242 A. I. RINGER AND EMIL J. BAUMANN 



absolute alcohol. It exhibits strong mutarotation in aqueous solution. 

 The specific rotations for a-1-arabinose, (3-1-arabinqse and the equilibrated 

 mixture are -f 76, +184 and -f 104 respectively. 



The most characteristic compounds of arabinose are parabromophenyl 

 hydrazone, diphenyl hydrazone and phenyl-osazone. The diphenyl hydra- 

 zone, melting at 218 C., is a colorless crystalline substance and is usually 

 used for identifying arabinose. 



d-Ribose. Unlike the other two pentoses which have been considered, 

 d-ribose does not appear as a pentosan, but is an important constituent 

 of plant nucleic acid, as proven by Leveue and Jacobs (1912). It seems 

 probable that the known plant nucleic acids are quite similar, and it has 

 been established that there are four molecules of d-ribose in those plant 

 nucleic acids that are known. 



Methyl Pentoses. Several of these have recently been isolated from 

 plants. They differ from pentoses in having a methyl radical replace one 

 of the hydrogens of the primary alcohol, CH 2 OH, forming the group 

 CHOH.CH 3 , as in 



COH 



HCOH 

 HCOH 



HOCH 



I 

 HOCH 



CH 3 



Rhamnose. 1-Rhamnose is a constituent of many glucosides and is 

 perhaps the most common of the methyl pentoses. It crystallizes with one 

 molecule of water and exists in a and (3 forms. 



Digitoxose is probably a reduced methyl pentose obtained from digi- 

 talis : 



CH 3 . CHOH . CHOH . CHOH . CH 2 . COH 



The methyl pentoses behave like the pentoses on the whole, but yield 

 methylfurfuraldehyde on distillation with acids. 



Dioses, Trioses, Tetroses, etc. The simplest sugar is the diose, glycol- 

 ic-aldehyde, COH but it has not been found in nature. It is of inter- 



CH 2 OH 



est, however, as a possible product of the intermediary metabolism of carbo- 

 hydrates. There are three trioses of interest, two aldoses, d-and 1-glyceroses 

 or glycericaldehydes, and one ketose, dihydroxyacetone. 



