Monosaccharide

The monosaccharides or simple sugars are the simplest carbohydrates; they do not hydrolyze, that is, they do not decompose into other simpler compounds. They have three to eight carbon atoms and their empirical formula is (CH₂O)_n , where n ≥ 3. They are named referring to the number of carbons (3-7), and end with the suffix -ose. The main monosaccharide is glucose, the main source of energy for cells.

Examples of monosaccharides include glucose (dextrose), fructose (levulose), and galactose. Monosaccharides are the building blocks of disaccharides (such as sucrose and lactose) and polysaccharides (such as cellulose and starch). Table sugar used in everyday vernacular is itself a sucrose disaccharide comprising one molecule each of the two monosaccharides D-glucose and D-fructose.

Each carbon atom that bears a hydroxyl group is chiral, except those at the end of the chain. This gives rise to a series of isomeric forms, all with the same chemical formula. For example, galactose and glucose are aldohexose, but they have different physical structures and chemical properties.

The monosaccharide glucose plays a critical role in metabolism, where chemical energy is extracted through glycolysis and the citric acid cycle to provide energy for living organisms.

Features

The carbon chain of monosaccharides is unbranched, and all but one carbon atom contains an alcohol (-OH) group. The remaining carbon atom has a carbonyl group (C=O) attached to it. If this carbonyl group is at the end of the chain, it is an aldehyde group (-CHO) and the monosaccharide is called aldose. If the carbonyl group is in any other position, it is a ketone (-CO-) and the monosaccharide is called a ketose.

All the monosaccharides are reducing sugars, since they have at least one free hemiacetal -OH, so they are positive in the reaction with Fehling's reagent, in the reaction with Tollens' reagent, in the Maillard Reaction and the Benedict Reaction.

Other ways of saying that they are reducing is to say that they present equilibrium with the open form, they present mutarotation (spontaneous change between the two cyclized forms α (alpha) and β (beta)), or to say that they form osazones.

Thus, for the aldoses of 3 to 6 carbon atoms we have:

• 3 carbons: trios, there is one: D-Gliceraldehyde.
• 4 carbons: tetrose, there are two, according to the position of the carbonyl group: D-Eritrosa and D-Treosa.
• 5 carbons: pentuous, there are four, according to the position of the carbonyl group: D-Ribosa, D-Arabinosa, D-Xilosa, D-Lixosa.
• 6 carbons: hexous, there are eight, according to the position of the carbonyl group: D-Alosa, D-Altrosa, D-Glucosa, D-Manosa, D-Gulosa, D-Idosa, D-Galactosa, D-Talosa.

The ketoses with 3 to 7 carbon atoms are:

• Trios: There is one: Dihydroxiacetona.
• Tetrosas: There is one: D-Eritrulosa.
• Pentuous: there are two, according to the position of the carbonyl group: D-Ribulosa, D-Xilulosa.
• Hexous: there are four according to the position of the carbonyl group: D-Sicosa, D-Fructose, D-Sorbosa, D-Tagatosa.

Like disaccharides, they are sweet, water-soluble (water-soluble), and crystalline. The best known are glucose, fructose and galactose.

These sugars make up the monomeric units of carbohydrates to form polysaccharides.

All simple monosaccharides have one or more asymmetric carbons, except dihydroxyacetone. The simplest case, that of glyceraldehyde, has a center of asymmetry, which gives rise to two possible conformations: the D and L isomers.

To find out if it is D or L, we can represent its formula in a Fischer projection and consider the configuration of the penultimate carbon (which is the asymmetric carbon furthest from the functional group). The position of your OH group to the right or to the left will determine the D or L series, respectively. The D and L isomers of glyceraldehyde are mirror images of each other and are therefore said to be chiral isomers, enantiomers, or enantiomorphs.

Representation of Fischer of the forms D and L of glucose. Both are symmetrical about a plane.

All aldoses are considered structurally derived from D- and L-glyceraldehyde. Similarly, the ketoses are considered to be structurally derived from D- and L-erythrulose. Almost all of the monosaccharides present in nature belong to the D series.

When the molecule has more than one asymmetric carbon, the number of possible optical isomers increases. The number of possible optical isomers is 2ⁿ, where n is the number of asymmetric carbons. In this case, not all optical isomers are mirror images of each other and several types of optical isomers can be distinguished:

• Epimers: two monosaccharides that differ in the configuration of one of their asymmetrical carbons. For example, D-Glucosa and D-Manosa only differ in the hydroxyl configuration in the C2
• Anomers: two cyclad monosaccharides that differ only in the -OH group of the anomeric carbon (which in principle belongs to the aldehyde or ketone group). They give rise to α and β configurations.
  • by alpha agreement down and beta above the projection plane of Haworth.
• Numbers: those monosaccharides that have a structure to speculate on the plane (D and L). In addition, they can be levogiras(-) or dextrogiras(+) depending on how the dissolved substance spins the plane in which the polarized light of the polarimeter is activated as it passes through the dissolution(or +) if the plane turns to the right and levogiro(or -) if it turns to the left), however, this nomination has nothing to do with the configuration L
• Diasteroisomers: monosaccharides that are not speculative images among themselves (see Nomenclature D-L)...