Boundary-condition dependent elasticity of short double-stranded DNA molecules

We have measured the entropic elasticity of ds-DNA molecules ranging from 247 to 1298 base pairs in length, using axial optical force-clamp tweezers. We show that entropic end effects and excluded-volume forces become significant for such short molecules. In this geometry, the effective persistence length of the shortest molecules decreases by a factor of two compared to the established value for long molecules, and excluded-volume forces extend the molecules to about one third of their nominal contour lengths in the absence of any external forces. We interpret these results in the framework of a modified wormlike chain model.