Chemistry of sugars

Chemistry of sugars

A. Reactions of the monosaccharides 

The sugars (monosaccharides) occur in the

metabolism in many forms (derivatives).

Only a few important conversion reactions

are discussed here, using D-glucose as an example.

1. Mutarotation.

In the cyclic form, as opposed

to the open-chain form, aldoses have a

chiral center at C-1 . The corresponding

isomeric forms are called anomers.

In the β-anomer (center left), the OH group at

C-1 (the anomeric OH group) and the CH2OH

group lie on the same side of the ring. In the α-

anomer (right), they are on different sides.

The reaction that interconverts anomers into

each other is known as mutarotation (B).

2. Glycoside formation.

When the anomeric

OH group of a sugar reacts with an alcohol,

with elimination of water, it yields an

O–glycoside (in the case shown, α –methylglucoside).

The glycosidic bond is not a normal

ether bond, because the OH group at C-1 has a

hemiacetal quality. Oligosaccharides and polysaccharides

also contain O-glycosidic bonds.

Reaction of the anomeric OH group with an

NH2 or NH group yields an N-glycoside (not

shown). N-glycosidic bonds occur in nucleotides

and in glycoprotein, for example.

3. Reduction and oxidation.

Reduction of

the anomeric center at C-1 of glucose (2) produces

the sugar alcohol sorbitol. Oxidation of

the aldehyde group at C-1 gives the intramolecular

ester (lactone) of gluconic acid (a glyconic

acid). Phosphorylated gluconolactone is

an intermediate of the pentose phosphate

pathway . When glucose is oxidized

at C-6, glucuronic acid (a glycuronic

acid) is formed. The strongly polar glucuronic

acid plays an important role in biotransformations

in the liver .

4. Epimerization.

In weakly alkaline solutions,

glucose is in equilibrium with the

ketohexose D-fructose and the aldohexose Dmannose,

via an enediol intermediate (not

shown). The only difference between glucose

andmannose is the configuration at C-2. Pairs

of sugars of this type are referred to as epimers,

and their interconversion is called epimerization.

5. Esterification.

The hydroxyl groups of

monosaccharides can form esters with acids.

In metabolism, phosphoric acid esters such as

glucose 6-phosphate and glucose 1-phosphate

(6) are particularly important.

B. Polarimetry, mutarotation 

Sugar solutions can be analyzed by polarimetry,

a method based on the interaction between

chiral centers and linearly polarized

light—i. e., light that oscillates in only one

plane. It can be produced by passing normal

light through a special filter (a polarizer). A

second polarizing filter of the same type (the

analyzer), placed behind the first, only lets the

polarized light pass through when the polarizer

and the analyzer are in alignment. In this

case, the field of view appears bright when

one looks through the analyzer (1). Solutions

of chiral substances rotate the plane of polarized

light by an angle α either to the left or to

the right. When a solution of this type is

placed between the polarizer and the analyzer,

the field of view appears darker (2).

The angle of rotation, α, is determined by

turning the analyzer until the field of view

becomes bright again (3). A solution’s optical

rotation depends on the type of chiral compound,

its concentration, and the thickness of

the layer of the solution. Thismethodmakes it

possible to determine the sugar content of

wines, for example.

Certain procedures make it possible to obtain

the α and β anomers of glucose in pure

form. A 1-molar solution of α-D-glucose has a

rotation value [α]D of +112°, while a corresponding

solution of β-D-glucose has a value

of +19°. These values change spontaneously,

however, and after a certain time reach the

same end point of +52°. The reason for this is

that, in solution, mutarotation leads to an

equilibrium between the α and β forms in

which, independently of the starting conditions,

62% of the molecules are present in the

β form and 38% in the α form.