This is a long way from our usual facile understanding of
photosynthesis. It turns out to be way more complicated than all
that. This could lead to better strategies for even photovoltaic
systems if only by the power of suggestion.
Read the last paragraph to capture the sense of all this.
We are now tuning our investigations in such a way as to establish
high resolution and sort out the real high points. A lot of new data
will flow from all this.
Study provides new
insight into photosynthesis
by Staff Writers
Toronto, Canada (SPX) Apr 09, 2013
Advanced optical
probes using femtosecond lasers enable light harvesting processes to
be examined in exquisite detail. Anticlockwise from top right: Purple
bacteria and the structure of the light harvesting complex that gives
these cells their distinctive purple color. This special protein
incorporates molecules of bacteriochlorophyll and carotenoid to
capture the energy from sunlight. The lower part of the figure shows
the protein data recorded from two-dimensional laser spectroscopy.
(Illustration: Evgeny Ostroumov). Credit: Evgeny Ostroumov.
Pigments found in
plants and purple bacteria employed to provide protection from sun
damage do more than just that. Researchers from the University of
Toronto and University of Glasgow have found that they also help to
harvest light energy during photosynthesis.
Carotenoids, the same
pigments which give orange color to carrots and red to tomatoes, are
often found together in plants with chlorophyll pigments that harvest
solar energy. Their main function is photoprotection when rays of
light from the sun are the most intense. However, a new study
published in Science this week shows how they capture blue/green
light and pass the energy on to chlorophylls, which absorb red light.
"This is an
example of how nature exploits subtleties that we would likely
overlook if we were designing a solar energy harvester," says
Greg Scholes, the D.J. LeRoy Distinguished Professor in the
Department of Chemistry at the University of Toronto and lead author
of the study.
A series of
experiments showed that a special "dark state" of the
carotenoid - a hidden level not used for light absorption at all -
acts as a mediator to help pass the energy it absorbs very
efficiently to a chlorophyll pigment.
The researchers
performed broadband two-dimensional electronic spectroscopy - a
technique used to measure the electronic structure and its dynamics
in atoms and molecules - on light-harvesting proteins from purple
bacteria. The aim was to characterize in more detail the whole
sequence of quantum mechanical states of carotenoids that capture
light and channel energy to bacteriochlorophyll molecules. The data
revealed a signature of a special state in this sequence that was
predicted decades earlier, and sought ever since. The results point
to this state's role in mediating energy flow from carotenoid to
bacteriochlorophyll.
"It is utterly
counter-intuitive that a state not participating in light absorption
is used in this manner," says Scholes. "It is amazing that
nature uses so many aspects of a whole range of quantum mechanical
states in carotenoid molecules, moreover, and puts those states to
use in such diverse ways."
\
The other significant
aspect of the work is that the existence of these dark states has
been speculated for decades and that the report by Scholes and his
colleagues is the clearest evidence to date of their existence.
"We found a
smoking gun for the state predicted decades ago and argued about ever
since," says Scholes.
"The energy
transfer processes in natural light-harvesting systems have been
intensively studied for the last 60 years, yet certain details of the
underlying mechanisms remain controversial. Our work really clears up
this particular mystery," says Richard Cogdell, the Hooker
Professor of Botany at the University of Glasgow, co-author of the
report.
"It makes us look
differently at the potential of molecules as building blocks,"
Scholes says. "Just imagine one molecule, a carotenoid, that
can be used to harvest light, photoprotect, convert to a 'safety
valve' in bright light to dissipate excitations, or even be employed
as a heat transducer by purple bacteria such as are found in the
black hole on the island of San Andros in the Bahamas."
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