MODES OF LINKAGE BY COVALENT BONDS 63 



present in orthophosphoric acid alone, so that these substances are stronger 

 acids than phosphoric acid itself. When phosphoric acid is liberated from 

 these esters and regains its three acid groups there is no appreciable change 

 in the reaction of the medium. Among the monoesters of orthophosphoric 

 acid, we might mention glucose-6-phosphate, a-glycerophosphoric acid 

 and fructose-6-phosphate. 



Orthophosphoric acid may be esterified, not only with alcohol groups, 

 but also with the pseudoaldehyde groups of sugars. Aldose derivatives in 

 which the reducing group of the sugar is combined with orthophosphoric 

 acid are very important in cell-chemistry. Sugar- 1 -phosphates are in this 

 category ; their general formula is as follows : 



PO3H2— O— C— (CHOH) , — CH— Pv 

 / 



H 



-0- 



R=H 



or CH2OH 



They are very easily hydrolysed in strongly acid solution. The sugar- 1- 

 phosphates of furanose sugars are more acid-labile than those of the 

 pyranose form. 



Phosphoric acid also forms esters with enols, the most interesting of these 

 is phosphopyruvic acid, in which the enolized pyruvic acid is combined 

 with H3PO4. Very acid-labile and very alkaline-labile, it is readily split by 

 oxidizing agents liberating phosphoric acid. 



Phosphoamides are also phosphoric acid esters. Their general formula is : 



HO R 



\ / 



P— N 



/w \ 



HO O R' 



Phosphoamides of the phosphoguanidine type are very acid- and alka- 

 line-labile and their rate of acid hydrolysis is increased by molybdic acid. 

 Phosphoarginine and phosphocreatine are phosphoamides. 



Orthophosphoric acid can associate with other molecules of the same acid 

 by means of anhydride linkages to form polyphosphoric acids, notably 

 pyrophosphoric and triphosphoric acids. 



Pyrophosphoric acid is made up of two molecules of ortho-phosphoric acid. 



HO OH 



\ / 



P— O— P 



/W !l\ 



HO O O OH 



