Scientists like to compare the light-harvesting antennae in biological photosystems to radio dishes. Both collect electromagnetic radiation and focus it on a “receiver.” |
Sometimes when people talk about solar
energy, they tacitly assume that we’re stuck with some version of the silicon
solar cell and its technical and cost limitations.
Not so.
The invention of the solar cell, in 1941,
was inspired by a newfound understanding of semiconductors, materials that can
use light energy to create mobile electrons—and ultimately an electrical
current.
Silicon solar cells have almost nothing to
do with the biological photosystems in tree leaves and pond scum that use light
energy to push electrons across a membrane—and ultimately create sugars and
other organic molecules.
At the time, nobody understood these
complex assemblages of proteins and pigments well enough to exploit their
secrets for the design of solar cells.
But things have changed.
At Washington
University in St. Louis’s Photosynthetic Antenna Research
Center (PARC) scientists are exploring native biological photosystems, building
hybrids that combine natural and synthetic parts, and building fully synthetic
analogs of natural systems.
One team has just succeeded in making a crucial
photosystem component—a light-harvesting antenna—from scratch. The new antenna
is modeled on the chlorosome found in green bacteria.
Chlorosomes are giant assemblies of pigment
molecules. Perhaps nature’s most spectacular light-harvesting antennae, they
allow green bacteria to photosynthesize even in the dim light in ocean deeps.
Dewey Holten, PhD, professor of chemistry
in Arts & Sciences, and collaborator Christine Kirmaier, PhD, research
professor of chemistry, are part of a team that is trying to make synthetic
chlorosomes. Holten and Kirmaier use ultra-fast laser spectroscopy and other
analytic techniques to follow the rapid-fire energy transfers in
photosynthesis.
His team’s latest results are described in
New Journal of Chemistry.
Chlorosomes
Biological systems that
capture the energy in sunlight and convert it to the energy of chemical bonds
come in many varieties, but they all have two basic parts: the light-harvesting
complexes, or antennae, and the reaction center complexes. The antennae consist
of many pigment molecules that absorb photons and pass the excitation energy to
the reaction centers.
In the reaction centers, the excitation
energy sets off a chain of reactions that create ATP, a molecule often called
the energy currency of the cell because the energy stored ATP powers most
cellular work. Cellular organelles selectively break those bonds in ATP
molecules when they need an energy hit for cellular work.
Green bacteria, which live in the lower
layers of ponds, lakes, and marine environments, and in the surface layers of
sediments, have evolved large and efficient light-harvesting antennae very
different from those found in plants bathing in sunlight on Earth’s surface.
The antennae consist of highly organized 3D
systems of as many as 250,000 pigment molecules that absorb light and funnel
the light energy through a pigment/protein complex called a baseplate to a
reaction center, where it triggers chemical reactions that ultimately produce
ATP.
In plants and algae (and in the baseplate
in the green bacteria) photo pigments are bound to protein scaffolds, which
space and orient the pigment molecules in such a way that energy is efficiently
transferred between them.
But chlorosomes don’t have a protein
scaffold. Instead the pigment molecules self-assemble into a structure that
supports the rapid migration of excitation energy.
This is intriguing because it suggests
chlorosome mimics might be easier to incorporate in the design of solar devices
than biomimetics that are made of proteins as well as pigments.
The photosystem in green bacteria consists of a light-harvesting antenna called a chlorosome and a reaction center. The energy of the light the pigments absorb is transferred to the reaction center (red) through a protein-pigment antenna complex called the baseplate (gold). The antenna (green) is made of rod-shaped aggregates of pigment molecules. Source: Blankenship/WUSTL” |
Synthetic pigments
The goal of the work described in the latest journal article was to see whether
synthesized pigment molecules could be induced to self-assemble. The process by
which the pigments align and bond is not well understood.
“The structure of the pigment assemblies
in chlorosomes is the subject of intense debate,” Holten says, “and there are
several competing models for it.”
