A Step Toward Artificial Photosynthesis
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Image courtesy of Martin Hohmann-Marriott and Robert Blankenship
Electron microscopic tomogram of dividing cells of the green sulfur bacterium Chlorobaculum tepidum, with chlorosomes rendered in simulated color.
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 ultimately create an electrical current.
Silicon solar cells have little in common with the biological photosystems in tree leaves and pond scum that use light energy to 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 biological photosystems, to build both hybrids that combine natural and synthetic parts as well as fully synthetic versions of natural systems. PARC is one of 46 Energy Frontier Research Centers (EFRCs) established by the DOE Office of Science in 2009 at universities, national laboratories, and other institutions around the nation to accelerate advanced basic research related to energy.
The PARC team has just succeeded in making a crucial photosystem component—a light-harvesting antenna—from scratch. The new antenna is modeled on the chlorosome, or biological antenna, found in green photosynthetic 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 of the deep ocean.
Dewey Holten, Professor of Chemistry at Washington University, and collaborator Christine Kirmaier, 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. The team's results are described in the New Journal of Chemistry.
Mimicking Biological Systems
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Image courtesy of Jianzhong Wen and Robert Blankenship
Schematic structural model of green bacterial chlorosome showing pathway of energy transfer.
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 the photons, or light, and pass the energy on to the reaction centers.
In the reaction centers, this collected energy sets off a chain of reactions that ultimately create adenosine triphosphate (ATP), a molecule often called the "energy currency" of the cell. Cellular processes use the stored energy in the chemical bonds of ATP molecules when needed to power activity within the cell.
Green photosynthetic bacteria, which live in the lower layers of ponds, lakes, and marine environments, as well as 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—called chlorosomes—consist of highly organized three-dimensional systems of as many as 250,000 pigment molecules that absorb light and funnel the captured 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 the baseplate in the green bacteria) photo pigments are typically bound to protein scaffolds, which space and orient the pigment molecules in such a way that energy is efficiently transferred between them. But the pigment molecules in the chlorosome don't utilize a scaffold – they simply self-assemble.
This is intriguing because it suggests that design of the chlorosome might be easier to mimic artificially than the more complex design of antennae that are based on protein scaffolds.
Building Pigments in the Laboratory
The researchers' goal was to see whether a collection of synthesized pigment molecules could be induced to self-assemble. The process by which the specific pigments in the chlorosome align and bond is not well understood.
"The structure of the pigment assemblies in chlorosomes is the subject of intense debate," Holten said, "and there are several competing models for it."
Given this uncertainty, the scientists wanted to study structural variations of the pigment molecule to see what favored and what blocked assembly.
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Image courtesy of Dewey Holton
The three major types of pigments are relatively large, circular molecules.
Nature provides three molecular templates for the design of synthetic pigments. All three are relatively large, circular-shaped molecules known as "macrocycles": porphyrin, chlorin, and bacteriochlorin. Each of these macrocyles has an alternating double-bond structure that gives the molecule its basic electronic properties, including the ability to absorb visible or near-infrared light. Hemoglobin is a porphyrin that lends blood its red color; chlorophyll, the pigment in green plants, is a chlorin; and the pigments in purple photosynthetic bacteria are known as bacteriochlorins.
Sunlight is made up different colors which can be seen when light passes through a prism and you see a rainbow cast on the wall. The three types of pigments (porphyrin, chlorin, and bacteriochlorin) absorb the different colors of sunlight. A chemist wishing to design pigments that mimic those found in photosynthetic organisms first builds one of these three molecular frameworks.
"One of the members of our team, Jon Lindsey, can synthesize analogs of all three pigment types from scratch," said Holten. Lindsey is Glaxo Professor of Chemistry at North Carolina State University, a partner of Washington University in the EFRC.
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 toward the red end of the visible spectrum, an area of the spectrum scientists would like to be able to harvest for energy.
Graduate student Olga Mass and colleagues in Lindsey's lab synthesized 30 different chlorins, systematically adding or removing chemical groups thought to be important for self-assembly. They added peripheral chemical groups to the core chlorin molecule that might make it harder for the molecules to stack, or alternatively, cause electrons to shift so that the molecules might stack more easily.
The powdered pigments were carefully packaged and shipped to Holten's lab at Washington University and to David Bocian's lab at the University of California, Riverside (another partner institution in the EFRC).
Scientists in both labs made up green-tinctured solutions of each of the 30 molecules in small test tubes and then "poked and prodded" the solutions using analytical techniques to see whether the pigment had aggregated into "stacks" or remained disaggregated. Holten's lab studied their absorption of light and their fluorescence—aggregates don't fluoresce while individual molecules do—while Bocian's lab studied their vibrational properties, which are determined by the way the molecules stacked or how the pigment aggregated as a whole.
In one crucial test, Joseph Springer, a graduate student in Holten's lab, compared the absorption spectrum of a pigment in a solution that would prevent it from self-assembling to the spectrum of the same pigment in a solution that would allow the molecules to interact with one another and form assemblies.
"You can see them aggregate," Springer said. "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."
Next Steps
Although this project focused on self-assembly, the PARC scientists have already taken the next step toward a practical solar device. "With Pratim Biswas, the Lucy and Stanley Lopata Professor and chair of the Department of Energy, Environmental & Chemical Engineering at Washington University, 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," said 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."
As biological knowledge has exploded in the past 50 years, mimicking nature has become a smarter, more realistic strategy. While biomimicry hasn't always worked as in the case of designing early flying machines, 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.
—Diana Lutz, Washington University in St. Louis, dlutz@wustl.edu
Funding
DOE Office of Science, Office of Basic Energy Sciences
Publication
O. Mass, D.R. Pandithavidana, M.P.K. Santiago, J.W. Springer, J. Jiao, Q. Tang, C. Kirmaier, D.F. Bocian, D. Holten, and J.S. Lindsey, "De novo synthesis and properties of analogues of the self-assembling chlorosomal bacteriochlorophylls," New Journal of Chemistry 35, 2671 (2011).
