Figure 3.

Force-extension data of short, double-stranded DNA attached to a surface and an atomic force microscope cantilever via a 5-kDa poly(ethylene glycol) linker for each strand. Data for 20 pulling experiments at a separation velocity of 1 μm/s was binned into 1-pN intervals and averaged (circles). The solid line is the corresponding fit of the model presented here. The dashed line represents the fit in the case where the DNA duplex remains in its canonical B-form. At <30 pN, the fit underestimates forces, an observation that we attribute to nonspecific interactions and entanglements with neighboring constructs on the surface. (Inset) For forces >30 pN, theory and experimental data agree within the experimental error.