(550b) Stretched Exponential Kinetics in Thermal Dissociation of Surface-Bound DNA Hairpins

Over the past decade, groups have used a variety of single molecule techniques to study the folding/unfolding kinetics of DNA, RNA, proteins and other biomolecules. In our work, we probe the force dependence of DNA hybridization kinetics by measuring the lifetime distribution of single DNA hairpins under thermal dissociation. We use a scanning line optical tweezers instrument to measure the bound lifetimes of two DNA-coated microspheres under negligible applied tension. The two microspheres share a user-specified potential along the scan direction and are strongly confined in the perpendicular dimensions. The trapping laser intensity is modulated synchronously with a resonant scanner to null all optical contributions to the pair interaction potential near contact. In addition, the laser polarization can be rotated to produce a continuously adjustable optical repulsion, allowing the instrument to double as a force clamp over a modest range of applied tensions. Contrary to expectations for an all-or-none transition, we measure a stretched exponential lifetime distribution for the closed state of a DNA hairpin. We suggest that this unexpected distribution is a result of a slowly varying entropic tension due to the tethered microsphere diffusion and develop a simple model that is capable of explaining both exponential and nonexponential lifetime distributions that have been reported in a number of biophysical experiments.