Direct laser trapping of single DNA molecules in the globular state

A sharply focused laser is able to trap small particles at the laser focal point due to the difference in refractive index of the particles and that of the surrounding medium. This technique, called laser trapping, can be used to manipulate animal or bacterial cells without any contact and has been widely applied in biological research. However, it has been difficult to trap biological macromolecules such as DNA molecules, because these molecules give a low difference in refractive index and cannot overcome Brownian motion. DNA molecules can be transformed to a condensed globular state. This transformation results in a higher refractive index of DNA due to its increased density. We demonstrate in this paper that a single DNA molecule can be optically trapped using a Nd:YAG laser (1064 nm wavelength) upon transformation from the coiled state to the globular state.