CARBOHYDRATES 



18 



The glycosides are distinguished from other ethers by their ease of hydrolysis. 

 Short boiling in dilute acid is usually sufficient to hydrolyze the sugar moiety from the 

 aglycone. Glycosides may be named by designating the attached alkyl group first and re- 

 placing the "-ose" ending of the sugar with "-oside" as in a-methyl-D-glucoside. Many 

 common glycosides are best known by trivial names which do not indicate their structures 

 (e.g. "arbutin" is hydroquinone /3-D-glucoside). Nearly all natural glycosides have the 

 13 configuration. Both chemically and physiologically the natural glycosides are distin- 

 guished more by their aglycone portions than by their glycosyl portions; and they are ac- 

 cordingly treated in this book --- for instance, under terpenoids, flavonoids, etc. Gly- 

 cosidic bonds are also found, of course, in the oligosaccharides and polysaccharides. 



Glucose is the sugar most frequently found in glycosides. However, other common 

 sugars are not found as often as one would expect. Some rare sugars are peculiar to 

 specific glycosides. In particular the cardiac glycosides regularly contain deoxy sugars 

 that are not encountered in any other place. Rhamnose is deoxy sugar that occurs widely 

 in glycosides but not in other forms. 



Nucleosides may be thought of as glycosides where the aglycone is an amine rather 

 than an alcohol. They share some of the properties of the other glycosides and are some- 

 times referred to as N-glycosides in contrast to the O-glycosides. Peculiar azoxy-gly- 

 cosides are found in cycad roots (2). 



Thioglycosides have a thiol rather than an alcohol as the aglycone. These are dis- 

 cussed under their aglycones in Chapter 14. 



ESTERS 



As alcohols the carbohydrates are capable of forming esters with acids. Many have 

 been synthesized and are important derivatives in characterizing sugars. A few esters of 

 aromatic acids also occur naturally and are evidently widespread (3, 4). They may be 

 important intermediates in the transformations of aromatic compounds (cf. Chapter 4). 

 More important are the hydrolyzable tannins which are complex esters of phenolic acids 

 and sugars. They are discussed under their acid components (e. g. gallic acid). Most 

 important physiologically are the phosphate esters, which are the prime intermediates in 

 transformations of the sugars. Phosphorylated sugar moieties also go to make up several 

 coenzymes and nucleic acid derivatives (q. v. ). The phosphates are strong acids which 

 are conveniently isolated as their slightly soluble barium salts. Their stability in water 

 varies with the location of the phosphate group. The glycosyl phosphates (phosphorylated 

 at the anomeric carbon, as glucose- 1-phosphate) are notably more easily hydrolyzed than 

 compounds phosphorylated in other positions. Phosphate adjacent to the carbonyl function 

 is also more readily hydrolyzed than phosphate farther removed. Thus in fructose- 1, 6- 

 diphosphate the 1-phosphate is hydrolyzed more than ten times faster than the 6-phosphate, 

 Triose phosphates are peculiarly unstable in alkali, and their determination may be based 

 on this characteristic. 



There has been some indication of the natural occurrence of sugar sulfates. Indeed 

 Benson and Shibuya (5) have found as many as 60 organic compounds containing labelled 

 sulfur after feeding radioactive inorganic sulfate to Chlorella. Some of these compounds 

 are evidently sulfur analogues of the better known phosphorylated sugars, but their func- 

 tion is at present quite obscure. Sulfates of certain polysaccharides are widespread in 

 nature but apparently not found in higher plants. A sulfonic acid derived from glucose 

 is present in chloroplast lipids (Chapter 5), 



