 Epimer (source code)

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= Epimer

Two \b[epimers] are <diastereomers> with opposite configurations at exactly one of the two or more tetrahedral <stereocenters> in their molecules. Their connectivity is the same, and configurations at the other centers agree. The definition is more specific than simply calling two compounds <stereoisomers>.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[Entry E02167, epimers: https://old.goldbook.iupac.org/html/E/E02167.html ; originating stereochemistry terminology recommendation.]{ref}

D-<glucose> and D-<mannose> illustrate the relationship: their configurations differ at carbon 2 while the other corresponding stereocenters agree. This local change can alter recognition by enzymes without changing the elemental composition or the carbon-chain connectivity.

For a hypothetical molecule with corresponding stereocenters 2 and 3, the configurations $(2R,3R)$ and $(2S,3R)$ differ at one center and can form an epimeric pair. Changing both to $(2S,3S)$ is a different comparison: it does not satisfy the one-center condition. These labels assume the same molecular connectivity and consistently assigned centers.

A molecule with only one stereocenter does not produce an epimer pair merely by reversing that center: its two configurations are <enantiomers>, whereas the epimer definition requires a diastereomeric relationship. <Epimerization> names a process that interconverts an epimeric pair; it does not by itself specify the reaction mechanism, rate or equilibrium ratio.

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