Quantum secrets of photosynthesis revealed
http://www.physorg.com/news95605211.html
Through photosynthesis, green plants and cyanobacteria are able to transfer
sunlight energy to molecular reaction centers for conversion into chemical
energy with nearly 100-percent efficiency. Speed is the key - the transfer of
the solar energy takes place almost instantaneously so little energy is
wasted as heat. How photosynthesis achieves this near instantaneous energy
transfer is a long-standing mystery that may have finally been solved.
A study led by researchers with the U.S. Department of Energy’s Lawrence
Berkeley National Laboratory (Berkeley Lab) and the University of California
(UC) at Berkeley reports that the answer lies in quantum mechanical effects.
Results of the study are presented in the April 12, 2007 issue of the journal
Nature.
"We have obtained the first direct evidence that remarkably long-lived
wavelike electronic quantum coherence plays an important part in energy
transfer processes during photosynthesis," said Graham Fleming, the principal
investigator for the study. “This wavelike characteristic can explain the
extreme efficiency of the energy transfer because it enables the system to
simultaneously sample all the potential energy pathways and choose the most
efficient one.”
Fleming is the Deputy Director of Berkeley Lab, a professor of chemistry at
UC Berkeley, and an internationally acclaimed leader in spectroscopic studies
of the photosynthetic process. In a paper entitled, Evidence for wavelike
energy transfer through quantum coherence in photosynthetic systems, he and
his collaborators report the detection of “quantum beating” signals,
coherent electronic oscillations in both donor and acceptor molecules,
generated by light-induced energy excitations, like the ripples formed when
stones are tossed into a pond.
Electronic spectroscopy measurements made on a femtosecond (millionths of a
billionth of a second) time-scale showed these oscillations meeting and
interfering constructively, forming wavelike motions of energy (superposition
states) that can explore all potential energy pathways simultaneously and
reversibly, meaning they can retreat from wrong pathways with no penalty.
This finding contradicts the classical description of the photosynthetic
energy transfer process as one in which excitation energy hops from
light-capturing pigment molecules to reaction center molecules step-by-step
down the molecular energy ladder.
"The classical hopping description of the energy transfer process is both
inadequate and inaccurate," said Fleming. "It gives the wrong picture of how
the process actually works, and misses a crucial aspect of the reason for the
wonderful efficiency."
Co-authoring the Nature paper with Fleming were Gregory Engel, who was first
author, Tessa Calhoun, Elizabeth Read, Tae-Kyu Ahn, Tomas Mancal and
Yuan-Chung Cheng, all of whom held joint appointments with Berkeley Lab’s
Physical Biosciences Division and the UC Berkeley Chemistry Department at the
time of the study, plus Robert Blankenship, from the Washington University in
St. Louis.
The photosynthetic technique for transferring energy from one molecular
system to another should make any short-list of Mother Nature’s spectacular
accomplishments. If we can learn enough to emulate this process, we might be
able to create artificial versions of photosynthesis that would help us
effectively tap into the sun as a clean, efficient, sustainable and
carbon-neutral source of energy.
Towards this end, Fleming and his research group have developed a technique
called two-dimensional electronic spectroscopy that enables them to follow
the flow of light-induced excitation energy through molecular complexes with
femtosecond temporal resolution. The technique involves sequentially flashing
a sample with femtosecond pulses of light from three laser beams. A fourth
beam is used as a local oscillator to amplify and detect the resulting
spectroscopic signals as the excitation energy from the laser lights is
transferred from one molecule to the next. (The excitation energy changes the
way each molecule absorbs and emits light.)
Fleming has compared 2-D electronic spectroscopy to the technique used in the
early super-heterodyne radios, where an incoming high frequency radio signal
was converted by an oscillator to a lower frequency for more controllable
amplification and better reception. In the case of 2-D electronic
spectroscopy, scientists can track the transfer of energy between molecules
that are coupled (connected) through their electronic and vibrational states
in any photoactive system, macromolecular assembly or nanostructure.
Fleming and his group first described 2-D electronic spectroscopy in a 2005
Nature paper, when they used the technique to observe electronic couplings in
the Fenna-Matthews-Olson (FMO) photosynthetic light-harvesting protein, a
molecular complex in green sulphur bacteria.
Said Engel, "The 2005 paper was the first biological application of this
technique, now we have used 2-D electronic spectroscopy to discover a new
phenomenon in photosynthetic systems. While the possibility that
photosynthetic energy transfer might involve quantum oscillations was first
suggested more than 70 years ago, the wavelike motion of excitation energy
had never been observed until now."
As in the 2005 paper, the FMO protein was again the target. FMO is considered
a model system for studying photosynthetic energy transfer because it
consists of only seven pigment molecules and its chemistry has been well
characterized.
"To observe the quantum beats, 2-D spectra were taken at 33 population times,
ranging from 0 to 660 femtoseconds," said Engel. "In these spectra, the
lowest-energy exciton (a bound electron-hole pair formed when an incoming
photon boosts an electron out of the valence energy band into the conduction
band) gives rise to a diagonal peak near 825 nanometers that clearly
oscillates. The associated cross-peak amplitude also appears to oscillate.
Surprisingly, this quantum beating lasted the entire 660 femtoseconds."
Engel said the duration of the quantum beating signals was unexpected because
the general scientific assumption had been that the electronic coherences
responsible for such oscillations are rapidly destroyed.
"For this reason, the transfer of electronic coherence between excitons
during relaxation has usually been ignored," Engel said. "By demonstrating
that the energy transfer process does involve electronic coherence and that
this coherence is much stronger than we would ever have expected, we have
shown that the process can be much more efficient than the classical view
could explain. However, we still don’t know to what degree photosynthesis
benefits from these quantum effects."
Engel said one of the next steps for the Fleming group in this line of
research will be to look at the effects of temperature changes on the
photosynthetic energy transfer process. The results for this latest paper in
Nature were obtained from FMO complexes kept at 77 Kelvin. The group will
also be looking at broader bandwidths of energy using different colors of
light pulses to map out everything that is going on, not just energy
transfer. Ultimately, the idea is to gain a much better understanding how
Nature not only transfers energy from one molecular system to another, but is
also able to convert it into useful forms.
"Nature has had about 2.7 billion years to perfect photosynthesis, so there
are huge lessons that remain for us to learn,” Engel said. “The results we’
re reporting in this latest paper, however, at least give us a new way to
think about the design of future artificial photosynthesis systems."
Source: Lawrence Berkeley National Laboratory
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