1 H‐ 15 N Spin–spin couplings in alumichrome

We report the determination of two‐ and three‐bond 1 H‐ 15 N spin–spin couplings in the nmr spectra of a polypeptide. The 1 H‐ and 15 N‐nmr spectra of 99.2% 15 N‐enriched alumichrome have been studied at 360 MHz and 10.1 MHz, repectively. While some 2 J     1 H   15 Nand 3 J     1 H   15 Ncoupling are of the order of 5 Hz, most splitting resulting from the heteronuclear interaction are ≲2 Hz, which introduces strigent requirements of spectral resolution. In the 1 H spectra these requirements were met by digital deconvolution with a sine bell routine combined with positive exponential filtering. Although the 15 N spectra clearly exhibit features of fine structure, mainly because of the intrinsic higher nmir sensitivity of protons, observation of 1 H‐ 15 N spin–spin couplings was found to be more practical in the 1 H than in the 15 N spectra. We find that the alumichrome data do not satisfy a simple cyclic relationship linking the heteronuclear couplings to the crystallographic ψ dihedral angles. It is suggested that a formal treatment of the ψ‐related interresidue 1 H‐ 15 N coupling might have to take into account a more complex dependence of the intervening 3 J on the overall local electronic structure, which is dependent on ϕ,ψ, and ω simultaneoulsy. In contrast, our analysis indicates that χ 1 can be readily determined from the measurement of the corresponding heteronuclear 3 J coupling in the 1 H β or in the amide 15 N resonances. Karplus relationships are proposed that relate this heteronuclear 3 J to the corresponding dihedral angle θ and which, on average, yield\documentclass{article}\pagestyle{empty}\begin{document}$$ {}^3 J\left( \theta \right)= - 4.4\,{\rm cos}^2 \theta + 1.2 {\rm cos}\theta + 0.1_5$$\end{document}