Bending Elasticity of the reversible Freely Jointed Chain

The freely jointed chain model with reversible hinges (rFJC) is the simplest theoretical model that captures reversible transitions of the local bending stiffness along the polymer chain backbone, e.g. helix-coil-type of local conformational changes or changes due to the binding/unbinding of ligands). In this work, we analyze the bending fluctuations and the bending response of a grafted rFJC in the Gibbs (fixed-force) ensemble. We obtain a recursion relation for the partition function of the grafted rFJC under bending force, which allows, in principle, exact-numerical calculation of the behavior of a rFJC of arbitrary size. In contrast to stretching, we show that under sufficiently stiff conditions, the differential bending compliance and the mean fraction of closed hinges are non-monotonic functions of the force. We also obtain the persistence length $L_p$ of the rFJC, the moments $⟨R^2⟩$ (mean-square end-to-end distance), and $⟨z^2⟩$ (mean-square transverse deflection) for the discrete chain and take the continuum limit. The tangent vector auto-correlation decays exponentially, as in the wormlike chain model (WLC). Remarkably, the expression of $⟨R^2⟩$ as a function of the contour length $L$ becomes the same as that in the WLC. In the thermodynamic limit, we have calculated the exact bending response analytically. As expected, for $L\gg L_p$, the boundary conditions do not matter, and the bending becomes equivalent to stretching. In contrast, for $L_p\gg L$, we have shown the non-monotonicity of the bending response (the compliance and mean fraction of closed hinges).