  Hydrophobic polyelectrolytes (HPE) can solubilize bilayer membranes, form
micelles or can reversibly aggregate as a function of pH. The transitions are
often remarkably sharp. We show that these cooperative transitions occur by a
competition between two or more conformational states and can be explained
within the framework of Monod - Wymann - Changeux (MWC) theory that was
originally formulated for allosteric interactions. Here we focus on the
pH-dependent destabilization and permeation of bilayer membranes by HPE. We
formulate the general conditions that lead to sharp conformational transitions
involving simple macromolecules mediated by concentration variations of
molecular ligands. That opens up potential applications ranging from medicine
to the development of switchable materials.
