Optical activity of carbohydrates

Optical activity of carbohydrates

Carbohydrates solutions have the property to rotate the plane-polarized light to the right or left. This property is called optical activity of the carbohydrates. This property is due to the presence of the asymmetric carbon atoms in their structures. The sugars rotating the plane-polarized light to left are termed as levorotatory while those which rotate the plane-polarized light to right are termed as dextrorotatory. The levorotation is represented by l or (–) sign while the dextrorotation is represented by d or (+) sign. There is no relation between D and L and (–) and (+) signs. A sugar may be a D sugar and levorotatory e.g. D-fructose. In the solution, glucose is dextrorotatory and sometimes glucose is called dextrose. Trend of use of “l” and “d” has become deleted. Now a days (–) and (+) signs are used for optical activity.

There are some conditions in which the plane-polarized will neither be rotated to right nor to left. Those are

1.      If the compound does not possess an asymmetric carbon atom in its structure.

2.      If equal quantities of the dextrorotatory and levorotatory isomers are present. Such mixtures are termed as racemic mixtures.

3.      If a compound is a meso compound. Such compounds contain asymmetric carbon atoms but do not rotate the light. This is due to the fact that such compounds have two halves, one rotating the light to the right and the other to the left. This phenomenon is called as internal compensation e.g. meso tartaric acid.

Meso tartaric acid

Asymmetric Centers and D- and L-Monosaccharides

All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms. That is why they are optically active. The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and therefore has two different optical isomers, or enantiomers. One of these two forms is designated the D isomer and other is L isomer depending on the sides on which H and OH are attached.

             

Structural representation of sugars

A: Fisher projection: straight chain representation

B: Haworth projection: simple ring in perspective

C: Conformational representation: chair and boat configurations

A: Fischer Projection Formulas

In case of Fischer projection formulas the carbon chain is written vertically with carbon No. 1 at the top and the –H and –OH are written to the left or right. Carbon No.2 in both the cases is an asymmetric carbon atom (the carbon atom to which four different atoms or groups is attached). All carbohydrates having more than 3 carbon atoms have two or more asymmetric carbon atoms. In such compounds, the most distant asymmetric carbon atoms from the carbonyl group having the same configuration as that of the asymmetric carbon atom of the D-glyceraldehyde are called D sugars. On the other hand, all sugars whose farthest asymmetric carbon atoms from the carbonyl group having the same configuration as that of the asymmetric carbon atom of the L-glyceraldehyde are called L sugars. The D and L forms of a sugar are stereoisomers and mirror images of each other and are termed as enantiomers. All sugars are compared with D- and L-glyceraldehydes and thus glyceraldehyde is called the reference sugar. D sugars are most abundantly available in nature than the L sugars. L isomer of monosaccharide are also found in human body e.g. L-xylulose.

Open chained form of D-glucose (Fischer projection)

B: Haworth Projection Formulas

Monosaccharides normally adopt a ring configuration. When a D-glucose adopts a six membered ring structure, a hemiacetal bond is formed between the aldehydic carbonyl group of carbon No 1 and hydroxyl group on carbon No 5 (Alcohols react readily with aldehydes to

form hemiacetal). Cyclization of glucose thus result in the formation of ring structure, which is analogous to the structure of pyran, six membered ring containing five carbon and one oxygen atom. Sugars with a six membered pyran ring are designated as pyranose. Cyclization of D-glucose thus yields a cyclic hemiacetal, i.e. glucopyranose. Fructose, on the other hand, forms a five membered furan ring with four carbon and one oxygen atom. The linear form of D-fructose thus cyclize between carbonyl carbon of second carbon (C2) and alcoholic group of the fifth carbon (C5), and yield the hehiketal, i.e. fructofuranose (ketones can react with alcohols to form hemiketals). The smallest monosaccharide having the fructofuranose structure is ribose. The ring structure of sugar was proposed by Haworth and is also called Haworth structures.

D-glucose is an aldohexose. The open chain structure of glucose was first suggested by Fischer (Fischer’s projection formula) in 1891.

C: Conformational representation (Chair and boat forms D-glucose):

Although Haworth projections are convenient for display of monosaccharide structures. Sometimes glucose can also be representing in the form of chair and boot. Chair form of glucose is more stable.

β-D-Glucose (Chair form)                                        β-D-Glucose (boat form)

Vant Hoff’s law

All aldohexose have 4 asymmetric carbon atoms and according to Vant Hoff’s law (the number of possible optical isomers of a substance are equal to 2ⁿ where n is the number of asymmetric carbon atoms) they are expected to have 2⁴ = 16 possible isomers (allose, altrose, glucose, mannose, gulose, idose, galactose, talose in D and L forms). However, in aqueous solutions many monosaccharides including glucose display the property of mutarotation. Due to this property these sugars behave as if they have an extra asymmetric carbon atom. The carbonyl carbon is responsible for this phenomenon. Due to this property, the number of possible isomers for aldohexose become 2⁵ = 32 (16 isomers occurring in α and β forms). α and β forms usually occur in ring form and do not show aldehyde group.