CHAPTER III 



MODES OF LINKAGE BY COVALENT BONDS 



I. "OSIDE" LINKAGE 



A. OSIDES 



On hydrolysis, osides yield one or more sugars or "oses". They are called 

 holosides if the products of hydrolysis are solely sugars, and heterosides if 

 on hydrolysis substances other than sugars are obtained. 



The holosides are designated di-, tri- or tetraholosides according to the 

 number of sugar molecules obtained on hydrolysis. 



The heterosides are very abundant in the vegetable kingdom: tannins, rube- 

 rythric acid (from the madder plant, hydrolysis Hberates glucose and the 

 aglucone, alizarin), anthocyanins (colours of many flowers), digitalis gluco- 

 sides (gitine, digitonine, digitaline, gitoxine, etc. . . .) cyanogenetic glucosides 

 (whose hydrolysis liberates hydrogen cyanide in addition to a sugar), etc. . . . 



The heterosides are called a or jS-heterosides according to whether they 

 contain the a or j8 form of the sugar. 



The most interesting osides in the biosphere are the diholosides or disaccha- 

 rides which are classed as reducing or as non-reducing disaccharides. 



(a) Non-Rediichig Diholosides 



CHjOH 



a-Glucopyranose 



HOH.C 



CH.OH 

 Q^ /3-Fructofuranose 



[a]^ - + 66,67° 

 sucrose 



57 



