定义Definition
差向异构体是含有多个手性中心、只有一个手性中心构型不同而其余都相同的一对非对映异构体。例如 D-葡萄糖与 D-甘露糖是 C-2 差向异构体。
Epimers are diastereomers with several stereocentres that differ in configuration at only one of them; D-glucose and D-mannose, for example, are C-2 epimers.
差向异构化Epimerisation
一个手性中心的构型发生翻转称为差向异构化，常发生在酸性 α-H 所在的碳上（经烯醇或烯醇负离子），如碱性条件下葡萄糖、甘露糖、果糖互变（洛布里·德布鲁因–范埃克斯坦重排）；肽合成中氨基酸的消旋也是此问题。
Inversion at one centre is epimerisation, often at a carbon bearing an acidic α-H via an enol or enolate — e.g. glucose, mannose and fructose interconvert in base (Lobry de Bruyn–van Ekenstein); amino-acid racemisation in peptide coupling is the same issue.
生物学Biology
差向异构酶专一催化某个中心的构型转换，如 UDP-葡萄糖 4-差向异构酶（葡萄糖 ↔ 半乳糖，缺乏导致半乳糖血症）。
Epimerases convert a single centre, e.g. UDP-glucose 4-epimerase (glucose ↔ galactose; its deficiency causes a form of galactosaemia).
与端基异构体的关系Relation to anomers
端基异构体是环状糖在半缩醛碳（端基碳）上的差向异构体，是差向异构体的一个特例。
Anomers are epimers at the hemiacetal (anomeric) carbon of cyclic sugars — a special case of epimers.
年表Timeline
- 1891
费歇尔确定葡萄糖、甘露糖等的构型关系
Fischer establishes configurational relationships among glucose, mannose and related sugars
- 1895
洛布里·德布鲁因与范埃克斯坦报道碱催化糖互变
Lobry de Bruyn and van Ekenstein report base-catalysed sugar interconversion