|
The molecular power plants that carry out
photosynthesis are at the root of a scientific quest to learn how they channel
energy from sunlight to split water into oxygen and hydrogen.
Understanding these fundamental processes
could help scientists develop technologies that replicate nature’s handiwork to
produce cheaper and more efficient fuels.
Now, an international research team led by
scientists at Lawrence Berkeley National Laboratory and SLAC National
Accelerator Laboratory has used a powerful X-ray laser to shine new light on a
tiny cluster of molecules that is integral to an important stage of photosynthesis
known as Photosystem II. The results were published in the Proceedings of the National Academy
of Sciences.
The team crystallized the molecular
clusters, suspended the crystals in liquid, and injected them into the path of
SLAC’s X-ray laser, the Linac Coherent Light Source (LCLS). Laser light
diffracting off the crystals formed patterns that were used to reconstruct the
composition and atomic structure of the clusters.
In Photosystem II, plants absorb photons
from sunlight to drive chemical reactions that oxidize water, splitting water
molecules into hydrogen and oxygen. Electrons extracted from the water power
the photosynthetic process, generating almost all the food and energy that life
on Earth depends on.
Junko Yano, staff scientist in the Physical
Biosciences Division at Berkeley Lab and one of three leaders of the research
team, said it is the very last step in Photosystem II—the point at which oxygen
is released—that has proved most difficult for researchers to observe.
The successful imaging of the crystallized
molecules at LCLS gives the team confidence that it can use the instrument to
study other steps of the photosynthetic process, she said. What’s more, the
imaging was done at room temperature and without the need to freeze samples,
which can distort the results.
Researchers achieved a resolution of 6.5 Å for
the structure of the crystallized clusters that they used in their data. Some
previous experiments have achieved higher resolutions, but with frozen crystals
that may have been altered by X-rays.
Jan Kern, a research scientist at SLAC and
Berkeley Lab who was the lead author on the paper, said, “We hope that
with improved samples, in the future we will be able to get to a higher
resolution.”
Yano said one of the goals of
photosynthesis research is developing a clean and affordable fuel out of common
molecules like water and carbon dioxide. “Is there any way to directly
make a liquid fuel using sunlight and water as a source? How are we going to do
it? How efficiently can we do it?” she said. “That’s what we’re going to learn
from nature.”
LCLS is proving a valuable research tool in
biological research because its ultrashort, ultrabright pulses can provide data
in the quadrillionths of a second before the sample is destroyed by the
powerful X-ray radiation.
“Having a probe where you can study
the Photosystem II mechanism in real time, with a technique where you can probe
before the sample is destroyed, might really be the key to solving this
question,” said team co-leader Uwe Bergmann, deputy director for the LCLS
and a senior staff scientist at SLAC.
He said the research highlighted in the
paper represents an expansion of a longstanding collaboration in Photosystem II
research between researchers at SLAC and Berkeley Lab.
Bergmann noted that there is still work to
be done to sharpen the resolution of Photosystem II using X-ray lasers, which
may be possible with improvements in the crystal samples and in the delivery
systems used to stream the crystals across the X-ray laser beam.
There is
an intense global scientific race to solve these long-kept secrets in
photosynthesis, particularly with the launch of new capabilities in research
made possible by X-ray lasers and the most advanced synchrotron sources.
