Reversible stretching of random heteropolymers.

We analyze the equilibrium response of random heteropolymers to mechanical deformation. In contrast to homopolymer response, the stress-induced transformation of a heteropolymer from globule to coil need not be sharp. For chain lengths relevant to biological macromolecules, intermediate necklace-like structures dominate over a range of applied force. Stability of these conformations is primarily a consequence of solvation: In a typical necklace, relatively solvophilic regions of the chain are extended, while solvophobic regions remain compact. In the long-chain limit, homopolymeric behavior is recovered. Our results suggest that only select polypeptide sequences should unfold reproducibly at a specific force, explaining recent experimental observations.