13C NMR Metabolomics: Applications at Natural Abundance

C-13 NMR has many advantages for a metabolomics study, including a large spectral dispersion, narrow singlets at natural abundance, and a direct measure of the backbone structures of metabolites. However, it has not had widespread use because of its relatively low sensitivity compounded by low natural abundance. Here we demonstrate the utility of high-quality C-13 NMR spectra obtained using a custom C-13-optimized probe on metabolomic mixtures. A workflow was developed to use statistical correlations between replicate D-1 C-13 and H-1 spectra, leading to composite spin systems that can be used to search publicly available databases for compound identification. This was developed using synthetic mixtures and then applied to two biological samples, Drosophila melanogaster extracts and mouse serum. Using the synthetic mixtures we were able to obtain useful (CC)-C-13-C-13 statistical correlations from metabolites with as little as 60 nmol of material. The lower limit of C-13 NMR detection under our experimental conditions is approximately 40 nmol, slightly lower than the requirement for statistical analysis. The C-13 and H-1 data together led to 15 matches in the database compared to just 7 using H-1 alone, and the C-13 correlated peak lists had far fewer false positives than the H-1 generated lists. In addition, the C-13 D-1 data provided improved metabolite identification and separation of biologically distinct groups using multivariate statistical analysis in the D. melanogaster extracts and mouse serum.