Stretching DNA: Role of electrostatic interactions

The effect of electrostatic interactions on the stretching of DNA is invest igated using a simple worm like chain model. In the limit of small force th ere are large conformational fluctuations which are treated using a self-co nsistent variational approach. For small values of the external force f, we find the extension scales as fr(D) where r(D) is the Debye screening lengt h. In the limit of large force the electrostatic effects can be accounted f or within the semiflexible chain model of DNA by assuming that only small e xcursions from rod-like conformations are possible. In this regime the exte nsion approaches the contour length as 1/root f where f is the magnitude of the external force. The theory is used to analyze experiments that have me asured the extension of double-stranded DNA subject to tension at various s alt concentrations. The theory reproduces nearly quantitatively the elastic response of DNA at small and large values of f and for all concentration o f the monovalent counterions. The limitations of the theory are also pointe d out.