Polypeptide helix lifetimes in the helix–random coil transition region. Application to NMR spectra

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Abstract
Two rate processes characterize the helix‐random coil transition in synthetic polypeptides. A slow rate is obtained by an interpretation of separate helix and random coil peaks in the NMR spectra. A faster rate is obtained from relaxation time measurements which utilize rapid reaction techniques. The present model describes the time dependent behavior of polypeptides in the transition region by combining random walk probabilities with conventional equilibrium distributions of conformations. Average conformational lifetimes for each peptide unit were calculated for molecules containing up to 150 units. These results indicate that in the transition region no helical unit is sufficiently long lived to be individually distinguishable by NMR. Spectra are simulated using a two site model for exchanging systems. On the basis of these model calculations the two peaks observed in the NMR spectra are attributed to (1) the all random coil species and (2) all other conformations containing at least one helix unit.