Page 109 - phytochemistry I - PharmD Clinical

In plants, glycosides have an important role in detoxification processes,
transfer water insoluble substances by using monosaccharide, as
a reserved source of energy (sugar reservoir) and regulation of certain
functions. Also, some have beautiful colors (pollination process). In
addition, some glycosides have antibacterial activity thus protect plant
against disease causing bacteria.

Physicochemical properties:

Glycosides mostly occur in the form of colorless, amorphous, non-volatile
solid. Flavonoid glycosides are yellow, while anthraquinone glycosides
are red or orange in color. Most glycosides are soluble in water and
insoluble in organic solvents and some are soluble in alcohol.
Glycone part is water soluble while the aglycone is soluble in organic
solvent.
Most are bitter in taste and odorless except saponins. Glycosides form
characteristic color with specific test solutions; they give positive Molish’s
and Fehling’s solution test. These results are only observed after
hydrolysis.
Glycosides are hydrolyzed using mineral acid, temperature or by using
enzymes.

Chemistry of glycosides:

Chemically, glycosides are usually mixed acetals in which the hydroxyl
group on the anomeric carbon is replaced by a moiety possessing
a nucleophilic atom, with the elimination of water. Formation of glycosides
is an example of acetal formation, which is a reaction between
a hemiacetal group and another hydroxyl group. The linkage resulting
from such a reaction is known as a glycosidic bond.

The following figure is an example of a glycosidic linkage formed between
the anomeric carbon of a glucose molecule and the alcoholic function of
methanol.

۱۰۷