Helix-Coil Transition of Polypeptides under an External Force

The change in the helical fraction of polyeptide is calculated on applying an external force at the ends of the chain in the helix-coil transition region. It is shown that when the mean square end-to-end distance of chain has a minimum at the transition region, the helical fraction increases at the helix-side and decreases at the coil-side of the transition region on applying the force. The critical value of the force to make the helical content almost vanish is estimated and the force to make the helical content almost vanish is estimated and the implication of this phenomenon in the mechanical denaturation of polypeptides such as polyglutamic acid and silk fibroin is discussed.