Reactions and derivatives of monosaccharides, disaccharides, O- and N-glycosidic bonds, examples

Monosaccharides are derivatives of hydrocarbons, or of furan and pyran heterocycles, from which their basic division into furanoses and pyranoses is derived . They are a basic component of the food of animals, including humans. They contain several chiral carbons , which allows the formation of epimers and enantiomers (right- and left-handed antipodes that turn the plane of polarized light to the opposite side in solution).

See the Carbohydrates page for more detailed information .

Reactions of monosaccharides

Mutarotation

A so-called mutarotation can occur within one carbohydrate cycle. It is a condition where a carbohydrate changes its optical rotation upon dissolution. This is made possible by the fact that the C-1 carbon of the aldose is the chiral center of the cyclic molecule. We call the corresponding isomers anomers.

Epimerization

An isomerization reaction that takes place in a weakly basic environment. In the presence of an optically active substance (e.g. glucose ), its configuration changes . D-glucose can thus transition to D-mannose, which only differs in configuration at C-2. An epimer of glucose is also D-galactose, which is distinguished by its configuration at C-4. Such pairs are then called epimers .

Epimers D-glucose and D-mannose

Redox reaction

The reduction of monosaccharides at the C-1 position (in the presence of Ni, Pt) produces the sugar alcohol glucitol (reduction of glucose → sorbitol).

Oxidation takes place preferentially on the first carbon, when the aldehyde group is oxidized to form a carboxyl. This is how sugar acids are formed → aldonic acid (in the case of glucose – gluconic acid). Oxidation can also take place on the last carbon to form uronic (glucuronic) acid, or it can take place on both carbons at the same time and thus aldaric (glucaric) acid is formed.

Esterification

Reaction of monosaccharides (hydroxyl groups) with acids. In the body, the most common reaction of monosaccharides is with phosphoric acid (phosphorylation), which produces, for example, glucose-6-phosphate.

Glycosidic Bond

Acetate bond of carbohydrates with -OH group (e.g. alcohols, lipids, other carbohydrates - formation of oligo- and polysaccharides) or with -NH 2 (e.g. nitrogenous bases in DNA and RNA ). All monosaccharides with a carbonyl group can react with -OH to form a hemiacetal . The reaction taking place is a nucleophilic addition. The hemiacetal can further react with another nucleophilic group to form a total acetal to form a glycoside and water. This stabilizes the sugar and loses its reducing effects. The bond is called O-glycosidic . The N-glycosidic bond is formed by the reaction of a monosaccharide with the -NH 2 or -NH group. The resulting compounds are called nucleosides. In the human body, they occur in conjunction with phosphoric acid (phosphoric esters of nucleoside → nucleotides ). It forms the basis for ATP , ADP, etc.

• 

  O-glycosidic bond
• 

  N-glycosidic bond

Carbohydrate Report

If a monosaccharide finds itself in a strongly acidic environment, a furan derivative is formed from the given monosaccharide, or pyran. Furfural is formed from ribose by dehydration , 5-hydroxymethyl-furfural is formed from glucose by dehydration . This product can subsequently react with phenols and aromatic amines, condensation occurs and a colored condensation product is formed. We use these reactions to identify carbohydrates and distinguish aldoses from ketoses and pentoses from hexoses.

reaction	card	reagents used	result and evaluation
Molischova	general carbohydrate test	H 2 SO 4 , naphthol	purple product (monosaccharides faster than oligo-/polysaccharides)
Bialova	distinguishing pentoses from hexoses	HCl, orcinol , FeCl 3	pentoses – blue-green colouring hexoses – brown colouring
Selivanova	distinguishing aldohexoses from ketohexoses	HCl, resorcinol	red coloring ketosis – 1-2 min. aldoses – more than 3 min. disaccharides (containing ketohexose) – 3 min.
Schiff	proof of the free aldehyde group	Schiff's reagent	the reaction of the reagent with an aldehyde produces a violet-red product, the reaction with aldoses is slower
Benedict's	reducing properties of carbohydrates	alkaline environment, Benedict's reagent	reduction of Cu 2+ to Cu + occurs monosaccharides – orange-red Cu 2 O precipitate reducing disaccharides – orange-red Cu 2 O precipitate non-reducing disaccharide y – no color change
Barfoed's	reducing monosaccharides from reducing disaccharides	acetic acid, copper acetate	they reduce Cu 2+ at higher temperatures to form a red-orange Cu 2 O precipitate monosaccharides – fast course disaccharides – slow progress
Lugol's	starch	polyiodide ions	dark blue coloration

