CARBOHYDRATES 37 



Arabinose is one of the most important of the pentoses. The 

 /-arabinose may be obtained from gum arabic, plum or cherry gum by 

 boiling for 10 minutes with concentrated hydrochloric acid. This 

 pentose is dextro-rotatory, forms an osazone and has reducing power, 

 but does not ferment. The z-arabinose has been isolated from the urine 

 and yields an osazone which melts at i66-i68C. 



EXPERIMENTS ON ARABINOSE 



1. Orcinol-Hydrochloric Acid Reaction (Bial). 1 To 5 c.c. of Bial's reagent 2 in 

 a test-tube add 2-3 c.c. of the arabinose solution and heat the mixture gently until 

 the first bubbles rise to the surface. Immediately or upon cooling the solution 

 becomes green and a flocculent precipitate of the same color may form. (For 

 further discussion see Chapter XXTV.) The test may also be performed by 

 adding the pentose to the hot reagent. 



It is claimed that this test is more delicate than the original orcinol 

 test (see 3) and more accurate, since menthol, kreosotal, etc., respond 

 to the original orcinol test but not to BiaPs. Sachs 3 has offered sug- 

 gestions as to modification of the test in order to avoid confusion 

 with glycuronic acid. 



2. Phloroglucinol-Hydrochloric Acid Reaction (Tollens) . To equal volumes of 

 arabinose solution and hydrochloric acid (sp. gr. 1.09) add a little phloroglucinol 

 and heat the mixture on a boiling water-bath. Galactose, pentose or glycuronic 

 acid will be indicated by the appearance of a red color. To differentiate between 

 these bodies make a spectroscopic examination and look for the absorption band 

 between D and E given by pentoses and glycuronic acid. Differentiate between 

 the two latter bodies by the melting-points of their osazones. 



Compare the reaction with that obtained with galactose (page 36). 



3. Orcinol Test. Repeat 2, using orcinol instead of phloroglucinol. A suc- 

 cession of colors from red through reddish blue to green is produced. A green pre- 

 cipitate is formed which is soluble in amyl alcohol and has absorption bands be- 

 tween C and D. 



4. Phenylhydrazine Reaction. Make this test on the arabinose solution 

 according to directions given under Glucose, 3, page 22. 



CH 2 OH 



I 



XYLOSE, (CHOH) 3 



CHO 



Xylose, or wood sugar; is obtained by boiling wood gums with dilute 

 acids as explained under Arabinose, (see above) . It is dextro-rotatory, 

 forms an osazone and has reducing power, but does not ferment. 



1 Bial: Deut. med. Woch., 28, 252, 1902, and Berl. klin. Woch., No. 18, 1903. 

 2 Orcinol 1.5 gram. 



Fuming HC1 500 grams. 



Ferric chloride (10 per cent) 20-30 drops. 

 'Sachs: Bioch. Zeit., i, 383, 1906, and 2, 245, 1906. 



