Classification and structure of carbohydrates
Carbohydrates, also called carbohydrates, are the most abundant organic substances on Earth. Their molecules are made up of oxygen, carbon and hydrogen atoms. From a chemical point of view, these are polyhydroxyaldehydes' and polyhydroxyketones. They contain functional aldehyde or keto groups in their molecule, as well as a larger number of hydroxyl groups.
Carbohydrate Classification[edit | edit source]
'According to the number of units in the molecule, we distinguish:
- monosaccharides - cannot be further hydrolyzed into simpler units;
- oligosaccharides - they form 2-10 units of monosaccharides by hydrolysis;
- polysaccharides' - hydrolyzing into more than 10 monosaccharides.
- Monosaccharides and oligosaccharides are generally called sugars. A synonym for polysaccharide is the word glycan.
'Monosaccharides are divided according to:
- Number of C-atoms: trioses, tetroses, pentoses, hexoses.
- Functional groups: aldoses and ketoses.
'Polysaccharides are divided into:
- Homopolysaccharides': these are polymers made up of the same type of monosaccharide. Examples are starch, glycogen or cellulose.
- Heteropolysaccharides': they are polymers made up of more than one type of monosaccharide. An example is hemicellulose.
Carbohydrate Structure[edit | edit source]
'The structure of a saccharide molecule can be expressed by different formulas:
- Linear' (Fischer) formula;
- Cyclic (Haworth's) formula that results from the formation of a heterocyclic structure.
- Cycle can be:
- six-membered - pyranose - by similarity to six-carbon pyran;
- five-membered - furanose - by similarity to five-carbon furan.
- The Tollens formula describes the creation of a cyclic structure from a linear formula. It shows the reaction of hydroxyl with a carbonyl group to form a so-called ``semiacetal (hemiacetal) structure.
Isomerism[edit | edit source]
It is a condition where compounds with the same general formula have a different structural arrangement of atoms in the molecule. The following types of isomerism are found in carbohydrate molecules.
- Template:D- and Template:L- series
- Indicated by the position of the −OH group' on the last chiral carbon. Assignment to the respective series is based on similarity with the starting compound of the carbohydrate series – glyceraldehyde. The −OH group in Fischer's formula is on the right for Template:D- and on the left for Template:L- isomers.
- Template:D- and Template:L- isomers are mirror images of the so-called enantiomers'' - optical isomers. They differ in the sign of optical rotation, or the direction in which they rotate the plane of polarized light.
- However, it is not generally true that the Template:D- series is right-handed and the Template:L- series is left-handed.
- Equimolar mixture of enantiomers is called a racemic mixture, or Template:DTemplate:L mixture, and it does not show optical activity.
- The Template:D- isomers' are more common in nature.
- Pyranoses and furanoses
- They are designated according to the similarity of the cyclic form of the respective monosaccharide 'with the cycle of pyran or furan. Glucose in solution is more than 99% in the form of gluco-pyranose, the rest of the molecules, less than 1%, then appears in the form of gluco-furanose.
- α- and β- anomers
- They are designated according to the position of the hemiacetal' or hemiketal −OH in the cycle. "Hemiacetals" are formed by the reaction of aldehyde and alcohol groups, hemiketals by the reaction of keto and alcohol groups.
- If the −OH group is oriented to the same side as the −OH group indicating belonging to the Template:D- or Template:L- series, it is an α-anomer. If the −OH group is oriented to the opposite side, it is a β-anomer'.
Anomers differ in optical rotation.
- They differ from each other by the position of one −OH group in the molecule'.
Examples are glucose' and mannose.
- Aldoses and ketoses
- They are designated by the different functional group on the 1st and 2nd carbons of the molecule.