302 PKACTICAL PHYSIOLOGY 



time, the mixture being briskly stirred between each addition. The precipitated 

 globulin is filtered off, and the filtrate, which reacts alkaline to litmus, is treated 

 with ammonium sulphate, drop by drop, until a faint haze of precipitated 

 albumin is obtained. A drop of water is added, so that the haze just disappears. 

 The solution is now treated with 10 % acetic acid, drop by drop, until a pre- 

 cipitate of albumin just forms. The flask is set aside ; in about twenty hours 

 it will .be found that a large number of needle-shaped crystals have become 

 deposited (see Fig. 224). 



V. Rotation of Light. All proteins are laevo - rotatory. Some 

 combined proteins, such as haemoglobin and nucleo-protein, are dextro- 

 rotatory, but their protein portion is laevo-rotatory. 



VI. Colour Reactions. This group of reactions is very important, as 

 each reaction yields information as to the constitution of the protein 

 molecule. The meaning of each test should therefore be carefully 

 noted. 



(a) The Biuret Reaction (Piotrowski's test). 



EXPERIMENT VII. Pour a drop of weak copper sulphate into a test 

 tube. Now add some 20% caustic soda until a pale blue colour is 

 obtained (about 15 c.c.). Divide this into three portions, A, B, C. Keep 

 A as control colour. To B add a few drops of diluted egg-white. To 

 C add the same number of drops of the commercial peptone provided. 

 Note the violet colour with albumin, the pink colour with the peptone 

 solution. 



It is important to keep control tube A, since in using very weak 

 solutions a slight change in colour can be detected by comparison with 

 the control. 



All proteins give either a purple or pink colour with this test. It 

 shows that the protein contains two or more CO - NH - groups linked 

 together. The same reaction is given by the body biuret formed when 

 urea is heated, hence the name. 



(b) Xanthoproteic Reaction. 



EXPERIMENT VIII. To about 5 c.c. of the solution of egg-white add 

 a few drops of strong nitric acid; a white precipitate results. Warm 

 this and the precipitate changes to a yellow curd. Cool under the tap 

 Add a few drops of strong ammonia ; the yellow colour changes to a 

 brilliant orange. The name xanthoproteic (yellow protein) will help 

 the student to remember the colour of the curd obtained. __ This test, 

 shows the presence of the benzene ring in the protein molecule ; hence 

 only proteins containing axSSRVFrng give this test. 



(c) Millon's Reaction. 



EXPERIMENT IX. Add a few drops of Millon's reagent (which 

 consists of a solution of mercurous and mercuric nitrates) to some of 



