18 



THE CARBOHYDRATE ECONOMY OF CACTI. 



(first) a series of compounds, the enols, in the following manner (M in the 

 formulae represents a metal or possibly enzyme), which replaces the ionized 

 hydrogen : 



H H OH H 



H H OH H H 



CH a OH 



CH : 0+MOET7CH a OH 



D-glucose. 

 H H OH H H 



H H OH 



CH a OH- 



CHOH- 



OH OH H 



-C=C OH 



OH OH H OH 



Common 1-2 dienol of D-glucose, D-mannose and 

 D-fructoie. 



This 1, 2 dienol, by taking up one molecule of water at the double bond 

 and rearrangement goes into either d-glucose, d-mannose, or into d-fructose, 

 according as to whether the OH of the HOH attaches to carbon atom 1 or 2. 

 In this manner the interconversions of the hexoses above referred to are 

 explained, the same principles holding for the other sugars as well as the 

 d-glucose. 



In a similar way there are formed : 



H H OH OH H H 



CH a OH 



C C CH 2 OH, 



CH 3 OH- 



-C = C C CH a OH 



OH OH 

 2.3 Dienol. 



OH OH OH OH 



3.4 Dienol. 



These dienols are also capable of rearrangement to form other mono- 

 saccharides and polysaccharides, as just indicated. However, the dienols 

 are capable of another and more drastic change. As is almost general in 

 carbon compounds, double carbon bonds break easily and are highly reactive. 

 The enols break apart spontaneously at the double bonds ; this results in the 

 formation of substances of very great reactivity, capable of intramolecular 

 rearrangement, action upon each other, and (if there is oxygen present), the 

 formation of the large variety of acids briefly described above. Thus, e. g., 



