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"Electrons have dual characteristics, sometimes
acting like billiard balls and sometimes like waves on a pond,"
Beratan said in an interview. "As a consequence, electrons do very
peculiar things. One thing they can do is tunnel through barriers
forbidden to them under the 'classical' rules of physics.
"Biology has to move electrons through proteins in
order to trap energy from the sun, capture energy from our food, and
control damage to living systems," he added. "So biology has had to
come to terms with this duality. Although electrons have the ability
to tunnel, it's very costly for them. But one thing that proteins seem
to do is to guide such electrons from place to place."
Scientists have already deduced that electron
movements are enhanced when proteins fold into complex
three-dimensional shapes in their active forms. "It is much easier for
electrons to tunnel quantum mechanically through a folded protein than
it is for them to penetrate empty space," he said.
Beratan said he and other Duke chemists have spent
years studying proteins' roles in electron transport. But only
recently has his group addressed how water between protein molecules
affects electron movement.
For instance, whenever two proteins that transfer
electrons interact strongly -- or "dock" -- they must exchange
electrons in a watery medium. What scientists didn't understand was
the role of water at this interface, he said.
According to Beratan, electrons cannot simply hop
over the very small half billionths of a meter gaps that separate such
docking proteins. Quantum mechanics requires that those electrons
instead follow pathways or conduits that are heavily influenced by the
positions of nearby atoms and gaps between atoms.
"What our study was about was probing how that
tunneling process changes if we begin pulling two proteins apart and
the gap between them fills with water," he said.
"What we show is that at the shortest separations
electrons take advantage of the proteins in tunneling between those
two molecules. But there is an intermediate distance where the
proteins are beyond contact and the water molecules start moving into
this interface.
"In this intermediate distance before the proteins
are too far apart, the water plays a very special role in mediating
the electron tunneling more strongly than might have been expected."
An illustration in their Science paper, derived
from massive computer studies by the authors, shows how a mere handful
of those water molecules can form an organized cluster under the
influence of the protein molecules on either side of the gap. This
cluster aids the electron transfer process, he said.
Electrons can then tunnel between "donor" atoms at
the tip of one protein to "acceptor" atoms on the other protein. Along
the way, the electrons follow multiple pathways through these water
molecules that facilitate the transport more strongly than expected.
"Before our study, expectations for electron
tunneling were that interactions between the electron donor and
acceptor through water would drop exponentially as a function of the
distance," Beratan said.
"What we found was that water is a better mediator
for electron transfer at intermediate distances than anybody had
expected. Another finding was that the water-mediated tunneling drops
only very slightly as a function of distance within this intermediate
length."
The Duke team's computations show tunneling
initially dropping off very rapidly when the proteins first start
separating -- just like scientists originally expected. But at
intermediate distances of a few tenths of a billionths of a meter "the
rates of tunneling don't change very much," he said. "Then, when the
proteins are separated somewhat further, the rates again drop
exponentially again as a function of their separation distance," he
added.
Experiments in the Netherlands as well as at the
University of California, Berkeley also suggest a special role for
water in promoting electron transfers between proteins, he said.
"You could think about the structure of the
proteins as well as the water as guiding or shepherding the electrons,"
Beratan said. "So evolution has had to come to terms with physics in
the way protein and water direct electrons through complex structures."
The study was the final Ph.D. project for Lin,
Beratan's former graduate student, who is first author of the Science
paper. Co-author Balabin helped the group calculate how the naturally
occurring motion of atoms in the protein might further influence the
electron transfer.
"We see pictures of proteins in fixed positions,
but in reality we should think of their atoms as wiggling all over the
place," Beratan said. |
