ELEMENTARY CHEMICAL PHYSIOLOGY 195 



albumin have, however, been crystallised. Certain vegetable 

 proteins, e.g. the globulin of hemp seed (edestin), crystallise more 

 easily. 



Extract hemp seed, which has been thoroughly pounded, with warm 5 per 

 cent, sodium chloride (50 C.) and place extract in a dialyser overnight. 

 As the result of dialysis, crystals of edestin become deposited in the tube. 

 Examine those placed under the microscope. Crystals of edestin may also 

 be obtained, on standing, by cooling the extract of hemp seed with ice. 



To obtain crystals of egg albumin the whites of several eggs are mixed 

 with an exactly equal amount of a fully saturated solution of ammonium 

 sulphate. This precipitates the globulins. The ammonium sulphate solution 

 must be exactly neutral in reaction and should be added to the egg-white in 

 small quantities at a 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 per cent, acetic acid, drop by drop, until a precipitate 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. 172). 



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

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

 dextro-rotatory, but their protein portion is lee vo -rotatory. 



VI. Electrical Properties. Although crystalloid free protein 

 solutions are practically non-conductors, it is found that when placed 

 in an electric field proteins differ in their reaction. Some migrate 

 towards the positive pole, some towards the negative and some 

 show no definite direction ; there are therefore protein cations and 

 anions. The third class are uncharged or isoelectric. Proteins of 

 opposite charges mutually precipitate one another. 



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

 tube. Now add some 20 per cent, 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. If too much copper has been 

 added filtration often gets rid of the excess copper hydrate. 



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 substance 

 biuret which is formed when urea is heated, hence the name, 



