Partial assignment of the 15N NMR spectrum of sulfomycin-I at natural abundance

Early efforts to utilize N-15 NMR spectroscopy in structure elucidation studies were often frustrated by the low gyromagnetic ratio (gamma(N)) and the low natural abundance (0.37%) of the nuclide. The advent of H-1 and inverse-detected 2D NMR methods has eliminated many of the difficulties inherent to the use of N-15 as a structural probe. This paper reports the partial assignment of the N-15 NMR resonances of the thiopeptide antibiotic sulfomycin-I produced by Streptomyces viridochromogenes. With the exception of two tertiary nitrogen resonances that had no two- or three-bond coupling pathways, assignments were made either through direct correlation H-1-N-15 GHSQC or one-bond optimized H-1-N-15 GHNMQC or via two or three bonds using H-1-N-15 GHNMQC spectra. Assignments are also reported for the heterocyclic nitrogen resonances of two thiazole and one oxazole moiety contained in the structure of the antibiotic via (3)J(N,H) coupling from the heterocyclic ring protons. Despite the suggestion that these coupling pathways, suspected to be ca. 2 Hz, might be difficult to observe since they are comparable to the linewidths of the thiazole and oxazole protons in question, they were still exploitable for assignment purposes. (C) 1998 John Wiley & Sons Ltd.