As double-stranded DNA is stretched to its B-form contour length, models of polymer elasticity can describe the dramatic increase in measured force. When the molecule is stretched beyond this contour length, it shows a highly cooperative overstretching transition. We have measured the elasticity and overstretching transition as a function of monovalent salt concentration by stretching single DNA molecules in an optical tweezers apparatus. As the sodium ion concentration was decreased from 1000 to 2.57 mM, the persistence length of DNA increased from 46 to 59 nm, while the elastic stretch modulus remained approximately constant. These results are consistent with the model of Podgornik et al. (2000, J. Chem. Phys. 113:9343-9350) using an effective DNA length per charge of 0.67 nm. As the monovalent salt concentration was decreased over the same range, the overstretching transition force decreased from 68 to 52 pN. This reduction in force is attributed to a decrease in the stability of the DNA double helix with decreasing salt concentration. Although, as was shown previously, the hydrogen bonds holding DNA strands in a helical conformation break as DNA is overstretched, these data indicate that both DNA strands remain close together during the transition.
Bibliographical noteFunding Information:
Funding for this project was provided by grants from National Institutes of Health (GM28093) and 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, and to Dori Henderson for taking the time to make a number of glass micropipettes for use in our experiments. We also thank the anonymous reviewers for helping us to clarify some points in the manuscript.