Determination of the relaxation time of the helix–coil transition of poly(γ‐benzyl‐L‐glutamate) by the nmr technique

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Abstract
NMR measurements of poly(γ‐benzyl‐L‐glutamate) are reported in several different strengths of magnetic field to determine the relaxation time of the helix–coil transition. Nmr spectra of various samples had line shapes varying from the double to single, depending on the extent of the polydispersity of the sample. This result indicated that the correct line shape of a polypeptide is obscured in the overlapping of multipeaks, which are due to the heterogeneity of the molecular weight in the sample. Thus, the conventional line‐shape analysis could not be applied to the kinetic study of the helix–coil transition of polypeptides without consideration of this polydispersity effect on the line shape.To overcome this difficulty, we measured linewidths of nmr spectra for fairly monodisperse samples, using various nmr spectrometers, having field strengths from 60 to 220 MHz. The results were analyzed by a quadratic equation, which involves an additional term proportional to the frequency difference of two sites. The equation differs from the conventional quadratic equation, usually utilized in the case of the fast‐exchange limit, only in this additional term. This modification is required to evaluate correctly the unusual broadening of the linewidth resulting from the polydispersity effect and to determine the relaxation time reflected in nmr.Nmr spectra of three samples (DP‐35, 85, and 250) were measured by 220‐, 100‐, and 60‐MHz spectrometers in trifluoroacetic acid/chloroform at 28°C and linewidths were analyzed. Relaxation times of the helix–coil transition obtained at the transition midpoint are 2.5 × 10−4, 7 × 10−4, and 1.1 × 10−3 sec, for DP‐35, 85, and 250, respectively.