They are organic compound in which there is usually a semiacetal linkage between the reducing group of a sugar and an alcoholic or phenolic hydroxyl group of a non-sugar compound called glycan. Most glycosides are derived from D-glucose are called glycosides, β type in plants.
Classification of Glycosides:
1) Cynophoric Glycoside or Cyanogenic Glycoside
2) Mustard Oil Glycoside or Glucosinolates
4) Phenolic Glycosides.
1) Cynophoric: Yield HCN on hydrolysis e.g. Amygdalin in bitter almond; α-peachs and plums and Dhurin in young seedling of sorghum plant.
2) Mustard Oil Glycosides: Yield iso-thiocyanates (-NCS) on hydrolysis e.g. sinigrin in black mustard; sinalbin in white mustard.
3) Saponins: Have ability to form colloidal solution in water which gives soapy foam and on hydrolysis yield variety of sugars e.g. Digitalis and Strophanthus, Nimbidin.
4) Phenolic Glycosides: Aglycon of this class is aromatic compound containing phenolic group e.g. arbutin, salicin, indican.
Occurrence of Glycosides: Widely occur in roots, barks, fruits, to small extent in leaves.
Properties of Glycosides: Crystalline solid, colourless with bitter taste, soluble in water & org. solvents except ether, stable, do not show reducing property like glucose; on hydrolysis yield sugar and agley (non-sugar).
Physiological Role of Glycosides:
Serve as reserve food material, means for removal of toxic substances inhibit disease producing microorganisms, repel harmful insects and animals. In certain cases some insects are attracted which may help in pollination, act as protective agent against wound.