isotopomers

Isotopomers are a class of chemical species that share the same molecular formula and the same connectivity of atoms but differ in the positions of isotopic substitutions. For example, in a partially deuterated molecule such as ethanol, substituting hydrogen with deuterium at different sites (for instance CH2D–CH2–OH, CH3–CHD–OH, or CH3–CH2–OD) yields distinct isotopomers. The total number of isotopic atoms is the same, but their locations within the molecule differ.

Isotopomers are a subset of isotopologues. Isotopologues differ in which atoms are isotopes and thus in overall isotopic composition, while isotopomers differ only in where those isotopes reside within an otherwise identical skeleton and stoichiometry. When a molecule has the same isotopic composition but the labeled atoms occupy different positions, these different distributions are isotopomers.

Detection and uses. Mass spectrometry often cannot distinguish isotopomers that have the same total isotopic content, because their nominal masses are the same. However, isotopomer distributions can alter fragmentation behavior and are often distinguishable by nuclear magnetic resonance (NMR) spectroscopy, infrared spectroscopy, or other methods sensitive to isotope placement. Isotopomer patterns arise in labeled-substrate experiments and can provide information about molecular structure, reaction mechanisms, and metabolic fluxes.

Applications and context. Isotopomer analysis is central to isotopic-labeling studies in chemistry and biochemistry, including tracer experiments, structural elucidation, and metabolic flux analysis. It helps researchers infer how atoms are incorporated into products and how labeling distributes through pathways.

See also: isotopologue, isotopic labeling, NMR spectroscopy, tracer studies.