Re: Could Photosynthesis Breakthrough Yield Solar Power Advance?

Text of press release in the journal Nature:

http://www.nature.com/nature/journal/v446/n7137/full/446740a.html

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News and Views

Nature 446, 740-741 (12 April 2007) | doi:10.1038/446740a; Published
online 11 April 2007

Biophysics: Quantum path to photosynthesis

by Roseanne J. Sension*

Knowing how plants and bacteria harvest light for photosynthesis so
efficiently could provide a clean solution to mankind's energy
requirements. The secret, it seems, may be the coherent application of
quantum principles.

The Sun bombards Earth with a steady stream of energy about 1017
joules per second, on average [ref 1]. If we could convert this energy
efficiently to a chemically useful form, our reliance on fossil fuels
and nuclear power and so our production of climate-change agents and
hazardous waste materials could be substantially reduced. But how can
we achieve such efficiency?

Of course, the photosynthesizing organisms on Earth already have the
answer. In higher plants and certain bacterial systems, the initial
steps of natural photosynthesis harness light energy with an
efficiency of 95% or more values that we can only aspire to with
artificial photocells. Elsewhere in this issue, Engel et al. [ref 2]
(page 782) take a close look at how nature, in the form of the green
sulphur bacterium Chlorobium tepidum, manages to transfer and trap
light's energy so effectively. The key might be a clever quantum
computation built into the photosynthetic algorithm.

Photosynthesis is initiated by the excitation, through incident light,
of electrons in pigment molecules chromophores such as chlorophyll.
This electronic excitation moves downhill from energy level to energy
level through the chromophores before being trapped in a reaction
centre, where its remaining energy is used to initiate the production
of energy-rich carbohydrates. In natural light-harvesting systems,
chlorophyll pigments are arranged together in an 'antenna', sometimes
with elegant symmetry and sometimes with apparent randomness, but
always with a precise structure that is supplied by a protein scaffold
(Fig. 1). Many studies indicate [refs 3, 4, 5, 6] that this strictly
defined structure, together with the close proximity of the
chromophores, is essential for the strong absorption of light, fast
downhill energy transfer and efficient trapping of the electron
excitation in a reaction centre, all of which are characteristic of
natural photosynthesis.

[ Image occurs here ]

But where does quantum mechanics, let alone quantum computing, fit in
here? The mechanism of energy transfer through chromophore complexes
has generally been assumed to involve incoherent hopping that is,
seemingly uncoordinated movement in a 'random walk' with a general
downhill direction either between individual chromophores, or between
modestly delocalized energy states spanning several chromophores. The
energy transfer is determined by quantum-mechanical states and their
overlaps, to be sure, but there is nothing inherently 'quantum' or
wave-like in the process itself.

Engel et al. [ref 2], however, performed two-dimensional Fourier
transform spectroscopy of the bacteriochlorophyll FennaMatthewsOlsen
antenna complex, and discovered regular variations in the intensity of
their signal. These 'quantum beats', which persist for hundreds of
femtoseconds, are characteristic of coherent coupling between
different electronic states. In other words, the electronic excitation
that transfers the energy downhill does not simply hop incoherently
from state to state, but samples two or more states simultaneously.
The data also suggest that the protein scaffold might itself be
structured to dampen fluctuations that would induce decoherence of the
electronic excitation.

Coherent energy transfer allows the 'wave-like' sampling of the energy
landscape to establish the easiest route for the electronic excitation
to the reaction complex much faster than the semi-classical hopping
mechanism allows indeed, it does so almost instantaneously. The
process is analogous to Grover's algorithm in quantum computing, which
has been proved to provide the fastest possible search of an unsorted
information database [ref 7].

Although the data were acquired at low temperature (77 kelvin), the
observation of electronic coherences in such a complex system is
remarkable. Assuming that the effect is general that similar
coherences occur in many different natural light-harvesting systems,
and are observed at non-cryogenic temperatures we may find that
nature, through its evolutionary algorithm, has settled on an
inherently quantum-mechanical process for the critical mechanism of
efficient light harvesting. This is an interesting lesson to be
considered when designing artificial systems for this purpose.

References

1. Grätzel, M. Chem. Lett. 34, 813 (2005).
2. Engel, G. S. et al. Nature 446, 782786 (2007).
3. van Amerongen, H., Valkunas, V. & van Grondelle, R.
Photosynthetic Excitons (World Scientific, Singapore, 2000).
4. Yoder, L. M., Cole, A. G. & Sension, R. J. Photosynth. Res. 72,
147158 (2002).
5. Shiang, J. J., Yoder, L. M. & Sension, R. J. J. Phys. Chem. B
107, 21622169 (2003).
6. Brixner, T. et al. Nature 434, 625628 (2005).
7. Grover, L. K. Phys. Rev. Lett. 79, 325328 (1997).
8. Ferreira, K. N., Iverson, T. M., Maghlaoui, K., Barber, J. &
Iwata, S. Science 303, 18311838 (2004).
9. Papiz, M. Z., Prince, S. M., Howard, T., Cogdell, R. J. &
Isaacs, N. W. J. Mol. Biol. 326, 15231538 (2003).
10. Camara-Artigas, A., Blankenship, R. & Allen, J. P. Photosynth.
Res. 75, 4955 (2003).

*Roseanne J. Sension is in the FOCUS Center and Department of
Chemistry, University of Michigan, 930 North University Avenue, Ann
Arbor, Michigan 48109-1055, USA. Email: sension@xxxxxxxxx

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