Monosaccharides are carbohydrates which consist of a single polyhydroxy aldehyde or ketone unit and are also called as simple sugars. These are classes of sugars that can’t be hydrolyzed to simpler sugars. However, they can polymerize to form oligosaccharides and polysaccharides. They have various structural aspects which include:
D and L-isomers
The D and L isomers are mirror images of each other. The spatial orientation of -H and -OH groups on the carbon atom (Cs for glucose) that is adjacent to terminal primary alcohol carbon determines whether the sugar is D- or L-isomer. If -OH group is on right side sugar is of D-isomer, and on left side, it is L-isomer. It may be noted that naturally occurring monosaccharides in mammalian tissues are mostly of D-configuration. The enzyme machinery of cells is specific to metabolism of D-isomers.
If D- and L-isomers are present in equal concentration, it is known as racemic mixture or DL mixture. Racemic mixture does not exhibit any optical activity, since the dextrorotatory and levorotatory activities cancel each other.
When two monosaccharides differ from each other in their configuration around a single specific carbon (other than anomeric), they are referred as epimers to each other. For example, D-glucose and D-mannose, which differ only in stereochemistry at C-2, are epimers, D-glucose and D-galactose (which differ at C-4).
The interconversion of epimers i.e. glucose to galactose and vice versa is called epimerization which is catalysed by epimerase enzyme.
This is a special type of stereoisomerism where two stereoisomers are mirror images of each other. The two members are designated as D- and L-sugar. Majority of the sugars in higher animals (including man) are of D-type. The stereoisomers that are not mirror images of one another are called diastereomers.
Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers, and the carbonyl carbon atom is called the anomeric carbon. It refers to geometric variation.
E.g. α and β-cyclic forms of D-glucose are known as anomers. In case of α anomer, -OH group held by anomeric carbon is on opposite side of the group -CH2OH of sugar ring. The reverse is true for β-anomer. The anomers differ in certain physical and chemical properties.
Note point: An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal, respectively, creating a new chiral center at the carbonyl carbon. Substitution of a second alcohol molecule produces an acetal or ketal.
Download Link: Structural Aspects of Monosaccharides- Part 2.pdf