7.5 Simple Carbohydrates

Carbohydrates are a class of polyols. There are simple carbohydrates, known popularly as sugars, and complex carbohydrates such as starch and cellulose. Simple carbohydrates have one oxygen atom for every carbon atom.

The most common sugar is D-glucose, also called dextrose, as it is dextrorotary. It is a monosaccharide, that is, a single unit of a sugar molecule. It occurs in two main forms: an open aldehyde and a closed ring form, called a pyranose:

aldehyde form of glucose.

α-D-glucopyranose.

β-D-glucopyranose.

Pyranoses are so called for their resemblance to the ring structure of pyran (see lesson 4.10). Glucose is also capable of forming a furanose, so called for its 5-membered ring that resembles furan, however, the pyranose form is the usual monomer in complex carbohydrates.

Like the amino acids, D- and L- glucose are named for their resemblance to d- and l-glyceraldehyde, which itself is also a sugar:

D-glyceraldehyde

L-glyceraldehyde

L-serine for comparison.

The numbering of carbon atoms in the aldehyde form is kept for the pyranose form; C1 combines with the oxygen of carbon 5 to form a hemiacetal. Therefore we know that the carbon atom bonded to two oxygens is carbon atom number 1 or C1. C2, C3, C4, and C5 continue around the ring toward the -CH₂OH group, which is C6.

Having glucose as the monomer for complex carbs is a good strategy. Its chiral centers are arranged so that all the hydroxy groups can lie in the equatorial position, keeping the relatively bulky hydroxy groups farther apart. There are seven other sugars analogous to glucose, each with its own D- and L- forms, differing only in the arrangements of the chiral centers.

But the hydroxy of C1 can be either equatorial or polar, since both configurations are possible from the hemiacetal formation. Further, as long as that oxygen atom retains that hydrogen, the two forms can interconvert - they are tautomers - so they cannot be isolated and purified, at least not in simple sugar form.

The tautomer with all equatorial hydroxy groups is called beta-glucose, and the tautomer with a polar hydroxy on C1 is called alpha-glucose.

Now you have all the information necessary to understand the notation of the glycosidic bonds that link glucose monomers together to form complex carbohydrates.

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