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In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. These can be activated by enzymatic hydrolysis, which causes the sugar part to be broken off, making the chemical available for use. Many such plant glycosides are used as . Several species of Heliconius butterfly are capable of incorporating these plant compounds as a form of chemical defense against predators. In animals and humans, poisons are often bound to sugar molecules as part of their elimination from the body.
In formal terms, a glycoside is any molecule in which a sugar group is bonded through its anomeric carbon to another group via a glycosidic bond. Glycosides can be linked by an O- (an O-glycoside), N- (a glycosylamine), S-(a thioglycoside), or C- (a C-glycoside) glycosidic bond. According to the IUPAC, the name " C-glycoside" is a misnomer; the preferred term is " C-glycosyl compound". (2025). 9780967855097 ISBN 9780967855097 The given definition is the one used by IUPAC, which recommends the Haworth projection to correctly assign stereochemical configurations.
Many authors require in addition that the sugar be bonded to a non-sugar for the molecule to qualify as a glycoside, thus excluding . The sugar group is then known as the glycone and the non-sugar group as the aglycone or genin part of the glycoside. The glycone can consist of a single sugar group (monosaccharide), two sugar groups (disaccharide), or several sugar groups (oligosaccharide).
The first glycoside ever identified was amygdalin, by the French chemists Pierre Robiquet and Antoine Boutron-Charlard, in 1830.
There are many ways to chemically synthesize glycosidic bonds. Fischer glycosidation refers to the synthesis of glycosides by the reaction of unprotected monosaccharides with alcohols (usually as solvent) in the presence of a strong acid catalyst. The Koenigs-Knorr reaction is the condensation of glycosyl halides and alcohols in the presence of metal salts such as silver carbonate or mercuric oxide.
There are four type of linkages present between glycone and aglycone:
Along with playing a role in deterring herbivores, in some plants they control germination, bud formation, carbon and nitrogen transport, and possibly act as antioxidants. The production of cyanogenic glycosides is an evolutionarily conserved function, appearing in species as old as ferns and as recent as . These compounds are made by around 3,000 species. In screens they are found in about 11% of cultivated plants but only 5% of plants overall; humans seem to have selected for them.
Examples include amygdalin and prunasin which are made by the bitter almond tree; other species that produce cyanogenic glycosides are sorghum (from which dhurrin, the first cyanogenic glycoside to be identified, was first isolated), barley, flax, white clover, and cassava, which produces linamarin and lotaustralin.
Amygdalin and a synthetic derivative, laetrile, were investigated as potential drugs to treat cancer and were heavily promoted as alternative medicine; they are ineffective and dangerous.
Some butterfly species, such as the Dryas iulia and Parnassius smintheus, have evolved to use the cyanogenic glycosides found in their host plants as a form of protection against predators through their unpalatability.
Among the important effects of flavonoids are their antioxidant effect. They are also known to decrease capillary fragility.
Flavonoid glycosides
Here, the aglycone is a flavonoid. Examples of this large group of glycosides include:
Phenolic glycosides
Here, the aglycone is a simple natural phenol structure. An example is arbutin found in the Bearberry Arctostaphylos uva-ursi. It has a urinary antiseptic effect.
Saponins
These compounds give a permanent froth when shaken with water. They also cause hemolysis of red blood cells. Saponin glycosides are found in liquorice. Their medicinal value is due to their expectorant, corticoid and anti-inflammatory effects. Steroid saponins are important starting material for the production of semi-synthetic glucocorticoids and other steroid hormones such as progesterone; for example in Dioscorea wild yam the sapogenin diosgenin, in the form of its glycoside dioscin. The ginsenosides are triterpene glycosides and ginseng saponins from Panax ginseng (Chinese ginseng) and Panax quinquefolius (American ginseng). In general, the use of the term saponin in organic chemistry is discouraged, because many plant constituents can produce foam, and many triterpene-glycosides are amphipolar under certain conditions, acting as a surfactant. More modern uses of saponins in biotechnology are as adjuvants in vaccines: Quil A and its derivative QS-21, isolated from the bark of Quillaja saponaria Molina, to stimulate both the Th1 immune response and the production of cytotoxic T-lymphocytes (CTLs) against exogenous antigens make them ideal for use in and vaccines directed against intracellular pathogens as well as for therapeutic but with the aforementioned side-effect of hemolysis. Saponins are also natural ruminal antiprotozoal agents that are potential to improve ruminal microbial fermentation reducing ammonia concentrations and methane production in ruminant animals.
Steroid glycosides (cardiac glycosides)
In these glycosides, the aglycone part is a steroid nucleus. These glycosides are found in the plant genera Digitalis, Scilla, and Strophanthus. They are used in the treatment of , e.g., congestive heart failure (historically as now recognised does not improve survivability; other agentsThis is an example. are now preferred) and Heart arrhythmia.
Steviol glycosides
These sweet glycosides found in the stevia plant Stevia rebaudiana Bertoni have 40–300 times the sweetness of sucrose. The two primary glycosides, stevioside and rebaudioside A, are used as natural Sugar substitute in many countries. These glycosides have steviol as the aglycone part. Glucose or rhamnose-glucose combinations are bound to the ends of the aglycone to form the different compounds.
Iridoid glycosides
These contain an iridoid group; e.g. aucubin, geniposidic acid, theviridoside, loganin, catalpol.
Thioglycosides
As the name contains the prefix thio-, these compounds contain sulfur. Examples include sinigrin, found in black mustard, and sinalbin, found in white mustard.
See also
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