Given this uncertainty, the scientists wanted to study many variations of a pigment
molecule to see what favored and what blocked assembly.
A chemist wishing to design pigments that mimic those found in
photosynthetic organisms first builds one of three molecular frameworks. All
three are macrocycles, or giant rings: porphyrin, chlorine, and
bacteriochlorin.
“One of the members of our team, Jon Lindsey, can synthesize analogs of all
three pigment types from scratch,” says Holten. (Lindsey, PhD, is Glaxo
Professor of Chemistry at North
Carolina State University.)
In the past, chemists making photo pigments have usually started with
porphyrins, which are the easiest of the three types of macrocycles to
synthesize. But Lindsey also has developed the means to synthesize chlorins,
the basis for the pigments found in the chlorosomes of green bacteria. The
chlorins push the absorption to the red end of the visible spectrum, an area of
the spectrum scientists would like to be able to harvest for energy.
Key to pigment self-assembly are the metal atoms and hydroxyl (OH) and
carbonyl (C=O) groups in the pigment molecules.
Doctoral student Olga Mass and coworkers in Lindsey’s laboratory synthesized
30 different chlorins, systematically adding or removing chemical groups
thought to be important for self-assembly but also attaching peripheral
chemical groups that take up space and might make it harder for the molecules
to stack or that shift around the distributions of electrons so that the
molecules might stack more easily.
Testing for aggregation
The powdered pigments were carefully packaged and shipped by Fed Ex (because
the Post Office won’t ship chemicals) to Holten’s laboratory at WUSTL and to
David Bocian’s laboratory at the University
of California at Riverside.
Scientists in both laboratories made up green-tinctured solutions of each of
the 30 molecules in small test tubes and then poked and prodded the solutions
by means of analytical techniques to see whether the pigment had aggregated
and, if so, how much had formed the assemblies.
Holten’s laboratory studied their absorption of light and their fluorescence
(which indicated the presence of monomers, since assemblies don’t normally
fluoresce) and Bocian’s laboratory studied their vibrational properties, which
are determined by the network of bonds in the molecule or pigment aggregate as
a whole.
In one crucial test Joseph Springer, a PhD student in Holten’s laboratory,
compared the absorption spectrum of a pigment in a polar solvent that would
prevent it from self-assembling to the spectrum of the pigment in a nonpolar
solvent that would allow the molecules to interact with one another and form
assemblies.
“You can see them aggregate,” Springer says. “A pigment that is totally in
solution is clear, but colored a brilliant green. When it aggregates, the
solution becomes a duller green and you can see tiny flecks in the liquid.”
The absorption spectra indicated that some pigments formed extensive
assemblies and that the steric and electronic properties of the molecules
predicted the degree to which they would assemble.
Up next
Although this project focused on self-assembly, the PARC scientists have
already taken the next step toward a practical solar device. “With Pratim
Biswas, PhD, the Lucy and Stanley Lopata Professor and chair of the Department
of Energy, Environmental & Chemical Engineering, we’ve since demonstrated
that we can get the pigments to self-assemble on surfaces, which is the next
step in using them to design solar devices,” says Holten.
“We’re not trying to make a more efficient solar cell in the next six
months,” Holten cautions. “Our goal instead is to develop fundamental
understanding so that we can enable the next generation of more efficient solar
powered devices.”
Biomimicry hasn’t always worked. Engineers often point out early flying
machines that attempted to mimic birds didn’t work and that flying machines
stayed aloft only when nventors abandoned biological models and came up with
their own designs.
But there is nothing predestined or inevitable about this. As biological
knowledge has exploded in the past 50 years, mimicking nature has become a
smarter strategy. Biomimetic or biohybrid designs already have solved significant
engineering problems in other areas and promise to greatly improve the design
of solar powered devices as well.
After all, nature has had billions of years to experiment with ways to
harness the energy in sunlight for useful work.
