much sun – for plants as well as people – can be harmful to long-term health.
But to avoid the botanical equivalent of "lobster tans," plants have developed
an intricate internal defense mechanism, called photoprotection, which acts
like sunscreen to ward off the sun's harmful rays.
"We knew that biomolecules called carotenoids participate
in this process of photoprotection, but the question
has been, how does this work?" said Iris Visoly-Fisher,
a postdoctoral research associate in the Biodesign
Institute at Arizona State University.
Carotenoids act as 'wires' to carry away the extra
sunlight energy in the form of unwanted electrons,
somehow wicking away the extra electrons across long
distances from locations that could damage plant
tissues and photosynthesis. During photoprotection,
the consensus school of thought was that carotenoids--the
source of the orange pigments in carrots and Vitamin
A -- become oxidized, or charged, losing an electron
in the process.
Now, Fisher and other ASU scientists have found
a way to measure for the first time the electrical
conductance within such an important biomolecule.
And in doing so, the team has produced a new discovery
which shatters the prevailing view. The research
team found that oxidation is not required for photoprotection,
but rather, carotenoids in a neutral, or uncharged
state, can readily handle the electron overload from
findings have been published in the prestigious
journal Proceedings of the National Academy of Sciences
(PNAS) under the title"Conductance of a Biomolecular
Wire" ( http://www.pnas.org/cgi/content/abstract/0600593103v1 ).
"This is a remarkable experimental tour-de-force
and the result is quite unexpected," said Lindsay,
who directs Fisher's work in the Biodesign Institute's
Center for Single Molecule Biophysics. "Carotene
was regarded as the poster child for this molecular
mechanism, but it turns out that a much simpler mechanism
works just fine."
The innovative work was a collaboration between
several ASU departments and the Univesidad Nacional
de Rio Cuarto in Argentina. In addition to Fisher,
who was lead author on the paper, contributions from
chemistry and biochemistry professors Devens Gust,
Tom Moore and Ana Moore of ASU's Center for the Study
of Early Events in Photosynthesis were instrumental
in the project.
"The initial interest was to more fully understand
how photosynthesis works," said Fisher. Because our
center focuses on electron transport in a single
molecule, Devens Gust and Tom and Ana Moore suggested
that we look at single molecule transport in carotene."
To get at the heart of the problem, Fisher had to
attempt an experiment that had never been done before
for any biomolecule: to control the charge of the
biomolecule while measuring its ability to hold a
By holding a carotenoid under potential control,
Fisher could control whether the biomolecule was
in a neutral state or in the charged state (the oxidized
state), while simultaneously measuring the electron
transport through a single molecule.
"The importance of this result is not only for understanding
natural systems and photosynthesis, but also for
the fact that technically, for the first time, we
could hold a molecule in a state pretty close to
the natural conditions found in the plant," said
make the experimental measurements, Fisher first
needed to work out several technically challenging
variations to a method first pioneered by electrical
engineering professor Nongjian Tao of ASU's Fulton
School of Engineering. In concept, it's much like
trying to measure the current of a wire found in
an everyday household appliance, only in this case,
the "wiring" is
a miniscule 2.8 nanometers long and less than a
single nanometer thick, or about 10,000 times smaller
than the width of a human hair.
One measurement problem is that carotenoids are
highly prone to react with water and oxygen, so all
measurements had to be performed in an environment
that would both protect the molecule and immerse
it in an environment mimicking a biological cell
membrane, where the carotenoids are found in nature.
Other innovations included developing a new insulating
coat of polyethylene for the probe tip of a Scanning
Tunneling Microscope (STM), which is used to measure
the electron flow across single molecules. Also,
the chemical ends of the carotenoids had to be modified
so they could chemically stick to the STM probe's
gold tipped electrodes.
make a single measurement, the carotenoid molecules,
which lie flat on the surface of a tiny reaction
chamber, are first picked up by the STM probe's gold
tip and chemically bound between these two electrodes,
forming a kind of nanoscale bridge. "Gold is a soft
metal, and when you pull it apart, eventually, you
can measure the conduction of a single carotenoid
molecule between the gold electrodes," said Fisher.
The research team found that, especially when compared
to metals, carotenoids are not very conductive, even
when measuring the most oxidized form. However, the
electrical conduction was two orders of magnitude
higher when compared to what is needed for the photoprotective
effect to work.
The group also measured how fast the electrons traveled
across the carotenoid bridge between the electrodes.
By measuring carotenoids of different chemical lengths,
the team showed that the travel rate was fast enough
to match or exceed measurements performed in the
of the greatest challenges of the experiment came
down to the human endurance of taking thousands
of measurements over an intense, six month period. "We
needed to keep this finicky molecule away from the
light, so sometimes, the microscope room became like
a cave, where I was sitting for hours and hours in
the dark," said Fisher.
For Fisher and the rest of the team, however, the
main satisfaction was being able to break down a
complex process to understand its simplest components
and produce a groundbreaking discovery.
Contact: Joe Caspermeyer
Arizona State University