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Sugars are structurally related to one of two compounds; glyceraldehyde or dihydroxyacetone. Thus they contain either an aldehyde group or a ketone group respectively. These groups are very important in sugar chemistry because they are involved in ring-closure and provide the chemical reducing potential of the molecule.

This distinction is reflected in the description of a sugar as being either an aldose or a ketose (-ose means sugar). Eg aldohexose and ketohexose are sugars containing six carbon atoms one of which is an aldehyde and the other a ketone respectively. Aldohexose will often just be referred to as hexose and the ketohexose as hexulose.

An aldehyde is a highly reactive organic compound produced by the oxidation of an alcohol and having a CHO group.

A ketone is an organic compound characterized by a carbon atom doubly bonded to an oxygen atom and to two carbon atoms.

What does D or L mean?

Nearly all the sugars in the body (and the polysaccharides made from them) are derived from the 3-carbon sugars glyceraldehyde (aldotriose) and dihydroxyacetone (ketotriose). The structures are shown opposite.

The sharp-eyed will have noticed that glyceraldehyde has the letter "D" in front of it while dihydroxyacetone does not. This is the same "D" which is often used in the names of sugars like D-glucose. Although it is very much less common in biology, there is also L-glucose.

So what does this D & L mean, why is it important and why doesn't dihydroxyacetone have one?

The prefixes D & L distinguish two different forms of glyceraldehyde. The molecular diagrams commonly used to depict sugars, amino acids and other compounds are 2-dimensional representations of 3-dimensional molecules. These 2-dimensional diagrams are called Fischer projections and, while very useful, can be confusing because they do not contain information about bond angles in the dimension perpendicular to the plane of the paper. This is explained in more detail below but, for the moment, take it as read that the two molecules shown opposite are, actually quite different and L-Glyceraldehyde is not simply a rotation of the middle carbon atom around the vertical axis.

In D-Glyceraldehyde the hydroxyl group is positioned to the right and in L-Glyceraldehyde it is positioned to the left. All the sugars derived from D-Glyceraldehyde are D-sugars and those from L-Glyceraldehyde are L-sugars eg D-Glucose and L-Glucose. Except that you will never come across L-Glucose because only the D-form is recognised in biological systems. A similar situation is found with amino acids except with these it is the L-form which predominates in biology.

D & L are confusing terms

It is a common misconception that D stands for dextrorotatory and L stands for laevorotatory (see below). They do not and the choice of which orientation is D and which is L is entirely arbitary. This will become clearer later.

This is important because

The D & L forms of compounds are different shapes and enzymes recognise substrates partly by their shape. Wrong shape means no reaction and enzymes are nearly always geared up to recognise the D-form of sugars and the L-form of amino acids.

So what about dihydroxyacetone and why are there not D and L forms of this compound?

The answer to these questions lies in understanding something called symmetry.

Symmetry, asymmetry & chirality

Now we need to move away from Fischer projections and think three dimensionally. Carbon can form 4 covalent bonds but these do not all lie in the same plane.

A line drawn between the 4 groups bonded to carbon describes a regular tetrahedron which is composed of four triangular faces, three of which meet at each vertex. See opposite.

Because this is a regular tetrahedron it is symmetrical and a mirror image is no different from the original. Compare A and B opposite.

If one of the hydrogens is replaced with, say, a chloride ion then it would be possible to rotate the mirror image and still produce an identical image. There would, in fact, be no difference between the two molecules.

The only situation in which the mirror image is not identical is if all 4 groups bonded to the carbon are different. The molecule is then said to be asymmetric.

In this example the mirror image is not identical. Try rotating version A to produce version B of the molecule.

Although the 2 versions opposite are apparently the same, the fact that the mirror image can not be superimposed on the original no matter how they are rotated means that they are actually quite different molecules with different properties, not least of which is their shape.

Because shape is important in biology this has enormous and far reaching implications.

Hands are a very good analogy. Placing a left hand on a mirror produces an image which is identical in shape to the right hand except that one can not be superimposed on the other no matter how they are rotated.

This property of "handedness" is called chirality in chemistry and the mirror images are called enantiomers.

Other types of symmetry

Naturally, because this is science, it's not quite as simple as this. There are a number of different ways a molecule can possess the property of symmetry. The most common is point symmetry. If atoms such as carbon have four different atoms or molecules attached there is no point symmetry. Other types include axial symmetry and planar symmetry but these don't have much relevance to sugars in the context of extracellular polysaccharides.

Optical rotation (+) & (-)

Chiral compounds have the property of rotating plane polarised light. The diagram opposite tries to illustrate the essential features of an instrument designed to measure this property.

Lightwaves from a source such as a conventional bulb are propagated at all angles relevant to the direction of travel. Passing this light through a polariser filters out all the light except for that in one particular plane. This is called plane polarised light. The plane polarised light is then passed through a sample compartment, usually 10cm long which contains the compound under investigation. If this compound is chiral and is present as just one of its enantiomers (mirror images), the plane of the polarised light will be rotated.

If the light is observed from the viewpoint of the detector and the plane of polarisation is rotated to the right the compound is referred to as dextrorotatory. If it is rotated to the left it is laevorotatory.

Dextrorotatory compounds are often labelled with (+) and laevorotatory with (-).

The two enantiomers of a compound always rotate light in opposite directions.

