1H-15N Spin–spin couplings in alumichrome

We report the determination of two- and three-bond ¹H-¹⁵N spin–spin couplings in the nmr spectra of a polypeptide. The ¹H- and ¹⁵N-nmr spectra of 99.2% ¹⁵N-enriched alumichrome have been studied at 360 MHz and 10.1 MHz, repectively. While some ²J and ³J coupling 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 ¹H spectra these requirements were met by digital deconvolution with a sine bell routine combined with positive exponential filtering. Although the ¹⁵N spectra clearly exhibit features of fine structure, mainly because of the intrinsic higher nmir sensitivity of protons, observation of ¹H-¹⁵N spin–spin couplings was found to be more practical in the ¹H than in the ¹⁵N 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 ¹H-¹⁵N coupling might have to take into account a more complex dependence of the intervening ³J on the overall local electronic structure, which is dependent on ?,ψ, and ω simultaneoulsy. In contrast, our analysis indicates that χ¹ can be readily determined from the measurement of the corresponding heteronuclear ³J coupling in the ¹H^β or in the amide ¹⁵N resonances. Karplus relationships are proposed that relate this heteronuclear ³J to the corresponding dihedral angle θ and which, on average, yield