protomer

A protomer is a member of a family of isomeric forms of a molecule that differ in the location of a proton or in the molecule’s protonation state. Protomerism encompasses forms that arise when a proton is moved within the molecule or added to or removed from it, leading to different charge distributions or bonding arrangements. The concept is closely related to prototropic tautomerism, in which proton transfer accompanies a change in bonding, but it is often used more broadly to include different protonation states as distinct protomers.

Interconversion between protomers occurs by proton transfer and may be catalyzed by solvent, acids, bases, or other catalysts. In solution, the relative populations of protomers depend on pH, pKa values, and the stability of each form. In gas-phase chemistry and mass spectrometry, separate protomeric forms can sometimes be observed as distinct species with different spectra or fragmentation patterns, reflecting differences in structure and charge.

Common examples include neutral bases and their conjugate acids, such as ammonia (NH3) and ammonium (NH4+), or other molecules that can exist in protonated and deprotonated states. Tautomeric pairs, such as keto and enol forms that arise from proton relocation, are often cited as protomeric manifestations of the same underlying molecule.

Protomer concepts are important in fields such as medicinal chemistry, catalysis, and analytical chemistry, where the protonation state and proton position influence reactivity, binding, solubility, and spectroscopic signatures.