HYDROLYZING ENZYMES 



43 



If we examine the structural forinula of a pentose, as for example, 



arabinose : 



H— C = O 



I 

 HO— C*— H 



I 

 H— C*— OH 



H— C*— OH 



I 

 H— C— H 



I 

 OH 



we see that each of the three carbon atoms marked * is united to four 

 different atoms or groups of atoms. Each of these carbon atoms is there- 

 fore asymmetric, and, with regard to it, there are two possible isomers 

 (see p. 9, Cole, 3, for stereoisomerism). It will be found on examination 

 that there are eight possible isomers of the formulae given above : 



HO- 

 HO- 



CHO 

 I 

 ~C— H 



i 



H 



C 



I 

 HO— C— H 



I 

 CH.OH 



Mlibose 



OHO 



I 

 -C— OH 



H_C— OH 



I 

 4— C— OH 



I 



CH.OH 

 f/-Ribose 



CHO 

 I 

 H— C~OH 

 I 

 HO C~H 



H— C OH 



I 



CH.OH 

 Z-Xylose 



HO 



CHO 



I 

 C— H 



i 

 HO— OH 



HO— C— H 



I 

 CH.OH 



rf-Xylose 



CHO 

 I 

 H— C— OH 



I 

 HO— C— H 



I 

 HO— C— H 



i 



CH.jOH 



/-Arabinose 



CHO 



1 

 HO— C— H 



I 

 H C— OH 



I 

 H— C— OH 



I 

 CH^OH 



t^- Arabinose 



CHO 



H— C— OH 



I 

 H— C— OH 



I 

 HO— C— H 



I 

 CH,,OH 



/-Lyxose 

 unknown 



CHO 



I 

 HO C— H 



I 

 HO— C— H 



I 

 H— C— OH 



I 

 CH.OH 



f/-Lvxose 



Of these only seven have been isolated. The two pentoses which 

 occur in plants are ^-arabinose and ^-xylose. These, however, are known 

 almost solely as condensation products, pentosans, in gums, woody tissue, 

 etc. The pentoses form osazones (see p. 49 for reactions and composition). 



Arabinose. This sugar occurs as the pentosan, araban, in various 

 gums, such as Cherry Gum, Gum Arabic, etc. (see p. 45). 



Some of the properties and reactions of the pentoses are demonstrated 

 in the following experiments. 



