Stretchy Insights

Pulling on a rubber band seems like a good analogy for stretching a bond, but chemists are learning that what happens in our world doesn't always hold up on a molecular scale. The findings, presented to the Division of Organic Chemistry on Aug. 19 at the ACS national meeting in Washington, D.C., illuminate how molecules respond to strain and might have implications in biology and materials science.

Analyzing how strain affects individual chemical bonds is difficult with established force measurement techniques such as atomic force microscopy because small-scale events tend to be obscured. So chemistry professor Roman Boulatov and coworkers at the University of Illinois, Urbana-Champaign, designed a miniature force probe. They connect a rigid hydrocarbon that he calls stiff stilbene to molecules of interest via inert organic linkers. Shining light on stiff stilbene triggers a dramatic change in its shape, which stretches the molecule under observation in a controllable manner (Nat. Nanotech. 2009, 4, 302).

Boulatov's probes allow reaction rates to be quantified as a function of restoring force. Similar to the force that develops when stretching a rubber band, the molecular restoring force describes how much a molecule was distorted and in what direction. "This technology should allow researchers to probe the structures of transition states, which are difficult to observe experimentally," says Duke University mechanochemistry researcher Stephen L. Craig, who did not attend the meeting.

The probe has already led to surprising observations. "Unlike a rubber band, which always breaks faster when stretched, pulling on some chemical bonds doesn't make them break any faster," Boulatov said. "Sometimes it's a bond that you don't pull on that will break instead of the one you do pull."

For instance, in preliminary work presented at the meeting, Boulatov showed that hydrolysis of one P–O bond in a triphosphoester speeds up when bonds perpendicular to it are stretched. Although P–O bond hydrolysis is highly biologically relevant, it's too early to speculate whether this kind of stretching happens in nature, Boulatov added.