Comment on "Theory of high-force DNA stretching and overstretching"
  Recently Storm and Nelson [1] (Phys.Rev. E67, 51906 (2003)) introduced the discrete persistent chain model which contains both features of the freely jointed chain (FJC) and the wormlike chain (WLC) models. Equation (20) of their paper is correct only in a special case of large l, the ratio of the persistence length to the monomer length. This special case is unnecessary because the general case can be studied just as easily. Working out the general case, we obtain the force extension relation correct for all values of the parameter l. This force extension relation reduces to the FJC result at small l and to the WLC at large l. At small force, it reduces to the result of Rosa et al (cond-mat/0307015). 
Pui-Man Lam