Ideas and perspectives: Same Carbon Different Elements – An Insight into Position-Specific Isotope Patterns Within a Single Compound

Abstract. It is expected that information on the source, reaction pathway, and kinetics of an organic compound can be obtained from its position-specific isotope compositions or intramolecular isotope distribution (Intra-ID). To retrieve the information, we could use its equilibrium Intra-ID as a reference for understanding the observed Intra-IDs. Historically, observed, apparently close-to-equilibrium carbon Intra-ID had prompted an open debate on the nature of biosystem and specifically the pervasiveness of reversible biochemical reactions. Much of the debates remain unresolved, and the discussion has not clearly distinguished two states of equilibrium: (1) the equilibrium among the bond-breaking/forming positions in reactant and product, and (2) the equilibrium among all carbon positions in a compound. For an organic molecule with multiple carbon positions, equilibrium carbon Intra-ID can be attained only when a specific reaction is in equilibrium and the sources of each position are also in equilibrium with each other. An Intra-ID provides limited information if the sources and pathways are both unconstrained. Here, we elaborate on this insight using examples of the Intra-IDs of hydroxyl-bearing minerals, N2O, and acetic acid. Research effort aiming at calibrating position-specific equilibrium and kinetic isotope fractionation factors for defined processes will help to interpret Intra-IDs of a compound accurately and fully.