Measuring molecular interactions

A simple instrument that uses the principle of back-scattering interferometry has been developed by chemists at Vanderbilt University.

A simple instrument that uses the principle of back-scattering interferometry (BSI) has been developed by chemists at Vanderbilt University to measure the interactions between free-floating, unlabelled molecules including proteins, sugars, antibodies, DNA and RNA.

By shining a red laser into a microscopic, liquid-filled chamber where two kinds of molecules are mixed, the instrument can measure the strength with which they react, even when the interactions are extremely weak. In fact, the researchers have demonstrated that it is sensitive enough to detect the process of protein folding.

‘Pharmaceuticals depend on reactions between proteins and small molecules or between pairs of proteins, or interactions between RNA and DNA or pairs of DNA molecules. So the ability to measure how that happens is very advantageous,’ said Darryl J. Bornhop, the Prof of Chemistry who headed the 12-year development process.

The equipment required for the sensor is surprisingly modest: a helium-neon laser, a mirror, a CCD detector like those used in digital cameras and a special glass microfluidic chip. The chip contains a channel about one-fiftieth the size of a human hair. There is a “Y” at one end of this channel that allows the researchers to inject two solutions simultaneously, each containing a different kind of molecule. It is followed by a serpentine section that mixes the two.

Finally, there is a straight observation section where the interactions are measured. An unfocused laser beam is directed through the channel at this point. The beam is reflected back and forth inside the channel about 100 times. Each time the light beam strikes the channel wall, some of the light is transmitted back up to the mirror, and from there it is directed to the detector. There the light forms a line of alternating bright and dark spots called an interference pattern.

Graduate student Amanda Kussrow injects a solution containing sugar molecules into the back-scattering interferometer

The interference pattern is very sensitive to what the molecules are doing. If the molecules begin sticking together, for example, the pattern begins to shift. The stronger the binding force between the molecules, the larger the shift. This allows the system to measure interaction forces that vary a million-fold. That includes the entire range of binding forces found in living systems.

Vanderbilt has applied for and received two patents on the process and has several other patents pending. The university has issued an exclusive license to develop the technology to Molecular Sensing. Bornhop is one of the founders of the start-up and serves as its chief scientist.