Conformational Study of Solid Polypeptides by 1 H Combined Rotation and Multiple Pulse Spectroscopy NMR. 2. Amide Proton Chemical Shift

The relationship between the amide proton chemical shift and the conformation of homopolypeptides in the solid state was studied using the 1 H combined rotation and multiple pulse spectroscopy (CRAMPS) NMR method. The main-chain amide proton signals are considerably broad due to the residual dipolar couplings between the quadrupolar 14 N nucleus and amide proton relative to other proton signals of solid polypeptides. We have prepared fully 15 N-labeled (99 at. %) poly( l -alanines) ([Ala*] n ) and poly( l -leucines) ([Leu*] n ), adopting an α-helix or β-sheet form to eliminate the effect of the quadrupolar 14 N nuclei, and then measured their 1 H CRAMPS NMR spectra by increasing the magic-angle spinning speed. Thus, the 15 NH proton chemical shifts of [Ala*] n and [Leu*] n were successfully determined to be 8.0−8.1 ppm (α-helix) and 8.6−9.1 ppm (β-sheet). Accordingly, it became apparent that the amide proton chemical shifts are sensitive to the conformation of solid polypeptides.