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In nature the acetal formation occurs via intermediate steps (activated sugars,
e.g. GDP-glucose) and enzymes (transferases), but chemically (and
thermodynamically) the result is the same as shown above. Note that a
molecule of water is released in the reaction.
The opposite reaction (acetal + H
2
O
→
hemiacetal) is called hydrolysis since
water is a reagent. The reaction is normally acid catalysed, and is normally
called acid hydrolysis of glycosidic linkages.
In contrast, glycosidic bonds are generally stable to alkali, although with some
notable exceptions, as discussed in a later section.
5.1.2. Acid hydrolysis –
cyclic mechanism
The most common mechanism of acid hydrolysis of glycosides – including
polysaccharides - is the ‘cyclic mechanism’. It involves the following steps:
CHO
OH H
H HO
OH H
OH H
CH
2
OH
D-glucose (open chain - aldehyde)
HC
OH H
H HO
OH H
H
CH
2
OH
OH
O
Hemiacetal
O
H
HO
H
HO
H
H
OH H
OH
OH
=
O
H
O
H
HO
H
H
OH H
OH
OH
Hemiacetal
O
H
HO
H
HO
H
H
OH H
O
OH
Alcohol
O
H
O
H
HO
H
H
OH H
O
OH
O
H
H
HO
H
H
OH H
O
OH
Acetal
H
2
O