142 PROTEINS AND AMINO- ACIDS [ch. 



animal nucleic acids, it is probable that all plant nucleic acids may prove 

 to have the same composition. The nucleic acid investigated is a complex 

 substance formed by the condensation of four nucleotides, each of which 

 consists of phosphoric acid, a pentose sugar and a purine. Thus yeast 

 nucleic acid is represented as: 



On hydrolysis, nucleic acid yields phosphoric acid, rf-ribose and the 

 four purines as ultimate products. Nucleic acid is insoluble in water 

 but soluble in dilute alkalies: owing to the difficulty of obtaining other 

 suitable material, nucleic acid is usually prepared from Yeast. 



Expt. 131. Preparation of nucleic acid from Yeast (from Bertrand, see p. 10). 

 Take 40 gms. of baker's yeast and add 30 c.c. of 30 % caustic soda solution. Break 

 up the mass thoroughly and allow it to stand for fifteen minutes. Then add 

 20 c.c. of water, shake well and at the same time add 10-20 c.c. of 10 ^/q solution of 

 ferric chloride which will produce a gelatinous precipitate. The mass, which should 

 be homogeneous, is drained upon a cloth placed in. a funnel, so that the almost clear 

 liquid can be collected in a beaker. The residue is washed with 50 c.c. of warm 

 water (at 60-70° C.) and again drained on a cloth. The brownish filtrate is added 

 to an equal volume of alcohol and enough hydrochloric acid is added to render the 

 mixture slightly acid. A precipitate of nucleic acid is produced. The liquid should 

 be allowed to stand until the precipitate has settled well. The supernatant fluid 

 is then decanted, and the precipitate filtered off on a small porcelain funnel using, 

 if possible, a hardened filter-paper. The precipitate is washed with a little alcohol 

 and dissolved in the minimum amount of 10 % caustic soda solution. This is re- 

 precipitated by pouring into acid alcohol and finally collected on a small funnel, 

 again using hardened filter paper. 



The nucleic acid is tested for the pentose (ribose) and the phosphoric acid com- 

 ponents as follows : 



(a) A portion of the precipitate is shaken up with a few c.c. of strong hydro- 

 chloric acid in a test tube, a little orcinol is added and the liquid tested for pentoses 

 (see Expt. 39). i 



