New molecular force probe stretches molecules, atom by atom

Mar 29, 2009

Chemists at the University of Illinois have created a simple and inexpensive molecular technique that replaces an expensive atomic force microscope for studying what happens to small molecules when they are stretched or compressed.

The researchers use stiff stilbene, a small, inert structure, as a molecular force probe to generate well-defined forces on various , atom by atom.

"By pulling on different pairs of atoms, we can explore what happens when we stretch a molecule in different ways," said chemistry professor Roman Boulatov. "That information tells us a lot about the properties of fleeting structures called transition states that govern how, and how fast, chemical transformations occur."

Boulatov, research associate Qing-Zheng Yang, postdoctoral researcher Daria Khvostichenko, and graduate students Zhen Huang and Timothy Kucharski describe the molecular force probe and present early results in a paper accepted for publication in Nature Nanotechnology. The paper is to be posted on the journal's Web site on Sunday (March 29).

Similar to the force that develops when a rubber band is stretched, restoring forces occur in parts of molecules when they are stretched. Those restoring forces contain information about how much the molecule was distorted, and in what direction.

The molecular force probe allows reaction rates to be measured as a function of the restoring force in a molecule that has been stretched or compressed.

This information is essential for developing a chemomechanical kinetic theory that explains how force affects rates of chemical transformations.

Such a theory will help researchers better understand a host of complex phenomena, from the operation of that underlie the action of muscles, to the propagation of cracks in polymers and the mechanisms by which living cells sense forces in their surroundings.

"Localized reactions offer the best opportunity to gain fundamental insights into the interplay of reaction rates and molecular restoring forces," Boulatov said, "but these reactions are extremely difficult to study with a microscopic force probe."

Microscopic force probes, which are utilized by atomic force microscopes, are much too large to grab onto a single pair of atoms. Measuring microns in size, the probe tips contact many atoms at once, smearing experimental results.

"By replacing microscopic force probes with small molecules like stiff stilbene, we can study the relationship between restoring force and reaction rate for localized reactions," Boulatov said. "The more accurately we know where our probe acts, the better control we have over the distortion, and the easier it is to interpret the results."

Using conventional methods, Boulatov and his students first attach stiff stilbene to a molecule they wish to study. Then they irradiate the resulting molecular assembly with visible light. The light causes the stilbene to change from a fully relaxed shape to one that exerts a desired force on the molecule. The chemists then measure the reaction rate of the molecule as a function of temperature, which reveals details of what caused the reaction to accelerate.

One type of chemical transformation the researchers studied is the breaking of one strong (covalent) chemical bond at a time. The experimental results were sometimes counterintuitive.

"Unlike a rubber band, which will always break faster when stretched, pulling on some chemical bonds doesn't make them break any faster; and sometimes it's a bond that you don't pull on that will break instead of the one you do pull," Boulatov said. "That's because experiences in the macroscopic world do not map particularly well to the molecular world."

Molecules do not live in a three-dimensional world, Boulatov said. Molecules populate a multi-dimensional world, where forces applied to a pair of atoms can act in more than three dimensions.

"Even small molecules will stretch and deform in many different ways," Boulatov said, "making the study of molecular forces even more intriguing."

Source: University of Illinois at Urbana-Champaign (news : web)

Explore further: Scanning tunnelling microscopy: Computer simulations sharpen insights into molecules

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laserdaveb
4 / 5 (1) Mar 30, 2009
Molecules do not live in a three-dimensional world, Boulatov said. Molecules populate a multi-dimensional world, where forces applied to a pair of atoms can act in more than three dimensions.

thats a stretch...
latersville
not rated yet Mar 30, 2009
Yeah, that statement stood out to me as well. Either he was misquoted or he's trying to apply quantum mechanics at the wrong scale. If molecules don't live in a three-dimensional world, would he mind explaining how us mere three-dimensional creatures create and manipulate them? He also says ...

"By replacing microscopic force probes with small molecules like stiff stilbene, we can study the relationship between restoring force and reaction rate for localized reactions," Boulatov said. "The more accurately we know where our probe acts..."

Those sentences scream with three-dimensional references. "...accurately we know WHERE..." implies a precise coordinate system. As far as I know, human scientific equipment is based upon XYZ axes.

Despite that momentary lapse in logical thought, the method is quite elegant. Bravo, Professor!
laserdaveb
not rated yet Mar 31, 2009
yea..i didnt mean to imply i thought any less of the work...ill second that bravo!
latersville
not rated yet Apr 01, 2009
No sir, I wasn't questioning your comment at all. Actually, I thought "that's a stretch" was a clever pun!
laserdaveb
not rated yet Apr 02, 2009
cool! glad you got the pun. i do think that the qm behaviour at atomic level does extend to molecular level,and even to substantive material properties. im looking forward to what more they learn with this technique.

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