PRACTICAL EXERCISES g 



precipitated. It can be redissolved at first, but rapidly becomes in- 

 soluble. 



2. Special Reactions of Certain Proteins (i) Heat-Coagulable Pro- 

 teins : (a) Albumins. (a) Heat a little of the solution of egg-albumin in 

 a test-tube; it coagulates. With another sample determine the tem- 

 perature of coagulation, first very slightly acidulating with a 2 per cent, 

 solution of acetic acid. 



To determine the Temperature of Coagulation. Support a beaker by 

 a ring which just grips it at the rim. Nearly fill the beaker with water, 

 and slide the ring on the stand till the lower part of the beaker is im- 

 mersed in a small water-bath (a tin can will do quite well). In this 

 beaker place a test-tube, and in the test-tube a thermometer, both sup- 

 ported by rings or clamps attached to the same stand. Put into the 

 test-tube at least enough of the albumin solution to completely cover 

 the bulb of the thermometer, and heat the bath, stirring 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. 



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

 obtained as on p. 65. 



(b) Globulins. Use serum- globulin (p. 65), or myosinogen (p. 819). 

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

 Verify the following properties of globulins: 

 (a) They coagulate on heating. 

 () They are insoluble in distilled water (p. 65). 

 (7) They are precipitated by saturation with magnesium sulphate or 

 sodium chloride (p. 65). 



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. 8), as with suitable modifica- 

 tions can be instituted on solid substances. Thus, in performing (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-coagu- 

 lated 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 redissolves. 



Try the general protein reactions (p. 8) on a dilute solution. In 

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

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

 of pure gelatin, since these ammo-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- 

 Proleins : (a) Acid-Albumin. To a solution of egg-albumin add a little 

 o - 4 per cent, hydrochloric acid, and heat to about body temperature 

 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 



