Carbohydrates

A six-membered ring (pyranose) forms when the carbonyl carbon (C1 in aldoses) reacts with the -OH group on C5. A five-membered ring (furanose) would involve a reaction with the -OH on C4. Glucose predominantly forms a pyranose ring.

This change in optical rotation is due to mutarotation. α-D-glucose (+112°) undergoes ring opening and reclosing to form an equilibrium mixture of β-D-glucose (+18.7°) and α-D-glucose, with the overall specific rotation settling at +52.7°.

Chitin is a linear homopolymer of N-acetyl-D-glucosamine, which is glucose with an N-acetylamino group at the C-2 position. It is linked by β-1,4 glycosidic bonds, analogous to the structure of cellulose.

Amylose forms a left-handed helix. Iodine (as I₃⁻ or I₅⁻ ions) fits into the central hydrophobic channel of this helix. The resulting charge-transfer complex absorbs light strongly, giving a characteristic deep blue-black color.

Glycogen's extreme branching creates a compact, highly soluble granule with thousands of terminal non-reducing ends. Enzymes like glycogen phosphorylase and debranching enzyme can work simultaneously at multiple ends, releasing glucose-1-phosphate far faster than from the less branched amylopectin of starch.

Starch digestion hydrolyzes starch to glucose, which is then phosphorylated to glucose-6-phosphate and isomerized to glucose-1-phosphate for entry into glycolysis. Cellulose is synthesized in plants from the activated monomer UDP-glucose, which is derived from glucose-1-phosphate.

Sucrose is dextrorotatory (+66.5°). Upon hydrolysis, the resulting fructose is strongly levorotatory (-92°), which outweighs the dextrorotation of glucose (+52.7°). The net optical rotation of the mixture (invert sugar) is negative (-19.8°), thus the rotation is inverted.

Trioses are 3-carbon monosaccharides. The simplest aldose (aldehyde-containing) triose is glyceraldehyde. The simplest ketose (ketone-containing) triose is dihydroxyacetone. Both are key intermediates in glycolysis and photosynthesis.

The osazone formation involves C-1 and C-2 of a reducing sugar. Glucose and fructose differ only in the configuration at C-1 and C-2 (glucose is an aldose, fructose is a ketose). The reaction eliminates these differences, forming an identical phenylosazone. The rest of the carbon skeleton is identical.

Monosaccharides are polyhydroxy carbonyl compounds. If the carbonyl group is at the end of the carbon chain (C1), it is an aldehyde and the sugar is an aldose (e.g., glucose). If the carbonyl group is on an internal carbon (C2), it is a ketone and the sugar is a ketose (e.g., fructose).

The nucleophilic addition of an alcohol (hydroxyl group) to a carbonyl group forms a hemiacetal (from an aldehyde) or a hemiketal (from a ketone). This intramolecular reaction converts the linear monosaccharide into its cyclic form, creating a new chiral center (the anomeric carbon).

A reducing sugar has a free anomeric carbon whose carbonyl group can be oxidized, thereby reducing another agent like Cu²⁺ in Benedict's reagent. The free aldehyde or α-hydroxyketone group is essential for this property. Non-reducing sugars lack this free group.

Enantiomers are a pair of molecules that are non-superimposable mirror images of each other. D-glucose and L-glucose are mirror images at all four chiral centers (C2, C3, C4, and C5), making them enantiomers. Diastereomers differ at one or more, but not all, chiral centers.

Epimers are sugars that differ in configuration at only one chiral center. Glucose and galactose are identical in structure except for the orientation of the hydroxyl group on C-4, making them C-4 epimers. Glucose and mannose are C-2 epimers.

In the chair conformation, bulky substituents preferentially occupy equatorial positions (pointing out from the ring) rather than axial positions (perpendicular to the ring). β-D-glucose has all its -OH and -CH₂OH groups in equatorial positions, making it the most stable and abundant hexose.

When a pure anomer (α or β) is dissolved in water, the specific rotation of the solution changes over time until a constant value is reached. This is mutarotation, resulting from the ring opening and reclosing, establishing an equilibrium mixture of α (36%), β (64%), and the open-chain form (<0.1%).

Maltose is a reducing disaccharide formed from two D-glucose units linked by an α-1,4 glycosidic bond. The C1 of the first glucose (in α-configuration) is linked to the C4 of the second glucose. The second glucose retains a free anomeric carbon, making maltose a reducing sugar.

Sucrose consists of α-D-glucose and β-D-fructose linked via a glycosidic bond between their anomeric carbons (C1 of glucose and C2 of fructose). Since both anomeric carbons are involved, neither unit can open to expose a free carbonyl group, making it a non-reducing sugar.

Lactose is a reducing disaccharide. It is formed from β-D-galactose linked to the C4 of D-glucose via a β-1,4 glycosidic linkage. The glucose unit has a free anomeric carbon, giving lactose its reducing properties.