Tutorial on How to Name Carbohydrates

Carbohydrates:

Carbohydrates are known as sugar and composed of monosaccharides, disaccharides, and polysaccharides. Monosaccharides cannot be broken down into simple sugar whereas disaccharides and polysaccharides can be broken into simple sugars. Saccharides mainly consist of carbon, hydrogen, and oxygen. A monosaccharide’s general formula is CnH2nOn, where the integer n is at least 3 and rarely more than 6. Monosaccharides also contain the hydroxyl (alcohol, OH) functional group, whereas disaccharides and polysaccharides contain both the hydroxyl (alcohol, OH) functional group and a glycosidic bond between two simple sugar molecules.

How to Name Acyclic Monosaccharide:

Acyclic monosaccharides have three different characteristics: the number of carbon atoms it contains, its D or L configuration, and the placement of its carbonyl group (aldehyde or ketone). These characteristics are combined to name monosaccharide carbohydrates.

Three carbon monosaccharides are called trioses, four carbon are called tetroses, five carbon are called pentoses, six carbon are hexoses, and so on. The number of carbon atoms in a molecule is used in the suffix of carbohydrate naming.

This sugar is pentose because This sugar is hexose it has 5 carbons. because it has 6 carbons.

The carbonyl group is placed at the beginning of the saccharide. If the carbonyl group is an aldehyde (more common in carbohydrates), then the monosaccharide is an aldose, and if the carbonyl group is a ketone, then the monosaccharide is a ketose.

This sugar has an aldehyde functional This sugar has a ketone functional group so it is an aldose. group so it is a ketose. A monosaccharide is given D configuration if the hydroxyl group is to the right of the last stereocenter in a Fischer projection, whereas L configuration is given if the OH is to the left of the last stereocenter carbon. D or L is usually put in the beginning of the carbohydrate when naming the molecule.

Note: Designation of D or L relates the configuration of a given molecule and does not specify the sign (+ or -) of rotation of plane-polarized light.

This sugar has L configuration because This sugar has D configuration because OH is to the left of the last stereocenter. OH is to the right of the last stereocenter. Putting all the concepts together:

L-aldopentose D-ketohexose More examples:

This is D-aldopentose because the OH (circled) is located to the right of the last stereocenter carbon. It is an aldopentose because there are 5 carbons, which gives it the suffix pentose and it contains an aldehyde, which gives it the prefix aldo.

This is D-aldohexose because OH (circled) is This sugar is L-aldohexose because OH (circled) is located to the right of the last stereocenter carbon. located to the left of the last stereocenter carbon. It is an aldohexose because It is an aldohexose because there are 6 carbons, which gives it the suffix there are 6 carbons, which gives it the suffix hexose and it contains an aldehyde, which gives hexose and it contains an aldehyde, which gives it the prefix aldo. it the prefix aldo.

How to name Cyclic Monosaccharide

Cyclic monosaccharides mostly consist of 5 or 6 carbon ring. 5 carbon rings have the suffix fruscose and 6 carbon rings have the suffix pyranose. Also, when acyclic molecules are turned into cyclic molecule, it creates a chiral center at the anomeric carbon and this gives rise to α and β anomers. α means that the anomeric OH and CH2OH groups are trans whereas β means anomeric OH and CH2OH groups are cis. These anomers are placed in the beginning in naming cyclic carbohydrates.

This concept is shown in the examples below:

This is β because OH and CH2OH are cis. This is α because OH and CH2OH are trans.

This sugar is β -galactopyranose This sugar is α -glucopyranose because because there are 6 carbons, which gives it the suffix there are 6 carbons, which gives it the suffix pyranose and since the molecule is galactose, pyranose and since the molecule is glucose, it has the prefix galacto. it has the prefix gluco. How to Name Cyclic Disaccharides A disaccharide is two monosaccharide units linked by a glycosidic bond. The linkage between these two bonds is labeled by the carbon that they are attached to. For example, in both maltose (Figure 1) and cellobiose (Figure 2), the glucose units are 1-4 linked, meaning that the C-1 of one glucose is linked by a glycosidic bond to the C-4 oxygen of the other glucose. The only difference between them is in the configuration at the glycosidic bond. Maltose exists in α configuration, whereas cellobiose is β. Figure 1: Maltose

This disaccharide is 1,4’-α-glucopyranosyl-glucopyranose. The 1-4’ comes from the carbons attached to the glycosidic bond. It is α because carbon 1 and carbon 4 are trans to CH2OH. It is glucopyranosyl because the first molecule is glucose. Glucopyranose comes from the second molecule because it is glucose.

Figure 2: Cellobiose

This disaccharide is 1,4’- β- glucopyranosyl-glucopyranose. The 1-4’ comes from the carbons attached to the glycosidic bond. It is β because carbon 1 and carbon 4 are cis to CH2OH. It is glucopyranosyl because the first molecule is glucose. Glucopyranose comes from the second structure because it is glucose.

Figure 3: Lactose

This disaccharide is 1,4’- β- galactopyranosyl-glucopyranose. The 1-4’ comes from the carbons attached to the glycosidic bond. It is β because carbon 1 and carbon 4 are cis to CH2OH. It is galactopyranosyl because the first sugar molecule is galactose. Glucopyranose comes from the second structure because it is glucose.

How to Name Cyclic Polysaccharides

The most common polysaccharides are cellulose (Figure 1) and starch (Figure 2).

Figure 1: Cellulose

This polysaccharide is 1,4’- β -glucopyranose polymer. All of the individual sugars are glucose so the prefix is gluco and they all contain 6 carbons so it’s a pyranose. It is β because carbon 1 and carbon 4 are cis to CH2OH. The word polymer is added because it’s a chain of glucose.

Figure 2: Starch

This polysaccharide is 1,4’- α-glucopyranose polymer. All of the individual sugars are glucose so the prefix is gluco and they all contain 6 carbons so it’s a pyranose. It is α because carbon 1 and carbon 4 are trans to CH2OH. The word polymer is added because it’s a chain of glucose.

Summary:

• To determine acyclic carbohydrate name:

1. Determine the number of carbons.

2. Determine whether the molecule contains ketone or an aldheyde functional group.

3. Determine D or L configuration by looking at the position of the OH bonded to the last stereocenter carbon.

• To determine cyclic carbohydrate name:

1. Determine the number of carbons.

2. Determine whether the molecule contains ketone or an aldheyde functional group.

3. Determine α or β configuration by looking at the position of the OH that is attached to the anomeric carbon and CH2OH.

Work Cited

Carbohydrates. <http://web.virginia.edu/Heidi/chapter7/chp7frameset.htm>.

Dr. Hardinger’s Thinkbook and Lecture Supplement

Metzker, Julia.Carbohydrates. <http://chemphys.gcsu.edu/~metzker/Survey/Notes/16.Carbohydrates.html>.

Molecule Images from Google Images

Wikipedia