(a) Anomers
Definition : A pair of cyclic sugars that differ in configuration only at the anomeric carbon (C-1 in an aldose, C-2 in a ketose) are called anomers.
How they arise : when the open-chain form cyclises to the hemiacetal, the carbonyl carbon becomes a new chiral centre. The newly formed $-OH$ can point either down or up, giving two forms :
$$\alpha\text{-D-glucopyranose} \quad ([\alpha] = +111^\circ) \qquad \beta\text{-D-glucopyranose} \quad ([\alpha] = +19 \cdot 2^\circ)$$
Mutarotation : in solution the two interconvert through the open-chain form until an equilibrium rotation of $+52 \cdot 5^\circ$ is reached.
(b) Invert sugar
Definition : The equimolar mixture of D-(+)-glucose and D-(−)-fructose obtained by the hydrolysis of sucrose is called invert sugar.
$$\underset{\text{sucrose}}{C_{12}H_{22}O_{11}} + H_2O \xrightarrow{H^+ \text{ or invertase}} \underset{\text{glucose}}{C_6H_{12}O_6} + \underset{\text{fructose}}{C_6H_{12}O_6}$$
Why "invert" : the sign of optical rotation reverses during the hydrolysis.
| Species | Specific rotation |
|---|
| Sucrose | $+66 \cdot 5^\circ$ (dextrorotatory) |
| Glucose | $+52 \cdot 5^\circ$ |
| Fructose | $-92 \cdot 4^\circ$ |
| Product mixture | $-19 \cdot 9^\circ$ (laevorotatory) |
Because fructose rotates light to the left far more strongly than glucose rotates it to the right, the mixture is overall laevorotatory — the rotation has been inverted from $+$ to $-$.
(c) Glycosidic linkage
Definition : The oxygen bridge $(-O-)$ joining two monosaccharide units, formed by the loss of a water molecule between their hydroxyl groups, is called a glycosidic linkage.
$$\text{Monosaccharide} + \text{Monosaccharide} \rightarrow \text{Disaccharide} + H_2O$$
Examples :
- Maltose — an $\alpha$-1,4 linkage between two glucose units
- Sucrose — an $\alpha$-1,2 linkage between glucose and fructose
- Lactose — a $\beta$-1,4 linkage between galactose and glucose
This is also the linkage that builds up the polysaccharides — starch, glycogen and cellulose.
