Conformational and kinetic studies of synthetic polypeptides by NMR.

Abstract

AbstractThe α‐helix–coil transition of poly‐L‐leucine, poly‐L‐alanine and poly‐L‐methionine in chloroform–trifluoroacetic acid system was studied by nuclear magnetic resonance (NMR) and optical rotatory dispersion (ORD). The kinetics of the hydrogen–deuterium exchange in the peptide was also followed in these polymers by means of NMR. Two types of the NMR spectra and the hydrogen–deuterium exchange reaction were found, corresponding to the high and low molecular weight polypeptides. In high molecular weights, the NH and α‐CH resonance lines gave single peaks and the hydrogen–deuterium exchange was expressed as a single first order reaction. In low molecular weights, the NH and α‐CH lines were separated into two peaks, corresponding to helical and random‐coiled states, respectively, and the exchange react ion was expressed as super‐position of a very rapid exchange reaction in the random‐coiled part and another slow exchange reaction of the first order in the helical part. These results suggest that the helix–coil interconversion of low molecular weight polypeptides has a longer relaxation time (⩾ 4.5 × 10−3 sec) than that of high molecular weight polypeptides.