Dancing the Light Fantastic

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Ultrafast spectroscopists like Walter Struve are just beginning to gain
a detailed understanding of the processes taking place in the leaves of the sunflower beside him.

“A miniature green machine” may one day stockpile energy for later use, perhaps by splitting water into hydrogen and oxygen. One of the two main components in this artificial photosynthesis system, the light antenna, is borrowed directly from nature.

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In a photosynthetic system, the antenna absorbs sunlight, converts it to electronic excitation and transfers the excitation to the reaction center. Here it is used to create positively and negatively charged molecules on opposite sides of a membrane. Walter Struve leads a group at Ames Laboratory that, with other groups at Ames and at Iowa State University, is developing the antenna. The device consists of molecules of phthalocyanine, a dye, attached to protein helices.

Thus far, Struve’s group has made and begun studying the energy transfer properties of scaled-down antennas. They eventually expect to construct a family of antennas that can be coupled to reaction centers next summer. The entire device might be several hundred Ångstroms across, invisible to the unaided eye. In the meantime, the understanding of natural photosynthetic systems will have been greatly advanced by the effort to build artificial ones.

Struve’s work is sponsored by ER’s Basic Energy Sciences’ Division of Chemical Sciences. The team’s research has benefitted from work done at Princeton University, the University of Pennsylvania, the University of Alabama and DuPont Merck Pharmaceutical Company, among others. More information on the antenna development can be found in the Fall 1995 issue of Inquiry, Ames Laboratory’s quarterly news magazine. (Contact Walter Struve, Ames Laboratory, phone (515) 294-4276, email wstruve@ameslab.gov)

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A Fast Natural Antenna

The chlorosome of the green photsynthetic bacterium Chlorflexus aurantiacus illustrates on extreme of antenna design. In most natural antennas, the pigments are held in place by the protein matrix, or scaffolding, but in the chlorosome, there’s virtually no protein, and the 10,000 bacteriochlorophyll c pigments that make up most of the antenna are stuck directly together.

This antenna has another interesting property. If the chlorosomes are torn apart with polar solvents, and the pigments are isolated and concentrated in solution, they spontaneously form aggregates that are very similiar spectro-scopically to the intact chlorosomes. Since aggregates of most other pigments act as light traps rather than as antennas, this suggests that bacteriochlorophyll c is capable of a unique kind of self-organization.

Because of the proximity of the pigments in the chlorosomes and their alignment, once energy is absorbed it is rapidly spread over about a thousand pigments. this extensive delocalization, known as exciton coupling, is a fast mechanism of energy transfer. Indeed the chlorosome is as close as nature comes to an ideal excitation conductor, or excitation wire.

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A Slow Natural Antenna

The phycobilisome found in blue-green bacteria, such as those in the genus Synechococcus, illustrates another extreme of antenna design. The phycobilisome is a multiprotein complex made up primarily of biliproteins, a family of brightly colored proteins. In Synechococcus, there are six rod substructures, each of which consists of three disks. The disks in turn are made up of six molecules of a biliprotein held together by a linker polypeptide.

Phycobilisomes have two unusual properties. One is that the pigments are much farther apart than those in other systems. for this reason, energy transfer between them is by the relatively slow mechanism of Forster transfer.

The second unusual property is that the pigments are arranged according to their spectral forms, with the pigments that fluoresce at the shortest wavelengths (the hightest energy) at the core end of the rods. Energy transfer within the rods is from the tips to the core, that is, down the energy gradient. The pigments are so far apart that energy transfer must be directional. If it were random, the energy would be lost to fluorescence or other dissipative processes long before it reached the core.

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