Monosaccharides that contain five or more carbon atoms form cyclic structures in aqueous solutions.
Many five- and six-carbon sugars can exist either as a linear chain or in one or more ring-shaped forms. These forms exist in equilibrium with each other, but equilibrium strongly favors the ring forms (particularly in aqueous, or water-based, solution). In the figure below, you can see (a) the two-dimensional Fischer projection of glucose, (b) a three-dimensional representation of the glucose molecule during ring formation, and (c) the final cyclic form of glucose in simple three-dimensional representation (called a Haworth projection).
When a straight-chain monosaccharide forms a cyclic structure, the carbonyl oxygen atom may be pushed either up or down, giving rise to two stereoisomers that are anomers. The anomeric carbon can take on either an α- or β-conformation when the ring forms. During ring formation, the O from the carbonyl, which is converted to a hydroxyl group, will be trapped either “above” the ring (on the same side as the CH₂OH) or “below” the ring (on the opposite side from this group). When the hydroxyl is down, glucose is said to be in its α form, and when it’s up, glucose is said to be in its β form.
The opening and closing of sugars repeats continuously in an ongoing interconversion between anomeric forms and is referred to as mutarotation. Cyclic compounds undergo mutarotation, in which they shift from one anomeric form to another with the straight-chain structure.
Practice Questions
Khan Academy
The structure of monosaccharides
Hemiacetal formation of carbohydrates
MCAT Official Prep (AAMC)
Chemistry Question Pack Question 53
Key Points
• Monosaccharides that contain five or more carbons atoms form cyclic structures in aqueous solutions.
• Two cyclic stereoisomers can form from each straight-chain monosaccharide; these are known as anomers.
• In an aqueous solution, an equilibrium mixture forms between the two anomers and the straight-chain structure of a monosaccharide in a process known as mutarotation.
Key Terms
Monosaccharides: The basic unit of carbohydrates that cannot be hydrolyzed to simpler chemical compounds with the general formula (CH2O)n.
Aqueous solutions: Solutions in which the solvent is water.
Fischer projection: A two-dimensional representation of a molecule that maintains information about its absolute configuration.
Haworth projection: A simple three-dimensional representation of a monosaccharide.
Stereoisomers: Each of two or more compounds differing only in the spatial arrangement of their atoms.
Anomers: A type of stereoisomer that differs in configuration at the hemiacetal or acetal carbon; they are a specific type of epimer.
Anomeric carbon: A carbon derived from the carbonyl carbon (the ketone or aldehyde functional group) of the open-chain form of the carbohydrate molecule.
α form: The configuration of a cyclic monosaccharide where the oxygen attached to the anomeric carbon is on the opposite side of the ring from the CH₂OH group.
β form: The configuration of a cyclic monosaccharide where the oxygen attached to the anomeric carbon is on the same side of the ring as the CH₂OH group.
Mutarotation: The change between two anomers in solution when the corresponding stereocenters interconvert.