Reducing sugar

A sugar that can donate electrons to another substance because it has a free anomeric carbon or can form one. This reactivity enables tests such as Benedict’s assay and contributes to browning during cooking.

Anomeric carbon: The carbon derived from a sugar’s carbonyl carbon that is bonded to two oxygen atoms in its cyclic form. A free anomeric carbon lets a sugar ring open into a reactive carbonyl form.
Benedict's test: A chemical assay that detects reducing sugars by reducing blue copper(II) ions to colored copper(I) oxide. It uses the sugar’s electron-donating ability to produce a visible result.
Monosaccharide: A simple carbohydrate unit that cannot be hydrolyzed into smaller carbohydrate molecules. Reducing behavior originates in the structures of individual sugar units.
Nonreducing sugar: A sugar whose anomeric carbon or carbons are unavailable for ring opening and reducing reactions. It lacks the accessible anomeric carbon that defines reducing sugars.
Glycation: The nonenzymatic attachment of sugars to proteins, lipids, or nucleic acids. Reducing sugars can react with amino groups and initiate glycation.
Mutarotation: The interconversion of a cyclic sugar’s anomeric forms through its open-chain form in solution. Ring opening and reclosing expose the carbonyl chemistry behind reducing behavior.
Fehling's solution: An alkaline copper(II) reagent used to test for aldehydes and reducing sugars. Its copper ions are reduced when heated with a reducing sugar.
Haworth projection: A way to draw cyclic sugars as planar rings with substituents shown above or below the ring. It makes the anomeric carbon and its substituents visible in cyclic sugar structures.
Sucrose: A disaccharide of glucose and fructose joined through both monosaccharides’ anomeric carbons. Its two anomeric carbons are tied up in the glycosidic bond, so sucrose is nonreducing.
Advanced glycation end product: A stable product formed through later stages of glycation and related oxidation reactions. Some glycation pathways initiated by reducing sugars produce these compounds.

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