STRUCTURE AND PROPERTIES OF BIOMOLECULES - A. E. Zemlyakov - 2017

10. CARBOHYDRATES: NATURAL GLYCOSIDES

❖ Glycosides — mixed acetals of CARBOHYDRATES. The Structure of a glycoside comprises a glycosyl residue and an aglycone (an R-oxy substituent corresponding to the hydroxyl-containing compound that has entered into the reaction). The bond connecting the glycosyl residue to the aglycone is called a glycosidic bond. Depending on the configuration of the glycosidic carbon atom, α- and β-glycosidic bonds are distinguished.

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Depending on The Nature of the heteroatom attached to the glycosyl residue, O-, N-, and S-glycosides are distinguished.

Glycosides sometimes also include so-called C-glycosides, such as pseudouridine or carminic acid. However, from the standpoint of chemical structure, they are not mixed acetals or their aza- and thia-analogues, but rather polyol ethers, and therefore exhibit different chemical properties.

Through the glycosidic bond, Monosaccharides can connect with each other to form oligo- and Polysaccharides, as well as with Proteins (Glycoproteins) and Lipids (Glycolipids).

O-Glycosides are widely distributed in nature; they contain alcohol residues (including Steroids), phenols and related substances, cyanohydrins, and other hydroxyl-containing compounds as aglycones. Accordingly, the array of natural glycosides includes groups of alkyl glycosides, steroid, phenolic, and cyanogenic glycosides, among others.

The N-glycosidic bond links monosaccharides and heterocyclic bases in nucleosides. S-Glycosides (glucosinolates) occur significantly less frequently in nature.

Glycosides also include A number of Vitamins (rutin), Alkaloids (for example, solanin, which forms in potato tubers upon exposure to light), and Antibiotics.

❖ Glycosides as natural pigments. Many natural pigments are either glycosides in their own right or are derived from corresponding glycosidic derivatives As a result of glycosidic bond Cleavage. Phenolic Compounds, as well as derivatives of anthraquinone, coumarin, flavone, isoflavone, pyrylium, and indole, are widely encountered as the aglycones of such pigments. Some of these pigments have been known since ancient times.

✵ Carmine (cochineal) is one of the oldest and most expensive natural pigments. It was obtained from cochineal insects. The name "carmine" derives from the Arabic *kirmiz* (cochineal) and Latin *minium* (cinnabar). The coloring principle of carmine is carminic acid, which is an anthraquinone C-glycoside. It is used in a limited capacity as a colorant — food additive E120.

✵ Anthocyanins — plant glycosides whose aglycones are anthocyanidins, which are derivatives of flavylium (phenylbenzopyrylium) salts. These glycosides serve as the red and blue coloring agents of many flowers (roses, cornflowers, poppies, etc.) and berries (plums, lingonberries, blueberries, etc.). For example, cyanin is the red pigment found in dahlia and rose petals.

In fact, anthocyanidins act as natural acid-base indicators: depending on the pH of the medium, the STRUCTURE OF THE aglycone changes, and consequently, so does its color.

✵ Alizarin — one of the oldest plant pigments, found in the form of a glycoside (ruberythric acid) in the roots of madder (common madder or dyer's madder). The rhizomes of these plants contain ~6% hydroxyanthraquinones and their derivatives.

In the presence of Metal Ions, the alizarin resulting from glycoside Hydrolysis produces a red (Al³⁺), violet (Fe³⁺), or bordeaux (Cr³⁺) color.

✵ Indigo. A blue dye known since antiquity. For a long time, it was sourced from plants of the genus Indigofera, which contain the glycoside indican. Upon hydrolysis of the glycoside, indoxyl is formed, which is then oxidized by air into the indigo dye.

❖ Phenolic glycosides. A vast number of phenolic compounds occur naturally in the form of glycosides. Previously, we examined the glycoside salicin, which is found (up to 0.5%) in the bark of the white willow.

✵ Arbutin. Hydroquinone monoglucoside, arbutin, is found in A wide variety of plants. For example, in the dry leaves of the thick-leaved bergenia (Bergenia crassifolia), its content reaches 22%, and in bearberry (kinnikinick), it accounts for 6%. It is also abundant in the leaves and berries of lingonberries. The diuretic and antiseptic properties of these medicinal plants are attributed to this glycoside.

✵ Syringin is a glycoside isolated from lilac (Syringa). It is also found in Siberian ginseng (Eleutherococcus senticosus), the extracts of which exhibit tonic and anti-stress effects. The aglycone moiety is sinapyl alcohol, a derivative of the trihydric phenol pyrogallol.

✵ Aloesin is one of the carbohydrate-containing components of tree aloe (Aloe arborescens). The juice of this plant promotes tissue regeneration and is used to improve METABOLISM. The aglycone part of aloesin is a resorcinol molecule containing methyl and pyran-2-one substituents.

✵ Rutin is a glycoside of the flavonol quercetin and the disaccharide β-L-Rhа-(1 —> 6)- β-D-Glс. It was first isolated in 1860 from common buckwheat. Its maximum concentration is found in buckwheat, common rue, and Japanese pagoda tree.

