Sorting fullerenes by their handedness.

Many organic molecules can exist as right- and left-handed versions, or enantiomers (SN: 5/29/93, p. 348), but chemicals consisting purely of one element generally come in only one form. For example, diamond and graphite -- both pure carbon - don't form enantiomers because their architectures are symmetrical, making handedness impossible.

However, some of the all-carbon molecular cages known as buckminsterfullerenes do exist as enantiomers. When researchers synthesize these 60-atom buckyballs, the soot they produce also contains a 76-carbon molecule shaped like a squashed sphere. This fullerene occurs as a 50-50 mixture of its right- and left-handed forms.

Now, two University of California, Berkeley, chemists have found a way to separate the two subtly different fullerene forms - a significant accomplishment considering that the two enantiomers of the 76-carbon "rugbyball" vary by an almost imperceptible twist. "Only when you look at the molecule from a particular orientation does it even appear lopsided;' notes Joel M. Hawkins, who with co-worker Axel Meyer reported the results of their experiment in the June 25 SCIENCE.

The researchers employed a chemical reaction called osmylation that selectively uses up one enantiomer faster than the other. They reacted a 50-50 mixture of carbon-76 molecules with an osmium compound and a plant alkaloid that preferentially grabs one version of the molecule (SN: 1/2/93, p.6). The alkaloid appears to tell one enantiomer from the other by how well it fits with the puckered surface at two of the molecule's 30 types of bonds, Hawkins says. He and Meyer stopped the reaction just before completion and found that the remaining unreacted fullerene contained 98 percent of one enantiomer. They then treated the osmylated product to recover the other enantiomer.

The researchers found that the pure right- and left-handed versions of the rugbyball dramatically rotated the direction of plane-polarized light rays. Such molecules may one day prove useful as optical materials (SN: 8/24/91, p. 127).