PROTEINS 



What is the order in which the four solutions coagulate? 



Repeat the experiment, adding to the first tube i drop of acetic acid, to the 

 second i drop of concentrated potassium hydroxide solution, to the third 2 drops 

 of a 10 per cent sodium chloride solution and leave the fourth neutral as before. 



What is the order of coagulation here? Why? See page 116. 



13. Precipitation by Alcohol. Prepare three test-tubes each containing 

 about 10 c.c. of 95 per cent alcohol. To the first add i drop of 0.2 per cent 

 hydrochloric acid, to the second i drop of potas- 

 sium hydroxide solution and leave the third 

 neutral in reaction. Add to each tube a few 

 drops of egg albumin solution and note the re- 

 sults. What do you conclude from this experi- 

 ment? 



If in acid or neutral solution alcohol 

 precipitates proteins unaltered, but if al- 

 lowed to remain under alcohol the protein 

 is transformed. The "fixing" of tissues for 

 histological examination by means of al- 

 cohol is an illustration of the application 

 of this transformation produced by alcohol. 

 It apparently is a process of dehydration. 



14. Crystallization of Egg Albumin. 1 Care- 

 fully remove the egg-white from a number of 

 absolutely fresh eggs. 2 Measure the volume of 

 the egg-white and add an equal volume of satur- 

 ated ammonium sulphate a small portion at a 

 time, beating the mixture vigorously after each 

 addition. 3 Filter the mixture through a large 

 pleated filter paper. 4 Measure the volume of 

 the filtrate. To 100 c.c. of the filtrate add very 

 carefully a 10 per cent solution of acetic acid from 

 a burette being certain to note the exact volume 

 of the acid used. The acid should be added drop 



Frc - ^'- 



by drop, the albumin mixture being gently shaken 

 during the process. Add acid until the precipi- 

 tate which forms at each addition is no longer dissolved when the albumin is 

 shaken, and an opalescent mixture is secured. (It is generally rather difficult 

 to determine this point, inasmuch as suspended air bubbles may simulate a 

 precipitate.) As soon as the solution is milky, indicating that a permanent pre- 

 cipitate has formed, run in from the burette i c.c. of the acetic acid. This should 

 produce a heavy white precipitate. Now take the burette reading to determine 

 the exact volume of acid used in the treatment of 100 c.c. of the albumin mixture. 



1 Hopkins and Pinkus: Jour. PhysioL, 23. 



2 If not perfectly fresh the albumin will not crystallize. 

 1 Note the odor of ammonia. What causes it? 



4 Sometimes better results are obtained by permitting the mixture to stand several 

 hours before filtering. 



