Researchers supported by the National Institute of Biomedical Imaging and Bioengineering (NIBIB) have developed a new method to study the behavior of molecules, particularly how they interact with each other.
In a study appearing in the September 21 issue of Science, chemists at Vanderbuilt University report that a new and deceptively simple technique – shining a red laser like those used in barcode scanners into a microscopic, liquid-filled chamber where two kinds of molecules are mixed -- can measure the interactions between free-floating biological molecules including proteins, sugars, antibodies, DNA, and RNA. In fact, the researchers have demonstrated that it is sensitive enough to detect the process of protein folding.
The method represents an entirely new application of interferometry, a powerful technique that combines light from multiple sources to make precise measurements. Interferometry is used in everything from astronomy to holography to geodetic surveys to inertial navigation. The researchers call the new method back-scattering interferometry or BSI.
The equipment required for the new biosensor is surprisingly modest: a helium-neon laser like those used in grocery store scanners, a mirror, a charge-coupled device or 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 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 some of the light is transmitted back up to the mirror where it is directed to the detector. There it forms a line of alternating light and dark spots called an interference pattern.
It turns out that 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.
The underlying physics of this highly sensitive measurement technique are still being worked out. The researchers know that it responds to minute changes in the index of refraction, which is a measure of how fast the light travels through the liquid in the chamber compared to its speed in a vacuum. They hypothesize that it has to do with the rearrangement in the water molecules that cover the surface of the proteins: When two proteins react they squeeze the water molecules out of the area where they bind together. This displacement changes the density of the liquid slightly which, in turn, alters its index of refraction.
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, Inc. Bo is one of the founders of the start-up and serves as its chief scientist. The company plans on completing a prototype system this fall.
Additional information including images can be found on Vanderbilt University's website.
Bornhop DJ, Latham JC, Kussrow A, Markov DA, Jones RD, Sørensen HS. Free-Solution, Label-Free Molecular Interactions Studied by Back-Scattering Interferometry. Science 21 September 2007 317: 1732-1736 [DOI: 10.1126/science.1146559].