SCHWALBE, C. G.--"Die Chemie der Cellulose," 665 pages, Berlin, 1911.
Strictly speaking, the term _glucoside_ should be applied only to such compounds as contain glucose as the characteristic basic group. But in common usage, it refers to any compound which, when hydrolyzed, yields a sugar as one of the products of the hydrolysis. In all the natural glucosides which occur in plant tissues, the other organic constituent, which is represented by the R in the formula for glucosides (R·C_{ 6}H_{ 11}O_{ 5}, or R·(CHOH)_{ 5}CHO) is some aromatic group, or closed-ring benzene derivative.[3] The different organic constituents of glucosides are of a great variety of types, such as phenols, alcohols, aldehydes, acids, oxyflavone derivatives, mustard oils, etc. It is noteworthy, however, that no nitrogenous groups of the protein type have been found combined with sugars in glucosides.
Some glucosides contain more than one saccharide group, possibly as di- or trisaccharides. Under proper conditions of hydrolysis, one or more of the saccharide groups can be removed from such compounds, resulting in glucosides of simpler structure.
Most of the common glucosides are derived from _d_-glucose. Some are known, however, which are derivatives of galactose or rhamnose; while in some cases the exact nature of the sugar which is present has not yet been determined.
HC // \ HC CH The structural formula for benzene, C_{ 6}H_{ 6}, | ¦ is one which it is HC CH \\ / CH
difficult and inconvenient to reproduce in type. On that account, it is
/\ customary to indicate this formula by a plane hexagon, thus | |. \/
It is understood, in all such cases, that the figure represents six carbon atoms arranged in a closed ring, with alternate double and single bonds, and with a hydrogen atom attached to each carbon. The printing of some other group as OH, CH_{ 3}, adjacent to an angle of the hexagon means that this group replaces the H atom in the compound which is being illustrated.
All natural glucosides are hydrolyzed into a sugar and another organic residue by boiling with mineral acids; although they vary widely in the ease with which this hydrolysis is brought about.
In most cases, the glucoside is easily hydrolyzed by an enzyme which occurs in the same plant tissue, but in different cells than those which contain the glucoside. Injury to the tissues, germination processes, and perhaps other physiological activities of the cells, result in bringing the enzyme in contact with the glucoside and the hydrolysis of the latter takes place. A large number of such enzymes have been found in plants, many of which hydrolyze only a single glucoside. However, two enzymes, namely, the emulsin of almond kernels, and _myrosin_ of black mustard seeds, each hydrolyze a considerable number of glucosides. In general, _emulsin_ will aid in the hydrolysis of any glucoside which is a derivative of [beta]-glucose, and myrosin will help to split up any sulfur-containing glucoside. Glucosides which are derivatives of rhamnose require a special enzyme, known as _rhamnase_, for their hydrolysis.