  We consider a biopolymer bundle consisting of filaments that are crosslinked
together. The crosslinks are reversible: they can dynamically bind and unbind
adjacent filament pairs as controlled by a binding enthalpy. The bundle is
subjected to a bending deformation and the corresponding distribution of
crosslinks is measured. For a bundle consisting of two filaments, upon
increasing the bending amplitude, a first-order transition is observed. The
transition is from a state where the filaments are tightly coupled by many
bound crosslinks, to a state of nearly independent filaments with only a few
bound crosslinks. For a bundle consisting of more than two filaments, a series
of first-order transitions is observed. The transitions are connected with the
formation of an interface between regions of low and high crosslink densities.
Combining umbrella sampling Monte Carlo simulations with analytical
calculations, we present a detailed picture of how the competition between
crosslink shearing and filament stretching drives the transitions. We also find
that, when the crosslinks become soft, collective behavior is not observed: the
crosslinks then unbind one after the other leading to a smooth decrease of the
average crosslink density.
