130 PHYSIOLOGICAL CHEMISTRY 



ence proceed as follows: Transfer the precipitate of nucleoprotein remaining 

 from the previous experiment to a small flask and add 25-50 c.c. of 5 per cent 

 H 2 SO 4 . Boil for an hour or more to decompose. Maintain the original volume 

 by adding water. The solution becomes brown due to formation of melanin- 

 like substances. The purine bases are set free. Retain one-fourth of the solu- 

 tion for the next experiment. Transfer the remainder to a casserole and add 

 ammonia with thorough mixing, a little at a time, until the fluid is nearly neutral. 

 Then make slightly alkaline with dilute ammonia and filter if not clear. Transfer 

 to a beaker and add about 10 c.c. of 5 per cent ammoniacal silver nitrate solution. 

 Purine bases if present will yield a brown flocculent precipitate of their silver 

 compounds. If a precipitate does not appear immediately, examine the solution 

 after it has been allowed to stand for some time undisturbed. 



4. To Show the Presence of Protein, Carbohydrate, and Phosphoric Acid 

 Radicals in Nucleoprotein. Filter the greater portion of the acid liquid which was 

 reserved from the preceding experiment. Apply the following tests to portions of 

 it: (a) The biuret test, (b) The xanthoproteic test, (c) Molisch test, (d) 

 Fehling's test, (e) Test for phosphate. 



5. Preparation of Thymus Nucleoprotein. About 100 grams of fresh thymus 

 gland (lymphatic glands may also be used) freed as nearly as possible from adherent 

 fat are run through a meat chopper. To this material in a flask add 300 c.c. of 0.9 

 per cent NaCl and allow to stand 24-48 hours in the cold. A little chloroform and 

 toluol should be added as preservatives, and the mixture shaken occasionally during 

 this period. Filter. A milk white liquid is obtained. Precipitate the nucleo- 

 protein from solution by the careful addition of dilute acetic acid. Excess of the 

 acid should be avoided. Ordinarily acetic acid to make a i per cent solution is 

 sufficient. Filter off the precipitate. Wash with alcohol and then with ether and 

 dry. 



6. Experiments on Thymus Nucleoprotein. Repeat the experiments given 

 under Yeast Nucleoprotein (page 129). 



7. Preparation of Yeast Nucleic Acid. Dilute 50 c.c. of i per cent NaOH 

 with 250 c.c. of water in a casserole and add to this solution 100 grams of com- 

 pressed yeast cut in small pieces. Heat on the water-bath for half an hour with 

 occasional stirring. Remove from the bath and filter at once through a folded 

 filter. To the cooled filtrate add acetic acid until faintly acid to litmus. Filter 

 again. Evaporate the solution to 100 c.c. or less and filter if necessary. Allow 

 to cool to 40C. or below, then pour with vigorous stirring into 200 c.c. of 95 per 

 cent alcohol containing 2 c.c. of concentrated HC1. Allow to settle and wash the 

 precipitate by decantation in a tall vessel, twice with 95 per cent, alcohol and 

 twice with ether. Transfer to a filter paper. Allow to drain and dry at room 

 temperature. 



8. Tests on Nucleic Acid from Yeast. 1 i. Test the solubility of nucleic acid 

 in cold and hot water, in alcohol, and in dilute acid and alkali. To the solution in 

 alkali add dilute HC1 drop by drop until the solution is acid, then add excess of 

 concentrated HC1. 



Does nucleic acid coagulate on boiling? Does the solution in hot water 

 gelatinize on cooling? 



2. Try xanthoproteic reaction and biuret test. 



3. Dissolve a little nucleic acid in water with the aid of heat. Test the re- 



1 A satisfactory preparation of yeast nucleic acid may be obtained from Merck and Co. 



