Persistence length

The persistence length $L_p$ is the length scale over which a polymer’s tangent direction stays correlated — roughly, how far you can travel along the chain before thermal bending randomizes its orientation.

It ties the bending stiffness $\kappa$ to thermal energy:

$L_p=\frac{\kappa }{k_{B}T}$

For double-stranded DNA, $L_p\approx 50\text{nm}$ (about 150 base pairs). Below this length DNA behaves like a stiff rod; far above it, like a flexible entropic coil — the regime made tangible in the wlc-force-extension explorer, where $L_p$ sets the scale of the entropic restoring force.