Classification and chemical structure of monosaccharides in plant agrochemistry

Carbohydrates form the basis of plant organic matter and serve as a key energy source for soil microflora. In agrochemistry and plant physiology, they are generally classified into three main groups:

• monosaccharides (simple sugars);
• oligosaccharides;
• polysaccharides.

Monosaccharides and their derivatives cannot be broken down in the plant organism without losing their fundamental carbohydrate properties. Their empirical formula is expressed as (CH₂O)_n, where the number of carbon atoms varies from 3 to 9. Depending on the length of the carbon backbone, monosaccharides are divided into several classes.

• Carbon atoms in the molecule — from 3 to 9
• Formula of monosaccharides — (CH₂O)_n
• Formula of hexoses — C₆H₁₂O₆

Chemical structure, nomenclature, and isomerism of simple sugars

In simple sugar molecules, hydroxyl groups are attached to all carbon atoms except one. This single free carbon atom is part of an aldehyde or keto group (oxo group). Depending on the structure of this group, monosaccharides are divided into aldoses and ketoses. The progenitors of these groups are the simplest trioses — glyceraldehyde (aldose) and dihydroxyacetone (ketose), from which all other sugars are derived.

For accurate identification of monosaccharide derivatives, the carbon atoms in the molecule are numbered. The counting starts from the aldehyde group or from the end of the chain closest to the keto group. The position of any attached substituents is indicated by the corresponding number. A typical example of such nomenclature is the 2-deoxy-2-amino-3,4-di-O-methylglucose molecule.

A crucial feature of monosaccharides is isomerism — the ability of substances with the same composition to have different properties. Structural isomerism is determined by the different order of bonds between atoms in the formula. For example, hexoses with the general formula C₆H₁₂O₆ — glucose, mannose, galactose, and fructose — represent structural isomers. In Emil Fischer’s linear projections, these differences are clearly reflected in the molecular structure.

The spatial orientation of monosaccharides depends on the presence of chiral (asymmetric) carbon atoms bonded to four different substituents. Because of this, molecules exist in two enantiomeric forms — the D- and L-series. Assignment to a specific series is determined by the position of the OH group at the last chiral carbon atom: to the right of the chain for the D-series and to the left for the L-series. D- and L-glyceraldehyde serve as the standards for this classification.