MIT researchers find that the key to purple bacteria’s light-harvesting prowess lies in highly symmetrical molecules.
Purple bacteria are among Earth’s oldest organisms, and among its most efficient in turning sunlight into usable chemical energy. Now, a key to their light-harvesting prowess has been explained through a detailed structural analysis by scientists at MIT.
A ring-shaped molecule with an unusual ninefold symmetry is critical, the researchers found. The circular symmetry accounts for its efficiency in converting sunlight, and for its mechanical durability and strength. The new analysis, carried out by professors of chemistry Jianshu Cao and the late Robert Silbey, postdoc Liam Cleary, and graduate students Hang Chen and Chern Chuang, has been published in Proceedings of the National Academy of Sciences.
“The symmetry makes the energy transfer much more robust,” Cao says. “Most biological systems are quite soft and disordered. You would not expect a regular structure, almost a perfect structure,” as is found in this primitive microbe, he says.
In these regular round complexes, Cao says, “nature only used certain symmetry numbers: mostly ninefold, some eightfold, very few tenfold. It’s very selective.” His group’s mathematical analysis shows there are good reasons for that, he says.
These ring-shaped molecules, in turn, are arranged in a hexagonal pattern on the spherical photosynthetic membrane of purple bacteria, Cao says.
“With these symmetry numbers, the interactions between all pairs of the symmetric rings are optimized at the same time. … We believe that nature found the most robust structures in terms of energy transfer,” Cao says. Both eightfold and tenfold symmetries also work, though not as well: Only a lattice made up of ninefold symmetric complexes can tolerate an error in either direction. “You want consecutive numbers so it can tolerate such mistakes,” Cao says.