BIOPHYSICAL THEORY AND MODELING |
DNA Overstretching Transition: Ionic Strength Effects
Olli Punkkinen 1, Per Lyngs Hansen 2, Ling Miao 2 and Ilpo Vattulainen 1*
1 Laboratory of Physics and Helsinki Institute of Physics, Helsinki University of Technology
2 Physics Department, University of Southern Denmark
* To whom correspondence should be addressed. E-mail: ilpo.vattulainen{at}csc.fi.
Submitted on March 17, 2005
Revised on May 4, 2005
Accepted on 25 May 2005
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Abstract |
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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 the contour length, it further shows a highly cooperative overstretching transition. We have developed a theoretical description for this transition by coupling the two-state model proposed by Ahsan et al. [Biophys. J. 74, 132-137 (1998] and the elasticity theory of Podgornik et al. [J. Chem. Phys.113, 9343-9350 (2000)]. Furthermore, we have extended this model to account for monovalent salt effects on elastic moduli during the transition. We find that the present theoretical description is in very good agreement with recent measurements by Wenner et al. for the salt-dependence of the overstretching transition [Biophys. J. 82, 3160-3169 (2002)], allowing us to gain insight into the mechanisms that govern the transition. In double-stranded DNA, the effective length per unit charge varies with salt in agreement with the Manning and Poisson-Boltzmann models for thin polyelectrolyte rods, while the other model parameters describing structural features have barely any salt dependence. The results thus suggest that the electrostatic component of force-induced overstretching is mediated mesoscopically via elasticity.
Key Words:
DNA, electrostatics, overstretching transition, salt effects, single-molecule behavior
