Quantum physics and plant biology
seem like two branches of science that could not be more different, but
surprisingly they may in fact be intimately tied.
Researchers at the U.S.
Department of Energy’s (DOE) Argonne National Laboratory and the Notre Dame
Radiation Laboratory at the University of Notre Dame used ultrafast
spectroscopy to see what happens at the subatomic level during the very first
stage of photosynthesis. “If you think of photosynthesis as a marathon, we’re
getting a snapshot of what a runner looks like just as he leaves the blocks,” said Argonne biochemist David Tiede. “We’re
seeing the potential for a much more fundamental interaction than a lot of
people previously considered.”
While different species of
plants, algae, and bacteria have evolved a variety of different mechanisms to
harvest light energy, they all share a feature known as a photosynthetic
reaction center. Pigments and proteins found in the reaction center help
organisms perform the initial stage of energy conversion.
These pigment molecules, or
chromophores, are responsible for absorbing the energy carried by incoming
light. After a photon hits the cell, it excites one of the electrons inside the
chromophore. As they observed the initial step of the process, Argonne scientists saw something no one had observed
before: A single photon appeared to excite different chromophores
simultaneously.
“The behavior we were able to see
at these very fast time scales implies a much more sophisticated mixing of
electronic states,” Tiede said. “It shows us that high-level biological systems
could be tapped into very fundamental physics in a way that didn’t seem likely
or even possible.”
The quantum effects observed in
the course of the experiment hint that the natural light-harvesting processes
involved in photosynthesis may be more efficient than previously indicated by
classical biophysics, said chemist Gary Wiederrecht of Argonne’s
Center for Nanoscale Materials. “It leaves us wondering: how did Mother Nature
create this incredibly elegant solution?” he said.
The result of the study could
significantly influence efforts by chemists and nanoscientists to create
artificial materials and devices that can imitate natural photosynthetic
systems. Researchers still have a long way to go before they will be able to
create devices that match the light harvesting efficiency of a plant.
One reason for this shortcoming,
Tiede explained, is that artificial photosynthesis experiments have not been
able to replicate the molecular matrix that contains the chromophores. “The
level that we are at with artificial photosynthesis is that we can make the
pigments and stick them together, but we cannot duplicate any of the external
environment,” he said. “The next step is to build in this framework, and then
these kinds of quantum effects may become more apparent.”
Because the moment when the
quantum effect occurs is so short-lived—less than a trillionth of a second—scientists
will have a hard time ascertaining biological and physical rationales for their
existence in the first place. “It makes us wonder if they are really just there
by accident, or if they are telling us something subtle and unique about these
materials,” Tiede said. “Whatever the case, we’re getting at the fundamentals
of the first step of energy conversion in photosynthesis.”
An article based on the study
appeared online in the Proceedings of the National Academy of
Sciences.
