Effect of temperature on the circular dichroism spectra of polypeptides in the extended state

The circular dichroism (CD) spectra of poly-L -proline and of poly-L -glutamic acid and poly-L -lysine in their charged states have been studied as a function of temperature. The variation of CD spectra with temperature is inconsistent with the assignment of the spectrum of such charged polypetides to an unordered chain conformation, but does support our earlier assignment to a locally ordered structure—what we have called the extended helix conformation. These results also strengthen our previous assignment of the CD spectrum of an unordered chain, and indicate that three conformational states (α-helix, extended helix, and unordered) should be incorporated in our thinking about conformational transitions in polypeptides.