Analysis of the 1H‐NMR spectra of ferrichrome peptides. II. The amide resonances

The high‐resolution 1H‐nmr study of the ferrichrome cyclohexapeptides, in d6‐DMSO solutions, has been extended to the amide NH spectral region. A total of ten diamagnetic analogues of ferrichrome that differ in the coordinated metal ion (Al3+, Ga3+ or Co3+), the primary structure, the nature of the bidentate hydroxamate moiety, or the isotope compositions (14N, 15N) have been investigated. The 3JαNH values reflect regiorous conformational isomorphism throughout the complete suite of analogues, quite independent of the residue occupancy of each site. Totally resolved amide multiplets have been obtained in most cases and the four‐line (doublet of doublets) appearances of glycyl NH resonances has been observed for the first times; these data enabled stereospecific assignment and accurate analysis of the NH‐CαH proton spin systems. The high resolution was made possible by the use of a suitable spectral deconvolution shceme at 360 MHz. The fine structure, extraordinarily well displayed in the 15N‐peptide spectra, provides a series of parameter values whose consistency has been checked by computer simulation. Since the crystallographic structure for two of the ferric peptides is known to 0.002‐Å resolution, a 3J vs θ correspondence could be confidently established. A Karplus curve was derived from the combined x‐ray and nmr data: \documentclass{article}\pagestyle{empty}\begin{document}$$ ^3 J_{\alpha {\rm NH}} = 5.4\cos ^2 \theta - 1.3\cos \theta + 2.2{\rm Hz} $$\end{document} It is suggested that seriously nonplanar amides can exhibit 3JαNH values higher than predicted by the ferrichrome curve.