Abstract
30.5 MHz 15N m.m.r. ( CP/MAS) spectra of various solid polypeptides were measured using the cross-polarization/magic angle spinning technique. In order to obtain optimum signal-to-noise ratios, relatively short contact times (1 ± 0.5 ms) are required, because the cross-polarization times ( T NH ) are short and because the proton rotating-frame relaxation times ( T 1p) are in the order of 20 ms. The 15N n.m.r. signals of copolypeptides may be sensitive to sequence effects; yet they are in most cases more sensitive to the nature of the secondary structure. The signals of α-helices absorb ca. 8–10 ppm upfield of β-sheet structures, whereas the polyglycine II helix absorbs downfield. The natural abundance spectrum of crystalline gramicidin-S exhibits a signal at −247 ppm, a characteristic chemical shift of the antiparallel pleated sheet structure.
Published Version
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