Scientists use a light beam to separate proteins on a chip.
Researchers have demonstrated novel use of a beam of light to trap proteins and make them "dance," similar to Star Trek's imaginary "tractor beam" projected at something to pull it from one spot to another. The demonstration is described in a paper that appears in a recent issue of the Proceedings of the National Academy of Sciences Early Edition.
The technique—developed by a team from ORNL, California Institute of Technology and Protein Discovery—is more than just a novelty that turns science fiction into science. The discovery is useful for separating, concentrating and analyzing proteins rapidly with high sensitivity and selectivity.
"With this technique, we can steer DNA, proteins and other biomolecules for transport in three dimensions," says Chuck Witkowski, a co-author and president and chief executive officer of Protein Discovery, a Knoxville startup company. "We can also separate biomolecules according to their size and isoelectric point. The ability to perform these functions with high efficiency and precision has applications for diagnosing disease and making new medical discoveries."
The technique, called photoelectrophoretic localization and transport (PELT), involves shining a focused beam of light on semiconductor material to move the proteins. Force-field traps are created by the localized photocurrent, which produces electric fields that induce protein motion. In contrast to traditional gel electrophoresis, which uses high voltage, this approach permits researchers to change dynamically characteristics of the electric field in three dimensions in real time using computer-controlled software and low voltage.
PELT is extremely versatile and offers several advantages over methods that use conventional electrophoresis, according to co-author Thomas Thundat, a group leader in ORNL's Life Sciences Division. "This technique provides an easier way to separate proteins and other biomolecules. In addition to applications for diagnostics, this discovery tool allows researchers to investigate photo-induced effects of a semiconductor-liquid interface."
The new method also overcomes limitations of conventional optical trapping techniques, commonly called optical tweezers. Although versatile, optical tweezers are unable to transport objects much smaller than the wavelength of light, such as DNA fragments, oligonucleotides, proteins and peptides. Instead, such small molecules must first be attached to larger particles, called handles. This and other techniques have significant limitations, according to authors of the paper.
PELT both separates and concentrates proteins. When light from a laser diode shines on a gel-coated semiconductor chip placed under a positive electric potential, charge carriers are produced inside the semiconductor where the light is falling. Negatively charged carriers rush to the semiconductor-liquid interface, which then attracts the positively charged proteins. The smaller the activated spot, the more concentrated the accumulated proteins are. When the light is scanned across the surface of a gel containing a sieving medium, the smaller proteins skip along faster than the heavier ones chasing the light, resulting in separation.
Researchers can assess how much two different molecules, such as a disease protein and potential drug, interact at the illuminated spot by measuring changes in the photocurrent level. The approach might be useful for drug discovery.
Although photoelectrophoretic localization and transport holds tremendous promise, Witkowski says that much work remains to commercialize the technology. In time, however, he envisions a significant commercial role for this technology in the medical field, specifically for disease diagnostics.
Other authors of the paper are Nathan Lewis, professor at the California Institute of Technology; Dean Hafeman and James Harkins IV, both of Protein Discovery; Bruce Warmack of ORNL's Engineering Science and Technology Division, and Gil Brown of ORNL's Chemical Sciences Division. The paper will appear in an upcoming issue of Applied Physical Sciences, Biophysics, published by the National Academy of Sciences.
Funding for this research was provided in part by the Department of Energy's Office of Biological and Environmental Research. Protein Discovery is a privately held life sciences company that is developing solutions for molecular research, drug discovery and development and medical diagnostics using high-throughput mass spectrometry.
Web site provided by Oak Ridge National Laboratory's Communications and External Relations