When a single molecule of double-stranded DNA is stretched beyond its B-form contour length, the measured force shows a highly cooperative overstretching transition. We have measured the force at which this transition occurs as a function of temperature. To do this, single molecules of DNA were captured between two polystyrene beads in an optical tweezers apparatus. As the temperature of the solution surrounding a captured molecule was increased from 11°C to 52°C in 500 mM NaCl, the overstretching transition force decreased from 69 pN to 50 pN. This reduction is attributed to a decrease in the stability of the DNA double helix with increasing temperature. These results quantitatively agree with a model that asserts that DNA melting occurs during the overstretching transition. With this model, the data may be analyzed to obtain the change in the melting entropy ΔS of DNA with temperature. The observed nonlinear temperature dependence of ΔS is a result of the positive change in heat capacity of DNA upon melting, which we determine from our stretching measurements to be ΔCp = 60 ± 10 cal/mol K bp, in agreement with calorimetric measurements.
Bibliographical noteFunding Information:
Funding for this project was provided by grants from the National Institutes of Health (GM28093) and the National Science Foundation (MCB9728165).
We thank Prof. Matthew Tirrell and the University of Minnesota Center for Interfacial Engineering for funding and assistance in starting the optical tweezers project. We are grateful to Drs. Steve Smith and Christoph Baumann for help with protocols and instrument-building advice. We also thank Dori Henderson for taking the time to make a number of glass micropipettes for use in our experiments.