D & L in relation to (+) & (-)

Glyceraldehyde, our simplest of sugars, is optically active because the central carbon atom is asymmetric. Furthermore, it is the D enantiomer which is (+) and the L enantiomer which is (-). However, this is entirely coincidental and the next section will take this a little further by showing how all 3, 4, 5 and 6 carbon sugars are related

3, 4, 5 and 6 carbon sugars

The optical activity of D-Glyceraldehyde is sometimes included in the name as shown opposite. Similarly the enantiomer is L-(-)-Glyceraldehyde.

Carbon numbering

It is helpful to identify the individual carbon atoms and the convention in sugar chemistry is to start numbering from the end adjacent to the aldehyde or ketone group.

Sugars comprising 4, 5, 6 or more carbons are constructed by adding CH2O residues immediately adjacent to the aldehyde or ketone as shown by the red arrows opposite. Addition of a CH2O residue adds a further chiral centre to the molecule for reasons described above, therefore two possibilities exist creating two further enantiomers. In this case these are the two 4-carbon sugars called erythrose and threose.

Note that these are both referred to as D because the configuration is related to D-Glyceraldehyde. There is also L-Erythrose and L-Threose. Note also, however, that Threose rotates plane polarised light in a direction opposite to that of Erythrose.

Each of these two 4-carbon sugars can now give rise to two further 5-carbon sugars and so on.

The diagram below shows the relationship between all the 3, 4, 5 & 6 carbon sugars based on D-Glyceraldehyde. Of these, ribose, glucose, mannose and galactose are probably the most familiar.

Note that glucose and galactose differ only in the configuration around carbon-4. Molecules like this are called epimers.

Similarly for the 4,5 and 6 carbon ketose sugars which follow the same carbon-numbering convention as the aldoses. Note, however, that because dihydroxyacetone is not chiral D-erythrulose is the only 4 carbon ketose. There is, of course, an L-erythrulose because the newly added CHOH introduces an asymmetric carbon.

Fructose is one of the four 6-carbon D-ketoses. Psicose and tagatose are less familiar but anyone interested in oral health might recognise sorbose. Reduction of sorbose gives rise to the sugar alcohol sorbitol which has been included in some confectionary as a non-cariogenic substitute for sucrose. Similarly, xylitol, the other and more commonly used non-cariogenic sweetener, is derived from the 5-carbon pentose xylose.

What about α and β?

Five and six carbon sugars (aldoses and ketoses) exist only momentarily as open chains. The aldehyde and ketone groups spontaneously react with neighbouring alcohol groups to form a cyclic hemiacetal or hemiketal. Glucose is shown opposite by way of example.

Ring formation makes the carbon associated with either the aldehyde or ketone group asymmetric and, therefore, there will be two forms. In this case they are called α and β. These two new forms are called anomers.

The more familiar Hawarth projections are shown below.

In solution these 5 and 6-carbon hemiacetals and hemiketals spontaneously revert to the straight-chain form and back again. While in the straight-chain form the aldehyde and ketone groups can revolve about the single carbon bond. This means that when the cyclic form is recreated there is a chance that the orientation of the aldehyde or ketone-associated carbon will be reversed. Thus, in solution, a dynamic equilibrium is established between α and β the anomers.

For glucose solutions, the ratios of the various forms are 36% α, 63% β, <1% open chain.

Similarly for ketoses using fructose as the example

And finally, what about pyranosyl and furanosyl?

Five membered sugars resemble furan, hence furanoses.

Six membered rings resemble pyran, hence pyranoses.

Although aldohexoses and ketohexoses favour the pyran and furan forms respectively, ring closure can happen between the adehyde or ketone-carbon and other CHOH groups. Though rare, glucofuranose and fructopyranose exist.

Chemical names of carbohydrates

So, to finish off this part, understanding the chemical names of carbohydrates should now be easier. Let's take a look at sucrose by way of an example.

This is how the chemical name of sucrose is often written down. The numbers 1,2 refer to the carbon atoms of the glucose and fructose moieties respectively which form the glycosidic bond.

α-D-glucopyranosyl- (1,2)-β-D-fructofuranoside

But technically this is not correct because the bond is deemed to be 2→1 rather than 1→2

so it should really be as shown opposite. This way of drawing the structure of sucrose is not common. In fact I've never seen it anywhere.

β-D-fructofuranosyl-(2→1)-α-D-glucopyranoside

or more commonly

β-D-fructofuranosyl-(2,1)-α-D-glucopyranoside

A more common representation is that shown here. As long as the carbons and linkages are correctly numbered and the orientation of the axial and equatorial bonds are correct, it doesn't matter how the structure is drawn because none of them properly represent the real structure anyway. They are each just convenient ways of looking at the spatial orientation of the various atoms so we can get a visual handle on the molecule.

Footnote

Reducing potential

The anomeric carbon atoms provide sugars with their chemical reducing potential. Disaccharides such as mannose which have a free aldehyde group are, therefore, described as reducing sugars. Similarly, most oligo- and polysaccharides will have a reducing end to the molecule.

Sucrose, on the other hand, has the anomeric carbon atoms of both its constituent sugars involved in the glycosidic bond. Sucrose is, therefore, a non-reducing sugar. Glycosidic bonds of this type require more energy for formation and, consequently, release more energy when broken. This has implications in extracellular polysaccharide synthesis.

Back to why some sugars are called aldoses and some ketoses.

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