二糖Disaccharide

二糖的性质取决于两个单糖的连接方式。若一个单糖的半缩醛羟基仍游离（如麦芽糖的 α-1,4 键、乳糖的 β-1,4 键），该端可开环成醛基，具有还原性和变旋现象，称为还原性二糖。蔗糖由葡萄糖 C1 与果糖 C2 相连（α1→β2），两个异头碳都参与成键，没有游离半缩醛，是非还原性糖。纤维二糖与麦芽糖的区别仅在于 β-1,4 与 α-1,4 键。

A disaccharide's properties depend on how its two units are linked. If one unit keeps a free hemiacetal hydroxyl, as in the α-1,4 link of maltose or the β-1,4 link of lactose, that end can open to an aldehyde, so the sugar is reducing and mutarotates. In sucrose the C1 of glucose joins the C2 of fructose (α1→β2), both anomeric carbons are tied up, and there is no free hemiacetal, so it is non-reducing. Cellobiose differs from maltose only in having a β-1,4 rather than α-1,4 link.

蔗糖来自甘蔗和甜菜，1747 年马格拉夫发现甜菜中含有与蔗糖相同的糖；麦芽糖由淀粉经淀粉酶水解得到，是麦芽制糖的基础；乳糖存在于哺乳动物乳汁中，1780 年舍勒确认其为一种糖。20 世纪初哈沃斯等确定了这些二糖的环状结构和连接位置，哈沃斯因糖类结构研究获 1937 年诺贝尔化学奖。

Sucrose comes from sugar cane and sugar beet; in 1747 Marggraf found that beet contains the same sugar as cane. Maltose arises from starch by amylase hydrolysis, the basis of malt sugar, and lactose occurs in mammalian milk; Scheele recognised it as a sugar in 1780. In the early twentieth century Haworth and others established the ring structures and linkage positions, and Haworth received the 1937 Nobel Prize in Chemistry for work on carbohydrate structure.

二糖必须在小肠中被蔗糖酶、麦芽糖酶、乳糖酶水解为单糖才能吸收。许多成年人乳糖酶活性降低，摄入乳糖后会出现腹胀腹泻，称为乳糖不耐受。蔗糖经酸或转化酶水解得到等量葡萄糖和果糖的混合物，旋光方向由右旋变为左旋，称为“转化糖”，蜂蜜的主要成分即转化糖。

Disaccharides must be split into monosaccharides by sucrase, maltase and lactase in the small intestine before they can be absorbed. Many adults lose lactase activity and suffer bloating and diarrhoea after lactose, which is lactose intolerance. Hydrolysis of sucrose by acid or invertase gives equal amounts of glucose and fructose; because the rotation changes from dextro to laevo, the product is called invert sugar, the main component of honey.