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Molecular ring mimics photosynthetic machinery

Spoke-like templates bend a linear chain of 24 porphyrins into a light-harvesting hoop

by Mark Peplow, special to C&EN
October 25, 2022


Structure of a molecular ring of 24 porphyrin units
Credit: Nat. Chem.
A chain of 24 porphyrins gets snapped closed to form a molecular ring by the coupling of two alkyne groups.

By bending a linear chain of porphyrin units into a molecular ring, researchers have mimicked a key structure that enables photosynthesis in purple bacteria (Nat. Chem. 2022, DOI: 10.1038/s41557-022-01032-w).

The natural light-harvesting system LH2 contains two concentric rings of bacteriochlorophyll molecules, each about 6 nm wide, which absorb sunlight and rapidly funnel the energy to a reaction center. Although the chlorophylls are not directly connected, proteins hold them very close together, and they can channel a cascade of energy in less than 200 fs.

Chemists have made various synthetic rings of porphyrins that serve as models for LH2, but most of them transport energy in a different way—or much more slowly—than the natural versions.

A team led by Harry L. Anderson at the University of Oxford has now created a closer mimic from a chain of 24 porphyrins, capped at either end by alkynes. A pair of asterisk-shaped template molecules bends the chain into a circle, before a palladium catalyst links the alkynes to close the ring. “Before this, we weren’t really sure that you could bend that sort of molecule into a ring at all,” Anderson says.

After freeing the 6 nm wide ring from its template, the researchers found that it could absorb light and funnel about 30% of the energy to the alkyne-linked portion within 350 fs, with the remainder following soon after.


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