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May 2013

Creation of the “Forbidden” Bacteriochlorophyll f

Light-harvesting pigment, undiscovered in nature, absorbs broader spectrum of light

Khuram Umar Ashraf

(A) Schematic structural representation of the green bacterial chlorosome, showing the energy transfer pathway. (B) Absorption of self-assembled BChls c, d, e and the human-made f in whole chlorosomes.

Understanding the process of how bacteria and other organisms absorb light and convert it into chemical energy is vital to creating solar cells and other renewable energy technologies. Photosynthesizing bacteria use pigments, what scientists call bacteriochlorophylls (BChl), that absorb wide wavelengths of light from the absorption spectrum. Within bacteria, especially green bacteria, bacteriochlorophylls c, d or e harvest light over a broad spectrum. Researchers from Washington University in St. Louis and The Pennsylvania State University, at the Photosynthetic Antenna Research Center, PARC, have shown that BChl f, not yet discovered in nature, can be synthesized to increase the range of light absorbed.

The lead investigator at PARC, Robert Blankenship, says, “Our group is interested in this particular research because the light-harvesting systems of green photosynthetic bacteria carry out some of the most efficient solar conversion processes we see in nature. Many scientists believe that the architecture of their system holds secrets that will help us build ultra-efficient solar cells that copy or mimic natural processes.”

One of the main issues scientists face is being able to design a system/device that can utilize a broader spectrum of light, thereby taking full advantage of more of the energy available in sunlight. Photosynthetic bacteria, in contrast to plants, absorb light not only in the visible spectrum, but also in the infrared light (IR) region. This unique feature of such bacteria among all known photosynthetic organisms explains the possibility of their thriving in ecological regions that have extremely low levels of solar radiation.

One such bacterium is the green photosynthetic bacterium that contains an organelle known as the chlorosome. Chlorosomes are light-harvesting “antenna” complexes, which naturally contain BChl c, d or e as the principal light-harvesting pigments. The team of PARC researchers has hypothesized that there may be yet another BChl, which they term the forbidden chlorophyll: BChl f! This led the team to show, by genetic manipulation and spectroscopic analysis, that if found in nature, BChl f-containing organisms would require a high irradiance niche. These organisms containing BChl f have had more than 2 billion years to adapt to this niche, allowing for the possibility that organisms producing BChl f may be found in nature.

The team of PARC scientists showed that the human-made BChl f had similarities to BChl e and d. This information allowed them to create a mutant within the green bacterium to produce BChl f. Through spectroscopic analysis such as fluorescence emission, they explained why the mutant organism containing BChl f grows slower than the wild type. The low energy transfer efficiency of BChl f compared to e, which is found in the natural system, could be caused by the increased range of wavelengths of light that the organism can absorb.

As the hunt for the organism containing the BChl f goes on, properties of this pigment may prove beneficial to artificial photosynthesis strategies. Greg Orf, one of the lead researchers, says, “This research reinforces the idea that, in many ways, nature has already come up with amazing ways to harness solar energy. If we could find a way to combine the beneficial properties of multiple natural pigments (or synthetic mimics of them) into a cohesive light-harvesting system, we could change the landscape of solar energy production.”

More Information

Orf GS, M Tank, K Vogl, DM Niedzwiedzki, DA Bryant, and RE Blankenship. 2013. “Spectroscopic Insights into Decreased Efficiency of Chlorosomes Containing Bacteriochlorophyll f.Biochimica et Biophysica Acta 1827(4):493-501. DOI: 10.1016/j.bbabio.2013.01.006.

Acknowledgments

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

About the author(s):

  • Khuram U. Ashraf is a Ph.D. student working for the Photosynthetic Antenna Research Center in Richard Cogdell's group at the University of Glasgow. His research is focused on the structure and function of the Chlorobaculum tepidum reaction center, in the pursuit of enhancing the absorption, using surface plasmons as a means of devising an artificial photosynthetic device.

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The energy transfer pathway for the green bacterial chlorosome, the small structures in bacteria that absorb light.

Green photosynthetic bacteria can absorb a much broader range or spectrum of light than plants, which may help bacteria function in environments where light is not plentiful or is outside the typical range of light on the spectrum. The bacteria absorb light using tiny structures known as chlorosomes. These complexes contain light-harvesting pigments, called bacteriochlorophylls. Scientists speculated about bacteriochlorophyll f, which was predicted, but never found in nature. Scientists created it in the laboratory in 2012 by adding a single carbon atom to bacteriochlorophyll e. Recently, they used spectroscopic analysis and computational modeling to characterize the pigment. They showed that the pigment has unusual fluorescence properties. Bacteriochlorophyll f absorbs light across a wider spectrum than the better-known bacteriochlorophyll e. The pigment has yet to be found in nature, but it could exist in land-locked desert salt lakes. Understanding the properties of pigments gives foundational insights for revolutionary solar energy technologies that can work in low or no light settings. Scientists at the Photosynthetic Antenna Research Center, led by Washington University in St. Louis and The Pennsylvania State University, conducted the study.

More Information

Orf GS, M Tank, K Vogl, DM Niedzwiedzki, DA Bryant, and RE Blankenship. 2013. “Spectroscopic Insights into Decreased Efficiency of Chlorosomes Containing Bacteriochlorophyll f.Biochimica et Biophysica Acta 1827(4):493-501. DOI: 10.1016/j.bbabio.2013.01.006.

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.