For the first time in 50 years, researchers say they have identified a new category of stereoisomers (Nat. Chem. 2018, DOI: 10.1038/s41557-018-0043-6).

The new class is characterized by a bond-angle inversion in which the central atom in a bent, singly bonded trio of different atoms flexes in the opposite direction. If this trio were an independent molecule, its two different isomers would be impossible to isolate because a simple rotation of the bonds would return the molecule to its original configuration. When placed in the middle of a larger molecule, however, that rotation is blocked, and the situation changes. Peter J. Canfield, a Ph.D. student advised by Jeffrey R. Reimers of Shanghai University and the University of Technology Sydney and Maxwell J. Crossley of the University of Sydney uncovered this isomerism while making porphyrin macrocycles with a boron-oxygen-boron bridge.

Unable to name the four versions of the product they had made using standard nomenclature, the researchers are arguing for a new category of stereoisomer. The porphyrin ring in the team’s compounds prevents rotation around the boron-oxygen bonds, enabling isomer isolation. The researchers call these akamptisomers, after the Greek word for inflexible. They synthesized four of the eight predicted akamptisomers and enantiomers of their product. Density functional calculations suggest the other four are less stable and may be impossible to produce.

Reimers says aspects of what the team discovered were already known but argues that recognizing these isomers as a defined group will help researchers determine whether the compounds have important new properties. Several patents have been filed related to the discovery. Like other kinds of stereoisomers, akamptisomers’s ability to assume different shapes with similar chemistry could be useful in making drug molecules. They could also act as molecular switches in electronics, although Canfield notes that commercial devices are likely years away. “At this stage we only dream about practical significance, but the mathematical significance of our discovery is clear and timeless,” Reimers says.