Determination of the conformation and stability of simple homopolypeptides using solid-state NMR

¹⁵N CPMAS, ¹³C CPMAS and ¹H CRAMPS spectra of several polypeptide samples were compared to determine the useful features of each technique. ¹³C CPMAS is the most well-established technique and is useful for quick determination of secondary structure. The ¹⁵N nucleus is more sensitive to exact hydrogen-bonding parameters, which complicates interpretation of the spectra. However, it is better for resolving end effects and structural types in short oligomers. ¹H CRAMPS spectra are similar to ¹³C CPMAS in the information obtained, but the resolution is not as good. Using ¹³C CPMAS, the conformation of polyglycine was investigated in detail. Precipitation from solvents such as DCA or TFA resulted in the rippled β-sheet structure (PG I), while 3 ₁-helix (PG II) was formed by precipitation from aqueous solutions of LiBr. Grinding the sample resulted in an increase in the amount of PG I, indicating that this form is more stable in the solid state. These results agree with previous work on poly(L-alanine) showing that the β-sheet form is more stable in the solid state. Homopolypeptides with larger side chains did not change conformation upon grinding due to the greater difficulty in disrupting van der Waals interactions and inertia of the large side chains.