A Sweet Little Quiz

Answer: CH2O

The chemical formula is the actual number of each atom in a molecule. The empirical formula is the simplest whole number ratio of atoms in a molecule. This is often written as (CH2O)n for carbohydrates, where n is any whole number integer. For example, a carbohydrate can technically be CH2O, C2H4O2, C3H6O3, etc.

In reality, the majority of sugars contain five or six carbons, though three and four carbon sugars do exist. Sugars can be named according to the number of their carbons. For example, six-carbon sugars are all hexoses, whilst five-carbon sugars are pentoses.

The empirical formula reveals the logic of the nomenclature of these biomolecules ("carbo-" (carbon) "-hydrates" (water) = CH2O).

Answer: Carbonyl

Aldehydes and ketones are very similar and both contain the carbonyl functional group, which is a carbon double bonded to an oxygen. If the carbon that is double bonded to this oxygen is itself bonded to only one other carbon, it is an aldehyde and if it is attached to two carbons, it is a ketone.

The suffix "-ose" is reserved for sugars and so these can be categorised as aldoses or ketoses. Sugars may be named according to whether they are an aldehyde or a ketone, as well as the number of carbons they have.

For example, glyceraldehyde (pictured left) is an aldotriose. This nomenclature gives most of the vital information biochemists need about a sugar - "aldo" (it's an aldehyde), "tri" (there are three carbons), "ose" (it is a sugar).

Answer: Glucose is an aldohexose. Fructose is a ketohexose.

As seen in the diagram, in glucose, the carbon atom double bonded to the oxygen as part of the carbonyl group is only attached to one other carbon atom, and is therefore an aldehyde group. Conversely, the equivalent carbon in fructose is attached to two carbon atoms and so is a ketose. Both contain six carbon atoms and so can be described as hexoses.

Answer: The chiral carbon that is furthest from the carbonyl group

Naturally occurring sugars are invariably D-isomers, whereas all naturally occurring amino acids are L-isomers. Optical isomers may be D (dextro, meaning right) or L (levo, meaning left) depending on the direction they rotate polarised light. Samples with equal amounts of each isomer are said to be racemic. Such a mixture can be harmful.

For example, only one isomer of a drug may be effective, whereas the other can cause unwanted side-effects. The best known example of this is thalidomide.

Answer: The aldose

Epimerisation occurs when a carbon is chiral (i.e. when it is attached to four different chemical groups). Whether there are 3, 4, 5, or 6 carbons, the aldose will always have more chiral carbons than a ketose of equivalent chemical formula, and so will be capable of existing as a greater number of epimers.

For example, in the image, D-Ribose (left) has three chiral carbons, whereas D-Ribulose (right) only has two. In Fischer projections (as are shown in the image), epimers vary only in whether the -OH group is facing left or right.

For example, D-mannose and D-glucose are epimers since the configuration of their groups differs at carbon 2. It should be noted that carbon atoms are numbered according to the end which has the carbonyl group closest to it.

For example, the terminal carbon nearest to the carbonyl group is carbon 1, the next one down is carbon 2, and so on.

Answer: Hemiacetals and hemiketals

The cyclisation is spontaneous and is an example of nucleohpilic attack. In the case of glucose (pictured) the -OH group of carbon 5 "attacks" the relatively positive carbon 1 atom, which is otherwise double-bonded to oxygen in the sugar's aldehyde group.

This reaction forms a covalent bond between carbon 1 and the O of carbon 5. The resulting group is called a hemiacetal or hemiketal. Acetal and ketal are formed when glycosidic bonds form between sugars.

Answer: Alpha or beta

The hemiketal or hemiacetal carbon is said to be anomeric. If the -OH attached to this carbon is below the plane of the sugar ring, it is the alpha anomer. If it is above the plane of the ring, it is the beta anomer. Since sugars are constantly switching between open and closed-ring forms, it is chance whether beta or alpha anomers will form upon each cyclisation.

This is called mutorotation. For glucose, the beta anomer is slightly more stable and so predominates in solution.

Answer: Furanose

These ring structures are represented in this quiz (as they are in the literature) as Howarth projections. This is in contrast with the open chain sugars, which are presented as Fischer projections.

So far we have looked at optical isomerism, anomerism, whether the ring is six or five-membered, and types of sugar. These can all be incorporated into the names of sugars to give accurate scientific descriptions. For example, glucose can be described as alpha-D-glucopyranose. Equally, it may be beta-D-glucopyranose. Saying "glucose" is therefore ambiguous and it may be necessary to elaborate on the structural aspects of the molecule in question by using this alternative naming system.

Answer: Chair

In the chair conformation, groups can be axial or equatorial to the ring. The most stable form is that which has the bulkiest groups in the equatorial position. This explains why the beta anomer of glucose is more stable (since its bulky -OH group on carbon 1 is equatorial rather than axial).

Answer: A glycosidic bond

A common type of glycosidic bond involves the linking of carbon 1 from one sugar and carbon 4 from another, via an oxygen atom (as shown in the picture). This bond would be called a 1,4-glycosidic bond. The carbon 1 of the first sugar will be anomeric, and whether it is alpha or beta is important to the type of bond which forms.

For example, when many glucose molecules join via alpha-1,4-glycosidic bonds, starch is made (or glycogen). However, when the same molecules join via beta-1,4-glycosidic bonds, cellulose (found in plant cell walls) is formed.