Why are all monosaccharides reducing sugars but not all disaccharides?
1 Answer
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Explanation:
To be a reducing sugar, you have to have either an aldehyde or a ketone functional group. I'm only going to talk about Aldehydes, but it is the same for Ketones. Monomer sugars have an equilibrium between their Aldehyde form and what is called their Hemiacetal form (Linear form and cyclic form). That means that the hemiacetal carbon can turn back into an Aldehyde....and this allows it to function as a reducing sugar. All monomer sugars have this equilibrium (Ketone to Hemiketal...if we are talking about ketone sugars).
Usually when a disaccharide forms (2 glucose units, for example), the bond that links them is between the hemiacetal of the first Glucose and the 4`Hydroxy of the 2nd glucose. The first sugar hemiacetal gets converted to a acetal (no equilibrium with straight form, and so can't reduce). This leaves the 2nd glucose with an intact hemiacetal, and so this end of the disaccharide can still reduce.
Some dissacharides form when 1 hemiacetal end combines with another hemiacetal end. These form a dissacharide in which both hemiacetals have been converted to acetals.....and there is no more equilibirum with the straight (aldehyde) form, so you've lost your reducing ability.
Sucrose is an example of this. Glucose hemiacetal links with fructose hemiketal and you get this strange acetal/ketal hybrid, but there is no more hemiacetal/hemiketal, and so you lose the equilibrium with the aldehyde/ketone and you lose your reducing sugar ability.
