protomers

Protomer is a term used in chemistry to describe any one of the isomeric forms of a molecule that differ only in the location of a labile proton. The collection of all distinct proton positions for a given compound forms its protomer ensemble. Protomerism overlaps with tautomerism and with protonation-state isomerism; in some contexts protomers are tautomeric forms that do not require extensive bond rearrangements, while in others the emphasis is on where the proton sits.

In practice, the relative populations of protomers depend on conditions such as pH, solvent, temperature, and intramolecular stabilization. Different protomers can exhibit distinct acid–base properties, spectroscopic characteristics, and reactivity, and they can influence biological activity and binding in pharmaceutical contexts. This makes protomer analysis relevant to drug discovery, catalysis, and analytical chemistry, where multiple protomeric forms may contribute to observed behavior or signals.

Experimental and computational approaches are used to study protomers. Experimental techniques include nuclear magnetic resonance, infrared spectroscopy, crystallography, and mass spectrometry, with some methods capable of distinguishing protomers on the basis of mobility or fragmentation patterns. Computational methods enumerate possible proton positions, estimate relative energies and interconversion barriers, and predict Boltzmann populations under specified conditions.

Commonly cited examples involve pairs such as 2-pyridone and 2-hydroxypyridine, which represent protomeric tautomers differing by proton location and associated bond representation. The term protomer is variably defined in the literature, but it is widely used to describe proton-position isomers across small molecules and biologically relevant compounds.