What Is the Structure of Fructose?

Fructose is a sugar found naturally in fruits and honey. It is also commercially manufactured. Fructose has the same chemical formula as glucose, the most common sugar in foods, but a different arrangement of the atoms gives it somewhat different properties.

Both glucose and fructose have the chemical formula C6H12O6, and both consist of a linear six-carbon chain that is preferentially formed into a ring structure. In the linear form, both have a reactive group called a carboxyl that enables the transition between ring and chain structures. The crucial difference between glucose and fructose is the location of that carboxyl group. In glucose, it is located at the end of the carbon chain, whereas in fructose it is located on the second carbon from the end of the chain.

The “natural” structure of fructose, the form that it preferentially takes in honey or fruit or when made into sugar crystals, is called furanose. This structure is depicted as a six-membered ring composed of five carbons and one oxygen. This structure is formed by the reaction of the reactive carboxyl with the carbon furthest away from it. The sixth fructose carbon is connected to the ring but is not part of it. In this form, fructose tastes twice as sweet as glucose or “regular” sugar.

Fructose is most familiar to chemistry students in its pyranose form: a five-membered ring made up of four carbons and one oxygen. This structure is formed when the reactive carbonyl at the second position reacts not with the carbon furthest away from it, but one carbon closer. In this case, there are two extra carbons that are attached to the ring but not part of it. This structure is favored in solution at higher temperatures and has a sweetness level about equal to that of glucose.

In solution, especially at higher temperatures, the furanose and pyranose forms of fructose are constantly interchanging, with the linear chain form as an intermediate. It is therefore not appropriate to refer to any fructose solution as either furanose or pyranose, regardless of the structure of the sugar crystals that were initially added. An additional wrinkle in the situation arises from the fact that the “extra” carbons that are not part of the ring can be arranged in two different ways, leading to “alpha” and “beta” forms of each ring structure. Between alpha and beta-furanose, alpha and beta-pyranose and the linear chain intermediate structure, there are usually five different fructose structures present in any solution at a given time!

It is worth noting that all of the fructose structures that occur naturally are of one “chirality,” or handedness. For all of the structures discussed above, there is a mirror image structure that has the same chemical properties but is metabolized differently in the body. The naturally occuring structures are called “D-fructose”, while the mirror image fructoses, which can be prepared synthetically, are “L-fructose.”

Fructose is also capable of forming chain structures of various lengths. Sugar chains made out of fructose are called fructans. In fructans, fructose almost always takes the pyranose form, and once the chain is formed, it is locked into that structure. Fructose can join with one other sugar molecule to make a “disaccharide.” Sucrose, for instance, the sugar most commonly found at supermarkets, is a disaccharide made of one fructose attached to a glucose molecule. Fructose can form a long-chain structure called inulin, just as glucose can form long chains to become the familiar carbohydrates starch and cellulose. Although much less common than its glucose analogs, inulin is found in Jerusalem artichokes, onions, leeks and bananas.

Fructose is sold in some grocery stores as a specialty sugar and because of its extra sweetness in the furanose form, it can save calories when used in cold drinks like iced tea. However, in baking applications and in hot drinks like coffee, the significance of fructose's changing chemistry becomes apparent. The pyranose form, which is only as sweet as glucose, dominates at temperatures above 140 degrees Fahrenheit, diminishing the calorie-savings effect. Similarly, the non-natural L-fructose has been proposed as a low-calorie sweetener but as described in "Advances in Sweeteners," it appears to be metabolized to some extent in the body, so the extent of calorie savings is questionable. However, fructose, in general, is metabolized more slowly than glucose, which has made it the sugar of choice for diabetics and other individuals with difficulty controlling their blood sugar levels.