  It has become clear in recent years that the simple uniform wormlike chain
model needs to be modified in order to account for more complex behavior which
has been observed experimentally in some important biopolymers. For example,
the large flexibility of short ds-DNA has been attributed to kink or hinge
defects. In this paper, we calculate analytically, within the weak bending
approximation, the force-extension relation of a wormlike chain with a
permanent hinge defect along its contour. The defect is characterized by its
bending energy (which can be zero, in the completely flexible case) and its
position along the polymer contour. Besides the bending rigidity of the chain,
these are the only parameters which describe our model. We show that a hinge
defect causes a significant increase in the differential tensile compliance of
a pre-stressed chain. In the small force limit, a hinge defect significantly
increases the entropic elasticity. Our results apply to any pair of
semiflexible segments connected by a hinge. As such, they may also be relevant
to cytoskeletal filaments (F-actin, microtubules), where one may treat the
cross-link connecting two filaments as a hinge defect.
