A nuclear magnetic resonance study of the helix-coil transition of poly(L-glutamic acid).

Abstract

AbstractThere have been many reports that the nuclear magnetic resonance (nmr) spectra of a large number of polypeptides exhibit peak doubling of the α‐carbon and the α‐carbon proton in the helix–coil transition region. One apparent exception to this generalization has been polypeptides with ionizable side chains, where the helix–coil transition is induced by changes in pH in aqueous solution. Because it is important to establish the proper theoretical reason for the peak doubling and its relation to the rate of conformational change of amino acid residues, we have reexamined the proton and carbon‐13 nmr spectra, at high field, for two polydisperse samples of poly(L‐glutamic acid). Doubling of the α‐carbon proton resonance as well as those of the α‐ and β‐carbon, and backbone carbonyl are observed for a low‐molecular‐weight sample (DP = 54), while a higher molecular weight sample (DP = 309), exhibits only single resonances. Thus, polydispersity by itself is not sufficient to observe peak doubling; low‐molecular weight is also required.