Disaccharides

Disaccharides

When a disaccharide is hydrolysed — using dilute acid or an appropriate enzyme — it splits into two monosaccharide units, which may be identical or different. The two monosaccharide units are not simply mixed together; they are joined covalently through an oxygen atom, in a bond formed by the loss of one water molecule between an $-\text{OH}$ group on one sugar and an $-\text{OH}$ group on the other. This oxygen bridge between two monosaccharide units is called a glycosidic linkage.

Reducing and non-reducing disaccharides

Whether a disaccharide behaves as a reducing sugar or not depends on exactly which $-\text{OH}$ groups are used to form that glycosidic linkage.

(i) Sucrose

Sucrose is one of the most familiar disaccharides. On hydrolysis it yields an equimolar mixture of D-(+)-glucose and D-(–)-fructose:

The two rings are joined by a glycosidic linkage running from C1 of $\alpha$-D-glucose to C2 of $\beta$-D-fructose. Notice that C1 of glucose is glucose's own anomeric (reducing) carbon, and C2 of fructose is fructose's own anomeric (reducing) carbon — so this linkage consumes the reducing group of both component sugars at once. With no free aldehyde or ketone left on either ring, sucrose is a non-reducing sugar.

Inversion on hydrolysis. Sucrose itself is dextrorotatory. But hydrolysing it produces dextrorotatory glucose together with laevorotatory fructose, and fructose's laevorotation ($-92.4°$) is numerically larger than glucose's dextrorotation ($+52.5°$). The net effect is that the product mixture is laevorotatory overall. So the act of hydrolysing sucrose actually flips the sign of optical rotation, from dextro (+) in the starting sugar to laevo (–) in the product mixture — which is why the resulting glucose–fructose mixture is given the special name invert sugar.

(ii) Maltose …