NUCLEAR MAGNEITC RESONANCE INVESTIGATION OF HELIX TO RANDOM COIL TRANSFORMATION IN POLY-ALPHA-AMINO ACIDS .I. POLY-L-ALANINE

High‐resolution nuclear magnetic resonance spectra at 100 MHz and 220 MHz have been obtained on two samples of poly‐L‐alanine of differing molecular weights (2500 and 42 500) in the chloroform–trifluoroacetic acid system under various conditions of solvent composition, temperature, and polypeptide concentration. Separate helix and random coil peaks are observed for the α‐CH and peptide NH backbone proton resonances, thereby permitting the determination of helix content. This observation of separate peaks demonstrates that the lifetimes of the helix and random coil portions of poly‐L‐alanine have lower limits of about 10−1 sec. It is suggested that solvent–peptide versus peptide–peptide hydrogen bond competition, coupled with a destabilizing effect of the trifluoroacetic acid on the helix, is responsible for the helix–random coil transformation. Copyright © 1969 John Wiley & Sons, Inc.