Core Concepts
This article will discuss the structure, function, naming, and properties of sugars.
Introduction
Sugars are carbohydrate molecules found in all living things. The sugar formula is denoted as (CH2O)n, where n is the number of sugars present. A sugar’s structure is composed of an aldehyde or ketone with one or more hydroxyl groups. The figure below shows a sugar with an aldehyde circled in blue and a sugar with a ketone circled in green.
Recall an aldehyde is a carbon double bonded to an oxygen, single bonded to a hydrogen, and single bonded to an R group. A ketone is a carbon double bonded to an oxygen and single bonded to two R groups.
Monosaccharides
Monosaccharides are the simplest sugars (3-9 carbons) and the building blocks of more complex carbohydrates. The figure below shows the six most common monosaccharides.
Enantiomers
In the D conformation, the hydroxyl group is on the right or wedged on the last asymmetric carbon and the hydrogen is to the left or dashed. In the L conformation, the hydroxyl group is on the left and wedged on the last asymmetric carbon and the hydrogen to the right and dashed. This is illustrated in the figure below.
Sugar Naming
Step 1:
To begin naming, count the number of carbons in the molecule. The number of carbons yields the prefix. To finish the naming, you end with “ose” to express the molecule is a sugar. Table 1 will illustrate the prefixes and names.
Step 2:
To further classify molecules, a sugar is called an aldose if the sugar contains an aldehyde. “Aldo” would go in front of the prefix from Table 1.
A sugar is called a ketose if the sugar contains a ketone. “Keto” would go in front of the prefix from Table 1.
Our example problem contains an aldehyde, so we know it is aldohexose.
Step 3:
Finally, The D or L configuration can be places in front of the names.
We can see that the aldohexose example problem is a D-aldohexose.
Cyclic Sugars
Sugars can often take cyclic forms. A cyclic sugar is lower in energy and therefore more thermodynamically stable. In cells, the cyclic form is most common although sugars can pass through their linear forms in aqueous solution.
A pyranose form is a sugar in a 6 membered ring. A furanose is a sugar in a five membered ring. The anomeric carbon is the carbon that has an aldehyde of ketone in its open chain form which is now a hemiacetal/hemiketal. As shown below, the anomeric carbon is labeled and is a part of the ring in cyclic glucose or glucopyranose.
When a hydroxyl group on an anomeric carbon is down, it is in the dashed or right position. An arrow is pointing to this molecule’s anomeric carbon. Since the hydroxyl group is down it implies that it is also dashed or to the right. This molecule is in the D conformation
The chair conformation is the cyclic structure with the lowest energy. When drawing a ring in boat and chair form, sugars are exclusively drawn as a chair.
Anomers
Alpha and beta sugars are anomers. Anomers are another subclass of diastereomers where a new asymmetric carbon forms as a ring closes. The figure below shows an example of anomers and how they form.
α – A downward hydroxyl group on an anomeric carbon
β – A upward hydroxyl group on an anomeric carbon
Glycosidic Bonds
- Sugar – Alcohol or Amine
- The oxygen on alcohol and anomeric carbon attach -> O-glycosidic bond
- The nitrogen on the amine and anomeric carbon attatch -> N-glycosidic bond
- Sugar – Sugar
- Creates disaccharides
- α and β glycosidic bonds shown in the figure below
- To name the bond, use the form α/β-#C,#C-glycosidic bond (#C denotes which carbons attach)
Disaccharide
Reducing Sugars
Basically, Fehling’s test takes a sugar with an aldehyde and oxidizes it. As a result, the aldehyde becomes a carboxylic acid. Aldoses are reducing sugars.
Preforming Fehling’s test with ketones, no carboxylic acid forms. With this in mind, ketoses are non-reducing sugars. Table 3 summarizes the results of Fehling’s test.
Sugar Examples
- Glycogen – a homopolysaccharide of glucose linked by alpha-1,4 and alpha-1,6 glycosidic bonds used to store glucose in animals.
- Starch – a homopolysaccharide of glucose broken into two forms called amylose and amylopectin used to store glucose in plants.
- Amylose: unbranched, alpha-1,4 glycosidic bonds
- Amylopectin: branched, alpha-1,4 and alpha-1,6 glycosidic bonds
Conclusion
To summarize, sugars are linear or cyclic molecules that contain a carbon chain with hydroxyl groups and either an aldehyde or ketone to store energy. Naming sugars through their length, functional groups, and isomer structure helps identify differences in sugars. Sugars can form glycosidic bonds to other sugars, alcohols, or amines to form larger molecules.