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Dongping Zhong
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The controversy, Zhong explained, stemmed from
the fact that researchers across different disciplines used different
methods to study the problem. Because of that, they got different
answers on the speed with which these essential biochemical reactions
take place.
"A biologist will tell you that water and proteins must interact on a
nanosecond [one billionth of a second] time scale, because that's how
fast proteins move," he said. "And a physicist will tell you that the
interaction would happen much faster - on the picosecond [one
trillionth of a second] time scale - because that's how fast water
molecules move. And someone who uses X-rays will give you a different
answer than someone who uses nuclear magnetic resonance and so on."
"My feeling is that there is no real controversy - everybody is just
looking at the same answer from different angles," he added.
The answer, revealed in Zhong's lab: water molecules do move fast on
their own, but they slow down - to a speed midway between the
nanosecond and picosecond scale - to connect with proteins.
Zhong, an assistant professor of physics, used ultra-fast laser pulses
to take snapshots of water molecules moving around a protein taken
from a common bacterium, Staphylococcus.
The key to getting a good view of the interaction was to precisely
locate an optical probe on the protein surface. They inserted a
molecule of the amino acid tryptophan into the protein as a probe, and
measured how water moved around it - a technique Zhong began to
develop when he was a postdoctoral researcher in Nobel laureate Ahmed
Zewail's lab at the California Institute of Technology 5 years ago.
Laser studies of the protein while it was immersed in water revealed
that far away from the protein - in a region Zhong called "bulk water"
- the water molecules were flowing around each other at their
typically fast speeds, with each movement requiring only a single
picosecond.
But the water near the protein formed several distinct layers. The
outermost layer flowed at a slower speed than in bulk water, and the
innermost layer even slower. In that innermost layer, each movement of
a water molecule to connect with the protein required at least 100
picoseconds to complete.
So when it comes to supporting life - on the molecular scale, anyway -
water has to move 100 times slower to get the job done.
"The fast-moving water has to slow down to connect with a slow-moving
protein - it's that simple," Zhong said.
"It sounds trivial, I know. But it should be trivial.
"It's an essential biological interaction that has to work just right
every time. If the water moved too slowly, it could get in the way of
proteins trying to meet - it would be a bottleneck in the process.
And if it moved too fast, it couldn't connect with the protein at all.
So I think this is nature's way of getting the interaction just
right."
Zhong and Zewail's coauthors on the paper included Weihong Qiu,
Ta-Ting Kao, Luyuan Zhang, Yi Yang, and Lijuan Wang of Ohio State and
Wesley E. Stites of the University of Arkansas . Zhong is now working
with Ohio State chemist Sherwin Singer to create computer simulations
of protein-water motions based on these results. That work is being
done at the Ohio Supercomputer Center.
This work was supported by the Petroleum Research Fund, the Packard
Foundation, the National Science Foundation, and the National
Institutes of Health.
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