PRACTICAL EXERCISES 9 



the water in the beaker occasionally with a feather or a splinter of 

 wood, or a glass rod, the end of which is guarded with a piece of 

 indiarubber tubing. Note the temperature at which the solution 

 becomes turbid, and then the temperature at which a distinct 

 coagulum or precipitate is formed. Repeat with the unacidulated 

 albumin solution. 



(/3) A similar experiment may be performed with serum- albumin 

 obtained as on p. 57. 



(b) Globulins. Use serum- globulin (p. 57), or myosinogen (p. 672). 

 Fibrinogen is also a globulin, but cannot easily be obtained in 

 quantity. Verify the following properties of globulins : 



(a) They coagulate on heating. 



(j8) They are insoluble in distilled water (p. 57). 



(y) They are precipitated by saturation with magnesium sulphate 

 or sodium chloride (p. 57). 



They give the general protein tests (i) to (8). 



Both the heat-coagulated proteins and such proteins as the solid 

 fibrin which is formed from fibrinogen in the clotting of blood give 

 such of the general protein tests, (i), (2), (3) (p. 7), as with suitable 

 modifications can be instituted on solid substances. Thus, in per- 

 forming (2), a flake of fibrin or a small piece of the boiled egg-white 

 should be soaked for a few minutes in a dilute solution of cupric 

 sulphate. Then the excess of the cupric sulphate should be poured 

 off, and sodium hydroxide added, when the coagulated protein will 

 become violet. Heat-coagulated proteins are insoluble in water, 

 weak acids and alkalies, and saline solutions ; fibrin is slightly 

 soluble in the latter. 



(2) Gelatin. Add some pieces of gelatin to cold water in a test- 

 tube. It does not dissolve. Immerse the tube in a boiling water- 

 bath till the gelatin goes into solution. Then cool the test-tube 

 under the tap ; the solution sets into a jelly. On heating it redis- 

 solves. 



Try the general protein reactions (p. 7) on a dilute solution. In 

 Piotrowski's test a violet colour is obtained. The tests which depend 

 on the presence of tryosin or tryptophane are not given by a solution 

 of pure gelatin, since these amino-acids are absent from the gelatin 

 molecule. Commercial gelatin may give a slight reaction due to 

 traces of other proteins. 



3. Reactions of Certain Derivatives of Native Proteins (i) Meta- 

 Proteins : (a] Acid-albumin. To a solution of egg-albumin add a 

 little o'4 per cent, hydrochloric acid, and heat to about body tem- 

 perature say 40 C. for a few minutes. Acid-albumin is formed. 

 It can be produced from all albumins and globulins by the action 

 of dilute acid. Make the following tests : 



(a) Add to a portion of the solution in a test-tube a few drops of 

 a solution of litmus ; the colour becomes red. Now add drop by 

 drop sodium carbonate or dilute sodium hydroxide solution till the 

 tint just begins to change to blue. A precipitate of acid-albumin is 

 thrown down. Add a little more of the alkali, and the precipitate 

 is redissolved. It can be again brought down by neutralizing with 

 acid. 



(/3) Heat a portion of the solution to boiling ; no precipitate is 

 formed. 



(y) Add strong nitric acid ; a precipitate appears, which dissolves 

 on heating, and the liquid becomes yellow. 



(b) Alkali-albumin. To a solution of egg-albumin add a little 

 sodium hydroxide, and heat gently for a few minutes. Alkali-albumin 



