PARTIAL ASSIGNMENT OF THE N-15 NMR-SPECTRUM OF SULFOMYCIN-I AT NATURAL-ABUNDANCE

Early efforts to utilize N-15 NMR spectroscopy in structure elucidatio n 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 di fficulties inherent to the use of N-15 as a structural probe. This pap er reports the partial assignment of the N-15 NMR resonances of the th iopeptide antibiotic sulfomycin-I produced by Streptomyces viridochrom ogenes. With the exception of two tertiary nitrogen resonances that ha d no two- or three-bond coupling pathways, assignments were made eithe r 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. A ssignments are also reported for the heterocyclic nitrogen resonances of two thiazole and one oxazole moiety contained in the structure of t he antibiotic via (3)J(N,H) coupling from the heterocyclic ring proton s. Despite the suggestion that these coupling pathways, suspected to b e 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.