V] HYDROLYZING ENZYMES 53 



Polysaccharides. 



These substances are formed by condensation, with elimination of 

 water, from more than three molecules of monosaccharides. 



Pentosans. 



It has already been mentioned that condensation products of the 

 pentoses, the pentosans, are widely distributed. The two most frequently 

 occurring pentosans are xylan and araban. No enzymes are known 

 which hydro lyze the pentosans. It is characteristic of xylan and araban 

 that they form copper compounds in Fehling's solution in presence of 

 excess of alkali. 



Araban. This pentosan may be regarded as a condensation product 

 of arabinose as already indicated. It occurs in various gums (Gum 

 Arabic, Cherry Gum) frequently in combination with other substances. 

 On hydrolysis with acids, araban yields arabinose. (See also gums and 

 arabinose.) 



Expt. 47. Preparation of araban from Oum Arabic. (Salkowski, 27.) Weigh out 

 20 gm.s. of gum arabic and dissolve in 500 c.c. of warm water in a large evaporating 

 dish. Then add 200 c.c. of Fehling's solution and excess of caustic soda solution. 

 The araban will be precipitated as a white gummy mass which will settle at the 

 bottom of the dish. Filter off" through muslin. Take up the precipitate in dilute hydro- 

 chloric acid (1 pt. of acid : 1 \>i. of water), and then add alcohol. The araban separates 

 out as a white precipitate. Wash away the copper chloride with alcohol. 



Expt. 48. Hydrolysis of araban. The araban from the last experiment is put 

 into a round-bottomed flask with about 200 c.c. of 2 % sulphuric acid and heated on 

 a water-bath for 2 hours, the flask being litted with an air condenser (see p. 45). 

 Then neutralize the liquid witli calcium carbonate, filter from calcium sulphate, and 

 concentrate on a water-bath. The sugar is extracted from the syrup with 90 % alcohol. 

 Arabinose crystallizes with difliculty but the process may be facilitated by sowing 

 the concentrated alcoholic solution with a few crystals of arabinose. Some of the 

 solution of arabinose should be tested with all the tests given in Expt. 38. 



Xylan. This pentosan occurs in lignified cell-walls, and is the chief 

 constituent of "wood gum." It is found in the wood of many trees (not 

 Coniferae), in bran, in wheat and oat straw, in maize cobs, in the shells 

 of coconuts and walnuts, in the testa of the cotton {Gussypiam) and in 

 many other tissues : also in some gums. On hydrolysis, xylan yields 

 xylose; hence wood shavings, bran, straw, etc., will give the pentose reac- 

 tions on hydrolysis. 



