Shining light on the fleeting interactions of single molecules

This method provides scientists a new, more direct way to measure binding and aggregation properties of single molecules in solution, and is applicable to numerous fields including biology, chemistry, materials science, and statistical physics and mechanics. Currently, the method is being used to assess the optical properties of photosynthetic antenna proteins in various states of assembly, and to measure other protein-protein interactions at the single-molecule level.

Many biological and chemical processes critically depend on molecules interacting in aqueous solution. However, watching these molecular encounter events is not easy due to the perpetual thermal agitation of the surrounding water molecules, and nanometer-sized biomolecules quickly diffuse out of the field-of-view of any conventional microscope. Building on the work that recently earned him a Nobel Prize in Chemistry, a Stanford University researcher used a microfluidic single-molecule trap to capture individual molecules in solution without any perturbations due to surface attachment. The trap compensates a single molecule's Brownian diffusive motion by continuously applying electric fields in solution that drive the molecule back to the center of the field-of-view.

While trapped, the molecule's residual movement can be precisely analyzed to yield size and charge sensitive motion parameters in real time (specifically, diffusion coefficient and mobility). Binding and dissociation events are directly visualized by fluctuations in those motion parameters. The methodology is demonstrated on two model systems: the time-dependent spontaneous dissociation of photosynthetic light-harvesting antenna complexes and direct observation of DNA strands binding and unbinding in solution.

This work was funded in part by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Additional funding was provided by Stanford University.