  The force laws governing the extension behavior of homopolypeptides are
obtained from a phenomenological free energy capable of describing the
helix-coil transition. Just above the melting temperature of the free chains,
T*, the plot of force, f, vs. end-to-end distance, R, exhibits two plateaus
associated with coexistence of helical and coil domains. The lower plateau is
due to tension induced onset of helix-coil transition. The higher plateau
corresponds to the melting of the helices by overextension. Just below T* the
f-R plot exhibits only the upper plateau. The f-R plots, the helical fraction,
the number of domains and their polydispersity are calculated for two models:
In one the helical domains are viewed as rigid rods while in the second they
are treated as worm like chains.
