单糖Monosaccharide

最简单的单糖是甘油醛（丙醛糖）和二羟基丙酮（丙酮糖）。以甘油醛为参照，距羰基最远的手性碳上羟基在费歇尔投影式右侧的为 D 型，左侧为 L 型；天然糖绝大多数是 D 型。一个 n 碳醛糖有 n−2 个手性碳，因而有 2ⁿ⁻² 个立体异构体，如己醛糖有 16 个。只在一个手性碳构型不同的两种糖互为差向异构体，如葡萄糖与甘露糖（C2）、葡萄糖与半乳糖（C4）。

The simplest monosaccharides are glyceraldehyde (an aldotriose) and dihydroxyacetone (a ketotriose). Taking glyceraldehyde as reference, a sugar is D if the hydroxyl on the stereocentre farthest from the carbonyl lies to the right in the Fischer projection and L if to the left; almost all natural sugars are D. An n-carbon aldose has n−2 stereocentres and 2ⁿ⁻² stereoisomers, so there are 16 aldohexoses. Sugars differing at only one stereocentre are epimers, such as glucose and mannose (C2) or glucose and galactose (C4).

单糖都具有还原性，能还原托伦试剂、斐林试剂和本尼迪克特试剂（酮糖在碱性条件下经烯二醇互变为醛糖，因此也有还原性）。在稀碱中单糖会发生差向异构化（洛布里·德布鲁因-范埃克斯坦重排）。与苯肼成脎、与醇成苷、被硝酸氧化成糖二酸、被 NaBH₄ 还原成糖醇（如山梨醇）。浓酸加热使戊糖脱水成糠醛、己糖成 5-羟甲基糠醛。

All monosaccharides are reducing sugars that reduce Tollens', Fehling's and Benedict's reagents; ketoses do too because in base they isomerise through an enediol to aldoses. In dilute base they epimerise (the Lobry de Bruyn–van Ekenstein rearrangement). They form osazones with phenylhydrazine and glycosides with alcohols, are oxidised by nitric acid to aldaric acids and reduced by NaBH₄ to sugar alcohols such as sorbitol. Hot concentrated acid dehydrates pentoses to furfural and hexoses to 5-hydroxymethylfurfural.

1891 年费歇尔以 D-甘油醛为参照任意规定了 D/L 构型，1951 年比伊沃特用 X 射线证明这一假定恰好正确。葡萄糖是细胞的主要能源；核糖和脱氧核糖构成 RNA 和 DNA；果糖存在于水果和蜂蜜中，甜度约为蔗糖的 1.7 倍；半乳糖是乳糖的组成部分。

In 1891 Fischer arbitrarily assigned D/L configurations relative to D-glyceraldehyde, and in 1951 Bijvoet's X-ray work showed the guess was correct. Glucose is the cell's main fuel, ribose and deoxyribose build RNA and DNA, fructose in fruit and honey is about 1.7 times as sweet as sucrose, and galactose is part of lactose.