  We derive the single molecule equation of state (force-extension relation)
for DNA molecules bearing sliding loops and deflection defects. Analytical
results are obtained in the large force limit by employing an analogy with
instantons in quantum mechanical tunneling problems. The results reveal a
remarkable feature of sliding loops - an apparent strong reduction of the
persistence length. We generalize these results to several other experimentally
interesting situations ranging from rigid DNA-protein loops to the problem of
anchoring deflections in AFM stretching of semiflexible polymers. Expressions
relating the force-extension measurements to the underlying loop/boundary
deflection geometry are provided and applied to the case of the GalR-loop
complex. The theoretical predictions are complemented and quantitatively
confirmed by MD simulations.
