Difference Between L and D Isomers

Main Difference – L vs D Isomers

Monosaccharides are the most basic form of sugars. Monosaccharides can combine with each other forming disaccharides, oligosaccharides, and polysaccharides. All monosaccharides are composed of C, H and O atoms arranged in aldehyde or ketone forms. The configurations of monosaccharides often have slight differences in their isomers. So it is important to name monosaccharides accurately in order to distinguish them. D, L convention is such a way of naming monosaccharides according to their configuration. The main difference between L and D isomers is that the OH- group of the penultimate carbon is positioned on the right side of the D isomer whereas, in L isomer, it is located on the left side.

Key Areas Covered

1. What is Fischer Projection
2. What is L Isomer
      – Definition, Properties
3. What is D Isomer
      – Definition, Properties
4. What is the difference between L and D Isomers
      – Comparison of Key Differences

Key Terms: Aldehyde, D Isomer, Fischer Projection, Isomer, Ketone, L Isomer, Monosaccharide, Penultimate Carbon, Polysaccharide

What is Fischer Projection

Fischer projection is the two-dimensional representation of a three-dimensional molecule. It was introduced by Hermann Emil Fischer originally to show the configuration of sugar molecules.

Figure 1: D-Galactose

The above image shows the Fischer projection of acyclic Galactose molecule. The naming of this kind of molecule is started with numbering the carbon atoms. Numbering is done according to the priority of the functional groups. For the above molecule, number 1 is given to the carbon atom of aldehyde group. So it has 6 carbon atoms from top to bottom. The asymmetric carbon atom present here is the 5^th carbon. So it is called the penultimate carbon. This is because it determines whether the molecule is D or L.

What is L Isomer

When the –OH group of the penultimate carbon is positioned on the left side, it is known as the L isomer. This definition is given according to the Fischer projection of an acyclic monosaccharide. L isomer is the mirror image of the D isomer. Physical properties are the same for two mirror images because the molecular masses are the same. Chemical properties are also similar because the same functional groups are present in both molecules. But their biological properties are different due to the different spatial arrangement. In addition, D and L isomers can be sometimes related to the rotation of plane polarized light. Both D and L isomers can change the direction of plane polarized light. L isomer can rotate plane polarized light anticlockwise. It is also called (-) enantiomer. (Now it is referred to S-enantiomer).

What is D Isomer

D isomer is the mirror image of the L isomer of a particular molecule. It has the –OH group of the penultimate carbon atom on the right side. D isomer can also rotate plane polarized light in the clockwise direction. It is also called (+) enantiomer. (Now it is referred to R-enantiomer).

Figure 2: D and L isomers of Galactose

The above image shows the mirror images of Galactose. The difference between D and L isomers is the position of –OH group in the penultimate carbon atom. D isomer and L isomer are non-superimposable mirror images of each other.

Definition

L Isomer: When the –OH group of the penultimate carbon is positioned on the left side, it is known as an L isomer.

D Isomer: When the –OH group of the penultimate carbon is positioned on the right side, it is known as a D isomer.

Mirror Images

L Isomer: L isomer is the mirror image of D isomer.

D Isomer: D isomer is the mirror image of L isomer.

Rotation of Light

L Isomer: L isomer can rotate plane polarized light anticlockwise.

D Isomer: D isomer can rotate plane polarized light clockwise.

Summary – L vs D Isomers

L and D isomerism is most commonly used with sugar molecules. It is a naming system used to name two dimensional configurations or Fischer projections of molecules. The main difference between L and D isomers is in the position of –OH group in the penultimate carbon atom. In D isomer, the OH- group of the penultimate carbon is positioned on the right side whereas, in L isomer, the OH- group of the penultimate carbon is positioned on the left side.

References:

1.”Difference between L and D forms of amino acids?” Prahran Health Foods. N.p., 29 Oct. 2014. Web. Available here. 19 June 2017.
2. “D and L Are Outmoded and Wrong.” D and L Configurations. N.p., n.d. Web. Available here. 19 June 2017.

Image Courtesy:

1. “D-galactose” By User:Rob Hooft – Own work by Rob Hooft (CC BY-SA 3.0) via Commons Wikimedia
2. “DL-Galactose num” By NEUROtiker – Own work, Public Domain) via Commons Wikimedia