Epimerize

Epimerize is a verb meaning to convert one epimer into another. An epimer is a stereoisomer that differs in configuration at exactly one stereogenic center among several. Epimerization, therefore, is the process of inverting the configuration at a single stereocenter in a molecule that contains more than one such center. Epimers are a subset of diastereomers.

Epimerization can occur by different mechanisms. Chemically, it often proceeds through reversible deprotonation at or near the stereogenic center to form an intermediate enol or enolate, followed by reprotonation from the opposite face, resulting in inversion. Other pathways include radical or rearrangement processes that interchange the configuration at a specific carbon. In biological systems, specialized enzymes called epimerases catalyze the selective inversion of one stereocenter in a substrate, often using acid–base catalysis or cofactors.

The concept is especially important in carbohydrate chemistry, where many sugars differ only by the orientation at a single carbon atom. For example, glucose and mannose are C-2 epimers, and glucose and galactose are C-4 epimers. A well-known enzymatic example is UDP-glucose 4-epimerase, which converts UDP-glucose to UDP-galactose as part of nucleotide-sugar interconversion.

Epimerization is a tool in synthesis and metabolism to access different stereoisomers, but it can also complicate purification when epimers co-occur. The term contrasts with racemization, which involves inversion at all relevant stereocenters, and with epimerase, the class of catalysts that mediate site-specific epimerizations.