13Clabeled

13C-labeled refers to molecules in which the stable isotope carbon-13 has been incorporated at one or more carbon positions. The term 13Clabeled is uncommon and likely a misnomer; there is no widely used stable chlorine-13 isotope for labeling. In chemistry and biology, 13C labeling is used to trace carbon flow, study metabolic pathways, and enhance nuclear magnetic resonance (NMR) detectability.

Labeling can be achieved in vivo by feeding cells or organisms with 13C-enriched substrates such as 13C-glucose, 13C-glutamine, or 13C-acetate, allowing downstream metabolites to become enriched. In chemical synthesis, 13C-enriched starting materials are used to construct 13C-labeled compounds with defined labeling patterns. Labeling can be uniform, where many carbon atoms are 13C, or positional, where labels occupy specific sites. Enrichment levels are typically high (often 70–99% or greater), depending on the substrate, organism, and experimental design.

Applications include structural and mechanistic studies by NMR, where 13C nuclei provide informative signals and long-range correlations; metabolic flux analysis and isotopomer mapping in metabolomics and systems biology; and quantitative proteomics, such as SILAC, which uses 13C- or 15N-labeled amino acids to differentiate samples. 13C labeling also enables tracing of carbon incorporation in biosynthetic pathways and the evaluation of enzyme activities in cells and tissues.

Considerations for 13C labeling include cost and availability of labeled precursors, the desired labeling pattern, potential metabolic perturbations from the label, and the complexity of data analysis required to interpret isotope distributions.