PENTOSES 93 



decolorized with animal charcoal. The filtrate is evaporated 

 under reduced pressure and the calcium sulphate precipitated 

 by the addition of alcohol ; the filtered solution is then 

 evaporated to a viscous syrup and crystallized from alcohol 

 or from glacial acetic acid. 



Ribose. 



This pentose has been obtained as a product of the 

 hydrolysis of yeast nucleic acid. According to Robinson * 

 it is probably not pre-existent in this substance but is pro- 

 duced by optical inversion during hydrolysis from the xylose 

 contained in the nucleic acid. 



Apiose. 



This is a rare pentose obtained by the hydrolysis of the 

 glucoside apiin contained in parsley ; it yields a bromo- 

 phenylosazone, m.p. 211-212°. Owing to its abnormal struc- 

 ture (see formula, p. 90) it does not yield furfural when 

 heated with hydrochloric acid and gives no colour with 

 phloroglucinol and hydrochloric acid. 



Methyl Pentoses. 



There is no evidence that methyl pentoses occur free in 

 the plant ; they are, however, associated with the pentoses 

 as cell wall constituents, and also occur as glucosides. Their 

 constitution is represented by the formula 



CH3CHOH(CHOH)3CHO 



When heated with 10 c.c. of concentrated hydrochloric acid 

 and 2 c.c. of acetone, the methyl pentoses give a violet colour 

 which is permanent, in contradistinction to pentoses, which 

 also yield a violet colour which, however, fades within one hour. 

 Heated with concentrated hydrochloric acid, methyl 

 pentoses give off methyl furfural which with aniline acetate 

 gives a yellow colour ; whereas furfural which would be 



* Robinson : " Nature," 1927, 120, 44, 656. 



