Frontiers in Energy Research: July 2012

Soaking Up the Sunshine

Biohybrid light antennas offer more efficient solar energy capture than natural systems

A: Subunit of a bacterial light-harvesting antenna complex showing the peptide scaffold (green and blue) and light-harvesting bacteriochlorophyll molecules (purple). The attachment between the peptides and the bacteriochlorophyll is shown in black. B: Side and top views of the full bacterial antenna complex showing the peptide scaffold. Light-harvesting molecules are hidden for clarity. C: Model of the biohybrid antenna subunit showing chromophore attachment sites (red and brown) and the bacteriochlorophyll molecules (purple). Adapted from Springer et al. (2012) DOI:10.1021/ja207390y.

Biohybrid light-capturing antennas capable of harvesting solar energy from a wider range of the solar spectrum than the antennas used by photosynthesizing organisms have been constructed by a group of researchers from the Photosynthetic Antenna Research Center or PARC.

Natural Light-Harvesting Antennas:  Plants and photosynthetic bacteria use light-harvesting antennas to absorb solar energy and funnel it to the reaction centers where photosynthesis takes place. These antennas are composed of repeating subunits that each contain two peptides or small proteins and light-harvesting molecules, called "chromophores," such as bacteriochlorophyll and carotenoids. The peptides form a scaffold to hold and orient the chromophores which makes the transfer of absorbed solar energy extremely efficient. The energy efficiency of the system is, however, inherently limited because natural antennas typically absorb less than half of the wavelengths available in the solar spectrum. To create more efficient synthetic solar energy harvesting systems, a team of PARC researchers is adopting a biohybrid approach, what Jonathan Lindsey, one of the lead researchers, calls "an effort to combine the best of biology with the best of chemistry to surpass nature."

Building a Biohybrid Antenna:  Bacterial light-harvesting antennas can be taken apart into their component peptides and chromophores. Under carefully controlled conditions, antenna subunits will reform and then self-assemble into the full antenna complex, and this characteristic was exploited in constructing the biohybrid antennas.

The team designed and synthesized two artificial peptides based on those present in the light-harvesting antennas of photosynthetic purple bacteria. Each peptide was engineered with an attachment (to the amino acid cysteine) to hold the chromophores in place. Four synthetic chromophores, which absorb sunlight from different regions of the solar energy spectrum, were selected, and bacteriochlorophyll was extracted from purple bacteria. To build their simple biohybrid antennas, a mixture of bacteriochlorophyll, the two artificial peptides and one of the four synthetic chromophores was placed into the conditions that yield natural antenna subunits and then into conditions that cause these subunits to self-assemble into a full antenna.

The researchers used static and time-resolved absorption and fluorescence spectroscopy to measure the wavelengths of light absorbed by the antennas and the efficiency with which this energy was transferred. The results showed that, in each case, the synthetic peptides associated with the bacteriochlorophyll molecules and the synthetic chromophore to form subunits that self-assembled into a complete biohybrid antenna. Energy transfer was observed in all biohybrid antennas at up to 90 percent efficiency, comparable to natural antennas.

The Future Is Bright: Effective solar energy capture systems must maximize the range of wavelengths absorbed and efficiently transfer (and ultimately trap) this absorbed energy. The formation of simple biohybrid light-harvesting antennas which accept synthetic chromophores that harvest sunlight from different regions of the solar energy spectrum, while displaying the self-assembly capabilities and energy transfer efficiencies of natural antennas, paves the way for the development of more complex biohybrid structures that maximize solar energy capture. "Extension of this research will allow absorption of all wavelengths of light available from the sun and the efficient transfer of this energy to a single or multiple traps," said Paul Loach, a lead researcher in the study. This development, together with the wide range of synthetic chromophores now available, offers an exciting step forward in artificial photosynthesis, making the future very bright indeed.

Acknowledgments: 

This work was sponsored by the Photosynthetic Antenna Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences.

More Information: 

Springer JW, PS Parkes-Loach, KR Reddy, M Krayer, J Jiao, GM Lee, DM Niedzwiedzki, MA Harris, C Kirmaier, DF Bocian, JS Lindsey, D Holten, and PA Loach. 2012. "Biohybrid Photosynthetic Antenna Complexes for Enhanced Light-Harvesting." Journal of the American Chemical Society 134(10):4589-4599. DOI: 10.1021/ja207390y.

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