194 PHOTOSYNTHESIS 



f. Methylpentoses. CH3. (CH.OH)4. CH:0. 



The occurrence of methylpentoses in plants corresponds in general 

 to that of the pentoses, though they are apparently not as widely dis- 

 tributed nor as plentiful. Free methylpentoses in plants are very rare 

 though no very careful nor extensive search has been made.^'^ They fre- 

 quently accompany pentoses as methylpentosans in the structural elements 

 of plants and are also components of a number of glucosides or methyl- 

 pentosides. Very little is known regarding the mode of formation or role 

 in the carbohydrate economy of the plant. 



l-Rhamnose, is a component of a number of glucosides, the best known 

 of which is probably quercitrin present in oak bark and also widely dis- 

 tributed in other plants. It is the methyl derivative of 1-lyxose. It forms 

 an insoluble b-naphthylhydrazone, melting at 192-193°. 



d-Isorhamnose (isorhodeose) is a component of the glucoside con- 

 volvulin ; it is a methyl derivative of d-xylose. 



Fucose as a methylpentosan is a component of many sea-weeds. It 

 was the first of these substances to be the subject of extensive investiga- 

 tion. It is the methyl derivative of d-arabinose and has the constitution 

 corresponding to 1-galactose. It forms a p-bromphenylhydrazone, melt- 

 ing at 178°. 



Rhodeose, the optical antipode of fucose, is also a component of the 

 glucoside convolvulin.^* 



Methylpentoses can be identified by means of a color reaction with 

 hydrochloric acid and acetone. The substance is heated with 10 cc. of 

 38 per cent hydrochloric acid and 2 cc. acetone. A violet color is pro- 

 duced which in the spectroscope covers the D (sodium) line. Pentoses 

 produce the same color, but this color disappears if the tube is allowed 

 to stand at ordinary temperature for one-half to one hour, while the 

 color of the methylpentose remains. ^^ 



For the quantitative determination of methylpentoses, which usually 

 occur accompanied by pentoses, the methylfurfuralphloroglucide can be 

 employed. In order to differentiate between the pentoses and the methyl- 

 pentoses advantage is taken of the fact that methylfurfuralphloroglucide 

 is soluble in 96 per cent alcohol at 60°, and can thus be separated from 

 the furfuralphloroglucide formed from the pentoses. ^*^ 



g. Hexoses. Asymmetric Synthesis. 



The hexose sugars are by far the most important carbohydrates which 

 occur in plants. Not only are these sugars the most common components 

 of plants, but also for the life activities of both plants and animals the 



"Requier, Jour. Pharm. Chini. (6), 22, 540 (1905). 



"Czapek, "Biochemie der Pflanzen," I, 270, 642 (1913). Lippmann, "Chemie 

 der Zuckerarten," I, 159 (1904). 



" Rosenthaler, Zeit. anal. Chem., 48, 167 (1909). 



"Ellett and Tollens, Zeit. Rubenzuckerind.. 42, 19 (1905) ; Ber. chem. Ges., 38, 

 492 (1905). Mayer and Tollens, Ber. chem. Gcs., 40, 2434 (1907). 



