NUCLEIC ACIDS AND NUCLEOPKOTEINS 131 



that can scarcely be improved by any method of purification. It is a soluble 

 sodium salt of thymus nucleic acid but is generally referred to simply as thymus 

 nucleic acid. Very similar or identical substances may be prepared by the same 

 procedure from other animal tissues, rich in cell nuclei such as the pancreas and 

 spleen." 



10. Tests on Thymus Nucleic Acid. 1-4. Repeat the experiments as given 

 under yeast nucleic acid, page 129. 5. Make a 4 per cent solution of thymus nucleic 

 acid in hot water (0.4 gram to 10 c.c.). Allow to cool. What happens? Divide 

 into two portions. To one add a little NaOH solution; to the other add acetic acid. 

 Then neutralize carefully in each case. 



Both acetic acid and NaOH decrease the viscosity of the nucleate solution. It 

 may be changed back and forth from the gelatinous to the fluid condition by the 

 alternate addition of acid and alkali. 



11. Tests on Purine Bases and Derivatives. (a) Xanthine. i. Silver 

 Nitrate Reaction. Dissolve a little xanthine in ammonia and add silver nitrate 

 solution. Examine a little of the precipitate microscopically. (See page 369.) 



2. Copper Sulphate Reaction. Dissolve a little of the substance in dilute 

 alkali, make faintly acid with acetic acid. Heat to boiling. Add i c.c. 10 

 per cent CuSO 4 and then a few drops at a time of sodium bisulphite (saturated 

 solution) until the precipitate becomes yellowish. All of the purines give this 

 reaction. 



3. Nitric Acid Test. Place a small amount of the substance in a small 

 evaporating dish, add a few drops of concentrated nitric acid, and evaporate to 

 dryness very carefully on a water-bath. The yellow residue upon moistening 

 with caustic potash becomes red hi color and upon further heating assumes 

 a purplish-red hue. Now add a few drops of water and warm. A yellow 

 solution results which yields a red residue upon evaporation. Compare with 

 similar reaction on other purine bases and uric acid. (See Murexide test. 

 Chapter XXEH.) 



4. Weidel's Reaction. Bring a small amount of the substance into solution in 

 bromine water. Evaporate to dryness on a water-bath. Remove the stopper 

 from an ammonia bottle and by blowing across the mouth of the bottle direct the 

 fumes of ammonia so that they come into contact with the dry residue. Under 

 these conditions the presence of xanthine is shown by the residue assuming a red 

 color. A somewhat brighter color may be obtained by using a trace of nitric acid 

 with the bromine water. By the use of this modification, however, we may get a 

 positive reaction with bodies other than xanthine. 



(b) Hypoxanthine. i. Repeat Experiments i and 3 under Xanthine. Ex- 

 amine the crystals of hypoxanthine silver nitrate under the microscope. (See 

 page 369.) 



2. Dissolve a little of the substance in a very small amount of hot 6 per cent 

 nitric acid and allow to cool. Characteristic whetstone crystals of hypoxanthine 

 nitrate should be formed. Examine under the microscope. (See Fig. 40, page 



1350 



(c) Adenine. i. Warm a few crystals of adenine hi a test-tube with a little 

 water. They should become cloudy at 53 C. 



2. Dissolve a little adenine in hot water and add a few drops of picric acid. 

 Examine the pale yellow crystals under the microscope. The picrate crystal- 

 lizes as needle clusters. 



3. Repeat Experiment 3 under Xanthine. 



