Research: Laser Tweezers and Single Molecular Biophysics
The discovery of optical trapping and laser tweezers in the 1980s offers an unprecedented opportunity to study chemical and biochemical processes at the single molecular level. Click here for the mechanism of the laser trapping. Unlike bulk assays where ensemble average information is obtained, single molecule experiments can reveal properties of subpopulations and the energetics of sub-trajectories. Laser tweezers have another unique capability to measure or apply forces from femto- to hundrends of pico-Newtons. These forces are in the same range of those generated by most protein motors, or needed to alter the rate or fate of many biochemical reactions. As a result, biological systems and processes have been studied using laser tweezers. However, there is a lag of laser-tweezers application in chemistry. This can be attributed to the following reasons. First, tiny amount of the material contained inside a trapped object prevents the use of many traditional detection methods, such as UV-vis and IR. Second, to build a strong optical trap, objectives with short working distance are often used. This leaves little accessible space to incorporate other detection methods. Our lab explores unique capabilities of the laser tweezers, i.e., force detection in the range of picoNewtons and spatial measurement down to Angstroms, to follow chemical and biochemical interactions, such as the binding events between receptors and ligands. In addition, highly sensitive biosensors are developed in the lab by a combination of laser tweezers and microfluidics.
Since 2009, we poineered the research on the mechanical stabilities of non-B DNA structures, G-quadruplex and i-motif in particular. Since the nineteen fifties, DNA has been known to serve as a storage for genetic codes only. Correponding to this sole physiological role is the predominant right-handed double helix conformation (a.k.a. B DNA) in vivo. This dogma has not been seriously challenged until people found non-B DNA conformations exist in vivo. Now, it has been recognized that the sites to host these non-B DNA strcutures are throughout the human genome, most of which are located in the vast non-coding regions. In vivo evidences have further pointed out that these structures will function as regulatory elements to control important cellular processes such as Transcription and Replication. Our studies have so far indicated that the mechanical strength of G-quadruplex and i-motif exceeds the maximal load force of DNA or RNA polymerase, which are key enzymes for DNA replication and RNA transcription, respectively. These provide a strong support, from mechanical perspective only, that the formation of non-B DNA structures may stop replication or transcription processes.
The experiment is performed as follows. First, we grab a non-B DNA structure using two optically trapped particles. By moving one of the particles away from the other, the strucutre is mechanically unfolded (see below). Notice the double stranded DNA (dsDNA) serves as handles to isolate the non-B DNA structure from the particle surfaces.
