Polysoaps and biopolymers: The effect of secondary structure on the extension elasticity

Extension of DNA and the protein titin reveals a plateau in the force vs. end-to-end distance diagram. A similar feature is predicted for the stretching elasticity of long polysoaps. These are linear hydrophilic chains that incorporate, at intervals, covalently bound amphiphilic monomers. In water, the amphiphilic monomers self-assemble into intrachain micelles thus endowing the chain with a secondary structure. The similarity in the force laws is traceable to the equilibration of the secondary structure. For polysoaps forming a linear string of micelles, the plateau is associated with a coexistence of weakly perturbed micelles and fully dissociated amphiphiles. Similar scenarios can be invoked to explain the behaviour of DNA and titin. The resulting functional form seems to depend only weakly on the precise nature of the system.