SIGNS OF HETERONUCLEAR SPIN-SPIN COUPLING-CONSTANTS IN ACETAMIDE-N-15

By using digital deconvolution to improve spectral resolution, earlier NMR studies on 15N‐enriched acetamide have been revised and extended to determine the signs of the heteronuclear spin‐spin coupling constants. 1J(13CO15N), 2J(13CH315N) and 3J(C1H315N) are negative while 3J(1H13CH3)>0. The results, interpreted on the basis of the ‘selective decoupling’ formalism, were confirmed by computer simulation of the double resonance spectra. It is shown that 2J(1H‐α13CO) is significantly larger than 2J(1HN 13CO). Thus, jointly with {1H‐β}‐13C′ double resonance experiments, {1H‐α}‐13C′ experiments ought to be most helpful when assigning peptide group carbonyl resonances. The study provides valuable information for the interpretation of heteronuclear coupling constants in polypeptides. Copyright © 1979 Heyden & Son Ltd.