Key difference - L vs. D isomers
Monosaccharides are the simplest form of sugars. Monosaccharides can combine with each other to form disaccharides, oligosaccharides, and polysaccharides. All monosaccharides consist of C, H and O atoms, which are arranged in aldehyde or ketone form . The configurations of monosaccharides often have slight differences in their isomers. It is therefore important to name monosaccharides precisely in order to be able to distinguish them. The D, L convention is one such 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 on the right side of the D isomer, while the L isomer is on the left.
Key areas covered
1. What is the 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 main differences
Key Terms: Aldehyde, D-Isomer, Fischer Projection, Isomer, Ketone, L-Isomer, Monosaccharide, Penultimate Carbon, Polysaccharide
What is Fischer Projection?
The Fischer projection is the two-dimensional representation of a three-dimensional molecule. It was originally introduced by Hermann Emil Fischer to show the configuration of sugar molecules.
Figure 1: D-galactose
The picture above shows the Fischer projection of the acyclic galactose molecule. The naming of this type of molecule starts with the numbering of the carbon atoms. The numbering is based on the priority of the function groups. For the above molecule, the carbon atom of the aldehyde group is given the number 1. So it has 6 carbon atoms from top to bottom. The asymmetric carbon, this is the 5th carbon. So it is called the penultimate carbon . This is because it determines whether the molecule is D or L.
What is L isomer?
If the -OH group on the penultimate carbon is on the left, it is referred to as the L-isomer. This definition is based on the Fischer projection of an acyclic monosaccharide. The L-isomer is the mirror image of the D-isomer. The physical properties are the same for two mirror images because the molecular weights are the same. The chemical properties are also similar, as the same functional groups are present in both molecules. However, their biological properties are different due to the different spatial arrangement. In addition, D and L isomers can sometimes be associated with the rotation of plane polarized light . Both D and L isomers can change the direction of plane polarized light. The L-isomer can rotate polarized light counterclockwise. It is also called the (-) enantiomer. (Now it is called the S-enantiomer).
What is D isomer?
D-isomer is the mirror image of the L-isomer of a given molecule. It has the -OH group of the penultimate carbon atom on the right. The D-isomer can also rotate planar polarized light clockwise. It is also called the (+) enantiomer. (Now it is referred to as the R enantiomer).
Figure 2: D and L isomers of galactose
The above picture shows the mirror images of galactose. The difference between D and L isomers is the position of the -OH group on the penultimate carbon atom. D-isomer and L-isomer are mirror images of each other that cannot be superimposed.
Difference between L and D isomers
definition
L-isomer: When the -OH group of the penultimate carbon is on the left, it's called the L-isomer.
D-isomer : When the -OH group of the penultimate carbon is positioned on the right-hand side, it is referred to as the D-isomer.
Mirror images
L-isomer: The L-isomer is the mirror image of the D-isomer.
D-isomer : The D-isomer is the mirror image of the L-isomer.
Light rotation
L-isomer: The L-isomer can rotate planar polarized light counterclockwise.
D-isomer : The D-isomer can rotate planar polarized light clockwise.
Summary - L vs. D isomers
The 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 lies in the position of the -OH group on the penultimate carbon atom. In the case of the D-isomer, the OH group of the penultimate carbon is on the right-hand side, while in the case of the L-isomer the OH group of the penultimate carbon is positioned on the left-hand side.
References:
1. “Difference between L and D forms of amino acids?” Prahran health food. Np, October 29, 2014. Web. Available here. June 19, 2017. 2. “D and L are out of date and incorrect.” D and L configurations. Np, nd web. Available here . 19th 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