We can demonstrate the reducing properties of carbohydrates due to their ability to reduce heavy metal ions (at higher temperatures) that are bound in a complex. The Tollens and Fehling reactions, as well as the Barfoed and Benedict tests (see table) , are based on this principle .

They reduce all monosaccharides (aldoses and ketoses). In an alkaline environment, ketoses isomerize to aldoses, which can be reduced. Disaccharides containing free hemiacetal hydroxyl (lactose, maltose) also give a positive reaction. Starch hardly reduces, only its terminal glucose containing hemiacetal.

In medicine, we use these tests for a simple urine examination (e.g. when a metabolic disorder of carbohydrates is suspected)

Derivatives of monosaccharides

Esther

They are formed by the reaction of a saccharide (-OH) with an acid. The most important carbohydrate esters in the body are esters formed by reaction with trihydrogenphosphoric acid (phosphates). Such esters include, for example, glucose-6-phosphate , fructose-6-phosphate , fructose-1,6-bisphosphate , which participate in, for example, the process of glycolysis . Phosphorus esters can also be found in nucleotides and nucleic acids.

Amino derivatives

Derivatives that have the -OH group at C-2 replaced by an amino group, which is usually acetylated. The compound N-acetylamino derivative (acetic acid amide) is thus formed. Among the most important amino derivatives are N-acetyl-D-glucosamine and N-acetyl-D-galactosamine , which are characteristic of heteropolysaccharides and glycolipids .

Uronic acids

Oxidation products of hydroxyl at C-6. They can thus form compounds - glycosiduronates. Uronic acids include, for example, D-glucuronic acid , which is a typical structural component of heteropolysaccharides. It also plays an important role in detoxification reactions in the liver.

Aldonic acids

They are formed by oxidation of the aldehyde group at C-1. A metabolically important aldonic acid is D-gluconic acid , which is formed as a metabolite of the oxidation part of the pentose cycle .

Aldiols

They are also called sugar alcohols. They are formed by the reduction of the carbonyl group of a saccharide. Both D-glucose and D-fructose enable the formation of D-sorbitol , which is used as an artificial sweetener.

Disaccharides

300px|náhled|vpravo|O- a N- glykosidová vazba

Disaccharides are carbohydrates that are formed by the condensation of 2 monosaccharides with the removal of water. Individual units are connected by O-glycosidic bonds. The opposite process is hydrolysis. Disaccharides taken in with food are broken down in the digestive system using glycosylases (e.g. maltase , lactase ).

Representatives of disaccharides

Maltose – a disaccharide composed of two glucose units. It is a reducing sugar (bond: α 1.4). Maltose is obtained by hydrolysis of starch.

Lactose – a disaccharide composed of glucose and galactose. It is also a reducing sugar (bond: β 1,4). It is contained in the milk of mammals (4.8% cow's, 6% human).

Sucrose – disaccharide composed of glucose and fructose. It is a non-reducing sugar (bonds α, β 1,2), as it does not have a free hemiacetal. The most widespread disaccharide that all plants contain (cane sugar).

Links

related articles

• Carbohydrates in human nutrition

Resources

• MATOUŠ, Bohuslav, et al. Fundamentals of medical chemistry and biochemistry. 1st edition. Prague: Galén, 2010. 540 pp.  ISBN 978-80-7262-702-8 .
• https://el.lf1.cuni.cz/p9og07ou0x7/

Category:Biochemistry