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Taco shell protein
It's called bacteriochlorophyl (BChl) a protein,
but Blankenship fondly calls it the taco shell protein because of its
structure : its ribbon-like backbone wraps around three clusters of
seven chlorophylls, just like a taco shell around ground beef. The
structure also is referred to as trimeric because of the three
clusters.
The protein, which comes from a photosynthetic
bacterium that lives in extremely high temperatures, enabled the
researches to discover that quantum mechanical effects appear to play
a role in photosynthesis.
The taco shell protein is arguably the most studied
and understood protein in a complex photosynthesis researchers refer
to as the antenna system, molecules that efficiently transfer energy
from light in a cascade.
Photosynthesis transforms light, carbon dioxide and
water into chemical energy in plants and some bacteria. The wavelike
characteristic of this energy transfer process can explain its extreme
efficiency, in that vast areas of phase space can be sampled
effectively to find the most efficient path for energy transfer.
"We have a very detailed molecular structure of
this protein and we understand the electronic properties of it very
well, too," said Blankenship. "It's taught us a lot about how
chlorophylls interact with proteins. It was ideal for this study."
Blankenship's colleague, Graham R. Fleming, Ph.D.,
Deputy Director of the U.S. Department of Energy's Lawrence Berkeley
National Laboratory and professor of chemistry at the University of
California, and colleagues used 2-D spectroscopy to study what happens
inside a bacteriochlorophyll complex, and detected a 'quantum beating."
The effect, described in the April 12, 2007 issue
of Nature, occurs when light-induced excitations in the complex meet
and interfere constructively much like the interactions that occur
between the ripples formed by throwing stones into a pond.
The collaboration is a good illustration of
interdisciplinary science. The Washington University group's expertise
is in photosynthesis, especially antenna systems, and the West Coast
group's specialty is advanced laser techniques. The quantum finding
would have been impossible without collaboration.
Good vibrations
"We have obtained the first direct evidence that
remarkably long-lived wavelike electronic quantum coherence plays an
important part in energy transfer processes during photosynthesis,"
said Fleming, the principal investigator for the study. "This wavelike
characteristic can explain the extreme efficiency of the energy
transfer because it enables the system to simultaneously sample all
the potential energy pathways and choose the most efficient one."
Fleming is also a professor of chemistry at UC
Berkeley, and an internationally acclaimed leader in spectroscopic
studies of the photosynthetic process. In a paper entitled, "Evidence
for wavelike energy transfer through quantum coherence in
photosynthetic systems," he and his collaborators report the detection
of "quantum beating" signals, coherent electronic oscillations in both
donor and acceptor molecules, generated by light-induced energy
excitations, like the ripples formed when stones are tossed into a
pond.
Electronic spectroscopy measurements made on a
femtosecond (millionths of a billionth of a second) time-scale showed
these oscillations meeting and interfering constructively, forming
wavelike motions of energy (superposition states) that can explore all
potential energy pathways simultaneously and reversibly, meaning they
can retreat from wrong pathways with no penalty. This finding
contradicts the classical description of the photosynthetic energy
transfer process as one in which excitation energy hops from
light-capturing pigment molecules to reaction center molecules
step-by-step down the molecular energy ladder.
"The classical hopping description of the energy
transfer process is both inadequate and inaccurate," said Fleming. "It
gives the wrong picture of how the process actually works, and misses
a crucial aspect of the reason for the wonderful efficiency."
The taco shell is a sort of "middleman" in the
antenna system, sandwiched in between a larger antenna and a molecule
called the Reaction Center, where all the chemistry in energy transfer
takes place, said Blankenship.
"Most of the absorption of light is carried out by
a complex called the chlorosome that then transfers the energy to the
trimeric protein that in turn transfers to the Reaction Center,"
Blankenship said. |
