Biosynthetic studies on microbial metabolites have been greatly simplified by the use of carbon-13-labeled precursors and carbon magnetic resonance (cmr) for detection and facile identification of the labeling patterns in the metabolite without recourse to chemical degradation. Our initial biosynthetic studies employed continuous wave cmr with proton noise decoupling at 25.15MHZ with 50 to 1500 spectral accumulations and acquisition times of 2 to 72hr, depending on the quantity and enrichment levels of the sample. Enhanced sensitivity by an order of magnitude results from the use of FT techniques in cmr, with appreciable benefits in shorter spectral acquisition times and/or smaller sample requirements. Application of FT cmr to unambiguously define the biosynthetic origin of all carbon atoms of the fungal metabolites, avenaciolide, aspyrone, and rubratoxin B, is presented. Also discussed is the first application of FT-cmr at 55.33MHz, which verifies the unique J^<13>C-^<13>C observable between C_9 and C_<15> in an enriched sample of sterigmatocystin.
