What does epimer mean?
An epimer is a term from stereochemistry describing one member of a pair of compounds that share the same molecular formula and connectivity but differ in the three-dimensional arrangement of their atoms at exactly one chiral centre. When a molecule has multiple stereocentres, flipping the configuration at just one of them produces its epimer rather than a mirror image. The concept matters most in carbohydrate chemistry, where sugars such as glucose, mannose, and galactose form well-known epimer pairs whose biological roles can differ dramatically despite the tiny structural change. Chemists usually specify the position involved, as in 'the C-2 epimer of glucose'. Related terms include epimeric (adjective), epimerization (the process of converting one epimer into another), and epimerase (an enzyme that catalyses it). Because the word belongs to specialist vocabulary, it appears mainly in scientific writing rather than everyday speech, yet it captures elegantly how minute molecular differences underpin major functional consequences in chemistry and biology.
nounIn chemistry, one of two diastereomers that differ in the spatial configuration of atoms at only one of several chiral centres. For example, glucose and mannose are epimers because they differ at a single carbon atom.
- One of two diastereomeric compounds that differ in configuration at only one chiral (stereogenic) centre.
"Glucose and galactose are epimers, differing in configuration only at the fourth carbon."
"Mannose is the C-2 epimer of glucose."
"The enzyme converts one sugar into its epimer without altering any of its other stereocentres."
Regular plural; commonly used when discussing pairs or sets of related sugars and other stereoisomers.
"These two epimers interconvert slowly in solution."
Swap the orientation of a single carbon atom in glucose and you get galactose instead — molecules that differ by just one flipped centre are called epimers.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of epimer
The word epimer was coined in the early twentieth century from the Greek prefix epi-, meaning 'upon' or 'in addition', combined with -mer, from the Greek meros meaning 'part'. It follows the pattern of earlier chemical terms such as isomer ('equal parts'), polymer ('many parts'), and monomer ('single part'), all built on the same root. The name reflects the idea of two molecular forms that differ in the configuration at one part of the molecule while remaining identical elsewhere.
Related word forms
How epimer is actually used
A technical term used almost exclusively in organic chemistry and biochemistry; it carries no figurative or informal use. It typically appears with a specifier such as 'C-2 epimer' to identify which chiral centre differs.
Easily confused with epimer
An isomer is any compound with the same molecular formula but different structure or arrangement, while an epimer is the narrower case of diastereomers differing at just one chiral centre.
An anomer is a special type of epimer that differs specifically in configuration at the anomeric carbon of a cyclic sugar.