What are isotopologues?

Isotopologues are molecules with identical elements and structure, but differ in the position and number of isotopes. While there are many types of molecules, in ACIR, we are primarily focused on organic compounds, and the isotopes of carbon, hydrogen, nitrogen, oxygen, and sulfur (CHNOS). These are the most common elements in living organisms and abiotic organic compounds.

Many processes potentially alter the isotopic patterns within molecules, including biosynthesis, sorption and phase changes, or abiotic reactions on Earth or elsewhere in the solar system. Organic isotopologue ensembles are the net result of such influences, and provide insights to the origin and history of a molecule, as well as the individual atoms within it.

The possible isotope substitutions increases dramatically with the number of atoms in a molecule. This means more complex organic compounds translate into ever larger numbers of possible isotopologue ensembles. In the special situation when two rare isotopes share a chemical bond, the isotopes are said to be 'clumped.' Far more commonly, isotopologues include only a single rare isotope. The frequency of a rare isotope occurrence at one position relative to another may differ considerably.

Molecular isotope abundances have been measured using chromatographic separation and mass spectrometry methods for many years. Compound-specific isotope analyses (CSIA) typically determine the isotope ratio for the entire molecule. New methods and analytical tools make it possible to measure precisely isotope abundances at specific positions within molecules. These include NMR spectroscopy, pyrolysis inlets for isotope-ratio mass spectrometry, and high-resolution/high-mass-accuracy mass spectrometry methods involving orbitrap technology. ACIR team members are working with all three methods, as well as powerful models of isotopologue ensembles involving Density-Functional Theory (DFT) and networked biotic and abiotic reaction systems.

Click below for a recent review article on organic isotopologues by ACIR team member, Alexis Gilbert:

Gilbert (2021) The Organic Isotopologue Frontier