What is L sugar?

A chemical compound that is represented with a molecular formula C₆H₁₂O₆ is called L-(-) sugar. At the carbon’s 5^th position, the hydroxyl group is placed to the compound’s left and therefore the sugar is represented as L(-)-sugar. It is capable of rotating the polarized light’s plane in the direction anticlockwise. L isomers are one of the 2 isomers formed by the configurational stereochemistry of the carbohydrates.

Carbohydrates

The polyhydroxy ketones or aldehydes or the compounds that are capable of creating such kind of units in the process of hydrolysis are termed carbohydrates. These are majorly manufactured by plants and create a significant category of organic compounds that are produced in a natural manner. It is collectively known as saccharides. Sucrose, starch, fructose, furanose, hexose, cane sugar are few examples of this macromolecule. Based on the hydrolysis process, the number of constituting units of sugar gets modified and is categorized into different categories namely monosaccharides, oligosaccharides, and polysaccharides.    

• Monosaccharides are defined as the smaller unit which will not generate any form of simple carbohydrates in the process of hydrolysis.
• Disaccharides, on the other hand, are capable of yielding 2 monosaccharides units in the hydrolysis process. Example: Lactose yields galactose and glucose on hydrolysis.
• Oligosaccharides are defined as sugar residues that are connected together with the help of glyosidic connections. This takes place in the ratio of about 3:10.

Isomers  

The phenomenon of exhibiting similarity in the chemical formula but differing in the compound’s structure is termed as isomerism. The particular compound that exhibits such similarity is referred to as isomers. Isomers can be classified as either structural or constitutional isomers and stereoisomers. The isomers that have the same molecular formula but vary in the structural connectivity of different atoms are called structural isomers. When 2 isomers have the same molecular formula but differ in the group’s spatial arrangement are termed stereoisomers. Stereoisomers are further divided into:

• Diastereomers are neither mirror images of each other nor superimposable. Diastereomers have different configurations at the stereoisomeric centers.
• Enantiomers are non-superimposable mirror images.

Methods for Representing D and L Isomers

There are 3 methods for the representation of D and L isomers. They are namely Haworth projection, Fischer projection, and chair conformation. Each conformation is unique in the way of representing the isomers.

Fischer projection: This particular projection is used for the representation of the biomolecule carbohydrates. The chiral carbons are the ones shown at the point of intersection of vertical and horizontal lines. The group that is most oxidized is generally placed at the top position. This specific method is the most simplest and common way of representing or drawing an isomer’s molecular structure. It is a two-dimensional linear structure. In simple words, this can be defined as a convention that is specifically used to represent a stereo-formula in 2D without affecting its stereochemical information.   

Haworth projection: This projection is used for exhibiting the cyclic structure of the monosaccharides. It is basically a 3D structure. The cycle is formed from the bridge that is formed by the oxygen of the aldehyde group. Thus, Haworth projections are used to represent sugars in their cyclic forms.

Chair conformation: The chair conformation provides the actual spatial structure of a particular compound. The D and L isomers are represented by α (alpha) and β (beta) isomers. It is considered the most precise method for the molecule’s representation.

Optical Activity

A particular substance is considered to be levorotatory when the substance rotates the plane-polarized light to the left side. This is usually represented by the (-) sign This can be explained with an example of L-fructose or (-)-fructose and L-tagatose. If the same plane-polarized light rotates in the opposite direction, i.e., to the right side, then it is called dextrorotatory. This is represented with a (+) sign. For example, D-sugars including D-fructose or (+)-fructose and D-tagatose or (+)-tagatose.

Molecules are generally considered to be optically active when they are capable of rotating the plane-polarized light. Most of the biologically significant molecules tend to be optically active and exhibit the property of chirality. Glyceraldehyde is one of the easiest monosaccharides and it exists in 2 forms as L-(-) glyceraldehyde and D-(+)-glyceraldehyde. L sugar is explained as the one whose greatest numbered stereocenter exhibits a similar arrangement as that of the L-(-)-glyceraldehyde.

D and L sugar

D and L isomers belong to the stereoisomerism category in which both isomers have a similar type of chemical structure. But in this case, the mirror images are non-superimposable to one another. D (+) and L(-) sugars are termed as enantiomers by considering an example of glucose which is nothing but sugar.

L-Glucose

L(-)-glucose exhibits a non-superimposable mirror image of the isomers D(+)-glucose. Thus, it acts as an enantiomer of the D(+)-glucose. Since they are mirror images, the hydroxyl group exists in the opposite direction in the Fischer projection. In the structure of L-glucose, one hydroxyl (-OH) exists on the opposite side, which is the left side, while the other 3 hydroxyl groups are present on the same side. But on the other hand, in D-glucose, the 3 hydroxyl groups exist on the left side concerning the main carbon chain and the other hydroxyl group exists on the right side. 

In Haworth projection of L-glucose, the 3^rd position hydroxyl group is attached in the downward direction while the other 3 hydroxyl groups are in the upward direction. L-glucose exists mainly in certain fruits and vegetables.  

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry, Biochemistry and Molecular biology.  

Practice Problem

What will be present in the open-chain structure of L-talose?

Answer: Talose is defined as aldohexose sugar. L-Talose open-chain structure has 1 hydroxyl group (-OH) pointing left.