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In chemistry epimers are stereoisomers that differ in configuration of only one stereogenic center. All other stereocenters in the molecules, if any, are the same in each. Thus epimeric molecules can be either enantiomers or diastereomers of each other depending on whether there are stereocenters in addition to the epimeric one.

In chemical nomenclature, one of the epimeric pairs is given the prefix epi- for example in quinine and epi-quinine. When the pairs are enantiomers, the prefix becomes ent-.

Examples[edit]

The sugars α-glucose and β-glucose are epimers. In α-glucose, the -OH group on the second (anomeric) carbon is in the direction opposite the methylene group on carbon C-6 (in the axial position). In β-glucose, the -OH group is oriented in the same direction as the methylene group (in the equatorial position).^[1] These two molecules are both epimers and anomers (as indicated by the α-glucose and β designation).


α-D-glucopyranose	β-D-glucopyranose

The stereoisomers β-D-glucopyranose and β-D-mannopyranose are epimers because they differ only in the stereochemistry at the C-2 position. The hydroxyl group in β-D-glucopyranose is equatorial (in the "plane" of the ring) while in β-D-mannopyranose the C-2 hydroxyl group is axial (up from the "plane" of the ring). These two molecules are epimers but, because not mirror images of each other, are also not enantiomers (enantiomers have the same name but differ in D and L classification). They are also not sugar anomers, since the wrong carbon is involved in the stereochemistry.


β-D-glucopyranose	β-D-mannopyranose

Doxorubicin and epirubicin are two closely related drugs and epimers.


Doxorubicin–epirubicin comparison

Other closely related compounds are epi-inositol and inositol and lipoxin and epilipoxin.


Epi-inositol	Inositol	Lipoxin	Epilipoxin

Epimerisation[edit]

Epimerisation is a chemical process where an epimer is transformed into its chiral counterpart. It can happen in condensed tannins depolymerisation reactions. Epimerisation can be spontaneous (generally a slow process), or catalyzed by enzymes, e.g. the epimerization between the sugars N-acetylglucosamine and N-acetylmannosamine, which is catalyzed by Renin-Binding Protein.

References[edit]

^ Structure of the glucose molecule

English Journal

Epimerization of Alanyl-Alanine Induced by γ-Rays Irradiation in Aqueous Solutions.

Munegumi T1,2.
Origins of life and evolution of the biosphere : the journal of the International Society for the Study of the Origin of Life.Orig Life Evol Biosph.2017 Mar;47(1):69-82. doi: 10.1007/s11084-016-9507-0. Epub 2016 May 31.
PMID 27245350

Dihydroanatoxin-a Is Biosynthesized from Proline in Cylindrospermum stagnale PCC 7417: Isotopic Incorporation Experiments and Mass Spectrometry Analysis.

Méjean A1, Dalle K1, Paci G1, Bouchonnet S2, Mann S3, Pichon V4, Ploux O1,5.
Journal of natural products.J Nat Prod.2016 Jul 22;79(7):1775-82. doi: 10.1021/acs.jnatprod.6b00189. Epub 2016 Jun 24.
PMID 27340731

Microbial biotransformation of DON: molecular basis for reduced toxicity.

Pierron A1,2,3, Mimoun S1,2, Murate LS1,4, Loiseau N1,2, Lippi Y1,2, Bracarense AP4, Schatzmayr G3, He JW5, Zhou T5, Moll WD3, Oswald IP1,2.
Scientific reports.Sci Rep.2016 Jul 6;6:29105. doi: 10.1038/srep29105.
PMID 27381510

Japanese Journal

Preparation of C-Glycoside Pendant β^2- and β^<2,2>-Amino Acids

Bulletin of the Chemical Society of Japan 81(5), 606-616, 2008-05-15
NAID 10021075522

Cloning and sequencing of the cellobiose 2-epimerase gene from an obligatory anaerobe, Ruminococcus albus.

Biochemical and Biophysical Research Communications 360(3), 640-645, 2007-08-31
NAID 80018288392

Poly[(R)-3-Hydroxybutyrate] Formation in Escherichia coli from Glucose through an Enoyl-CoA Hydratase-Mediated Pathway(ENZYMOLOGY, PROTEIN ENGINEERING, AND ENZYME TECHNOLOGY)