epimerization

Epimerization is a chemical process that converts one epimer into another by changing the configuration at a single stereogenic center in a molecule with multiple such centers. Epimers are stereoisomers that differ at only one chiral center and are diastereomers of each other.

Epimerization can occur under a variety of conditions, including acid- or base-catalyzed reactions, or enzymatic processes. Often it proceeds via a temporary, more planar or open intermediate such as an enol, enolate, or oxocarbenium ion, which permits inversion at the targeted center and subsequent reformation of the stereochemistry.

In carbohydrates, epimerization commonly involves sugars or sugar nucleotides. For aldoses, epimerization at C-2 interconverts glucose and mannose. In nucleotide-sugar metabolism, UDP-glucose 4-epimerase converts UDP-glucose to UDP-galactose by forming a 4-keto intermediate at C-4 and then selectively re-reducing it to yield inversion. Enzymatic epimerases often require cofactors such as NAD+.

A key distinction is that epimerization changes configuration at only one stereocenter, whereas racemization would invert all stereocenters to produce the enantiomer. Thus, epimers are diastereomers, not mirror images (except in trivial cases with a single stereocenter).

Applications and relevance include access to different stereoisomers in carbohydrate metabolism, synthesis of rare or modified sugars, and pharmaceutical development where specific epimers may have distinct biological activities. In carbohydrate chemistry, selective epimerization enables interconversion among related monosaccharides and can influence enzyme specificity and metabolic flux.