Expedited Structure Elucidation of Novel Cyclic Peptides Using 15 N Stable Isotopic Labeling

Natural products have historically been a rich and diverse source of biologically active compounds. Some of the most interesting members of this category are cyclic and acyclic peptides. Although this structural class has a high potential for development into medicines and application to numerous therapeutic indications, the elucidation of their structures is not always trivial. The occurrence of nonstandard amino acids and extensive overlap of aliphatic 1 H α and amide 1 H N resonances can make the assignment of individual amino acid spin systems and connectivity between them difficult. Experiments like 1 H– 1 H TOCSY, 1 H– 13 C HSQC‐TOCSY, and 1 H– 13 C HMBC can be implemented to help disentangle these congested resonances, but the limited spectral dispersion of 1 H and restricted chemical shift differences in the 13 C dimension can limit the effectiveness of these experiments. To circumvent this issue, the larger spectral dispersion of 15 N (~900 ppm) can be leveraged with the use of 1 H– 15 N HSQC‐TOCSY and 1 H– 15 N HMBC experiments. Although these NMR experiments have been shown to be effective structure elucidation tools at natural isotopic abundance (0.4%), we demonstrate here that the quality of those data can be significantly enhanced, and their acquisition accelerated, through the production of 15 N‐labeled analyte.