Quercetin, rutin, and a number of their analogs are collectively referred to as "Vitamin P" (derived from permeability). The term bioflavonoids is also widely used in the literature. This group comprises over 150 compounds that improve capillary vessel condition and can partially mitigate Vitamin C Deficiency. THE CONCEPT OF "substances with vitamin P activity" is likewise frequently employed.

❖ Cyanogenic glycosides. Approximately 800 plant species from over 70 families contain glycosides whose aglycone moiety is represented by cyanohydrins (α-hydroxynitriles). Because the Enzymatic cleavage of these glycosides yields hydrogen cyanide (prussic acid), they are designated as cyanogenic glycosides. Consequently, a number of plants exhibit toxic properties.

Examples of such compounds include amygdalin, a disaccharide glycoside of benzaldehyde cyanohydrin found in bitter almond kernels—where its concentration can reach 3%—as well as in apricots, peaches, cherries, and the glycoside dhurrin from sorghum.

The roots of cassava (manioc, yucca), used for food in tropical countries, contain the toxic glycoside linamarin, which necessitates special Processing of this product prior to consumption.

❖ Triterpene glycosides (saponins). The name originates from the Latin sapo (genitive saponis), meaning soap. This is due to the surfactant properties of saponins. For instance, the roots of the soapwort plant (Saponaria) contain 13–15% saponins (primarily saponaroside) and have been utilized as a cleansing agent since ancient times.

A distinctive structural feature of this group of glycosides is the presence of a penta- or tetracyclic triterpene (C₃₀) aglycone. Saponins with various aglycone structures occur in nature, with glycosides derived from oleanane, ursane, and dammarane being the most widespread.

Saponins may contain either one (monosides) or two (bismosides) carbohydrate attachment sites. The sugar moieties can be represented by both monosaccharides and short oligosaccharide chains.

Saponins are widely distributed among plants and are highly toxic to cold-blooded animals. In some plants, their concentration is quite high; for instance, the roots of Ural liquorice (Glycyrrhiza uralensis) contain up to 22% glycyrrhizin. Its carbohydrate moiety consists of a D-glucuronic acid disaccharide. This substance is 50 times sweeter than sucrose.

At low concentrations, a number of triterpene glycosides are found in medicinal plant extracts, such as ginseng (panaxoside B) or Manchurian aralia (araloside A).

Triterpene glycosides have also been discovered in certain marine animals, particularly sea cucumbers (trepangs) and sea urchins. These substances exhibit significant ichthyotoxicity, thereby providing protection against predatory fish.

❖ Steroidal saponins. Within the large group of steroidal glycosides, a distinct group of saponins is distinguished by aglycones of the spirostanol type (e.g., dioscin from Dioscorea nipponica) and furostanol type (e.g., alliumoside from onion). Both types of aglycones contain 27 carbon atoms, but In the second type, the ketal ring is open and a glucose residue is attached to it.

Steroidal saponins, particularly those of the spirostanol series, inhibit fungal growth (exhibiting fungistatic activity). Several of them display anti-sclerotic activity, lower Blood pressure, and help regulate cardiac function.

❖ Cardiac glycosides - a group of steroidal glycosides used to regulate Heart function. In nature, cardiac glycosides are found in 45 species of medicinal plants belonging to 9 families (such as Apocynaceae, Liliaceae, Ranunculaceae, Fabaceae, etc.), as well as in the Skin secretions (venom) of certain amphibians. The therapeutic effect of foxglove herb tincture on a patient with cardiac asthma was first documented in 1785 by the English botanist William Withering.

The aglycones in these glycosides are steroidal alcohols containing unsaturated five-membered (cardenolides) and six-membered (bufadienolides) lactone rings, while the carbohydrate moiety is notably rich in deoxy sugars.

The best-known cardenolide glycosides are isolated from purple foxglove (digitoxin), lily of the valley (convallatoxin), pheasant's eye (adonis), and Strophanthus gratus (K-strophanthin-β). The carbohydrate portion of steroidal glycosides is rich in deoxy sugars, including rare ones. The structure of digitoxin contains a trisaccharide based on D-digitoxose, whereas K-strophanthin-β incorporates a methylated deoxy sugar, D-cymarose.

The bulbs of the sea squill (Urginea maritima), used in traditional medicine as a cardiotonic and diuretic agent, contain about 4% glycosides, including the bufadienolide glycoside scillaren A.

❖ S-Glycosides. Compared to O-glycosides, thioglycosides are significantly less common in nature. They are mainly represented by glucosinolates—sulfated hydroxyimino derivatives that are enzymatically cleaved to yield mustard oil derivatives.

Such glycosides are most widely distributed in the mustard family (Brassicaceae). For example, black mustard, black radish, and horseradish contain the S-glycoside sinigrin, which breaks down into allyl isothiocyanate. It is this specific compound that gives these plants their pungent taste.

White mustard contains the glycoside sinalbin, while radishes and cabbage contain brassidin, a glycoside featuring an indole moiety in its aglycone part.