Cyclic structure of glucose

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Cyclic structure of glucose
(Alpha and beta isomers/Haworth projections (lower) and ring conformers (upper) (Laghi.l/Wikipedia))

Glucose (C6H12O6) is the principal source of energy for most living things. The body converts most dietary carbohydrates into glucose. Glucose is a monosaccharide, or simple sugar, a class of molecules that forms 5- and 6-sided rings, not unlike a benzene ring. In solution, the glucose molecule prefers a ring (cyclic) shape. Unlike benzene, not all points of its hexagon are carbon atoms.

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Cyclic Shape

Outside of the open-chain, or dry, form, glucose is circular in structure. Its hexagonal shape has five carbon atoms and one oxygen atom at its six corners. Hydrogen and hydroxide groups (-OH) extend off four of these corner carbons. This circular version of glucose is usually in the "chair form" -- as opposed to the boat form -- so called because its sides fold in opposite directions (see diagram).

Glucose prefers the cyclic (circular) shape in solution, but at equilibrium some 0.02 per cent of glucose rings are open, i.e., in the "dry form."

Cyclic structure of glucose
Glucose with carbons numbered (Red_Hooft/Wikipedia)

Polymers of Glucose Rings

Glucose is one of the simple sugars, along with galactose, fructose, xylose, and ribose, among others. Technically, they don't hydrolyze (break down by addition of water) into simpler carbohydrate units. They are as simple as sugar gets.

Many complex carbohydrates (starch, glycogen, cellulose) are polymers (long chains) of the glucose ring, repeating over and over. This is easily demonstrated using acid to hydrolyze them into monosaccharides.

More Than One Cyclic Form

Glucose has two isomers (angular forms) with the chair form. The alpha and beta chair forms differ from each other by the orientation of one of the hydrogen-hydroxide orientations (see top diagram). A third cyclic isomer is called the "boat form," because the hexagon shape is cuplike.

Cyclic structure of glucose
Chair and boat forms (Mamorrow5/Wikipedia)

Ring Points

Monosaccharides favour 5- and 6-sided rings because they bring the angles between the carbon bonds closest to the ideal of 109.5 degrees. This is the natural angle for them, as can be seen from the separation angle of bonds in a methane (CH4) molecule, for example.

Six-membered cyclic structures like glucose are called "pyranose," after the hexagonal pyran molecule, which has 5 carbons and an oxygen in its ring just like glucose.

Cyclic structure of glucose
Methane with angles at 109.5 degrees (Benjah-bmm27/Wikipedia)

Representation of Cyclic Form

Representation of the cyclic form can be done as a flat structure or in a way that indicates the true angles of the bonds. The flat structure is called a "Haworth projection." "Ring conformers" present side views to make the angle difference, and the resulting puckering of the ring, clearer. A comparison can be found on the Michigan State University website.

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