassium and valinomycin, or it can be
expressed as ca. 200 mV in the presence of nigericin. In the absence
of drugs, it is the sum of pH
and a membrane potential. The pmf has been used to drive the synthesis
of ATP by CFO-F1 ATP synthase where it poises
the ADP + Pi = ATP reaction at a chemical potential similar to that
observed in natural energy transducing membranes. A variety of pmf/ATP-dependent
biochemical processes can be driven by the artificial photosynthetic
membrane including the pyruvate carboxylase reaction, various transporters
and molecular motors.
Keywords: energized
membranes, artificial photosynthesis, photosynthesis, protonmotive force
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