Signs of heteronuclear spin–spin coupling constants in 15N-acetamide

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(¹H^{N 13}CO). 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.