

THE PROTEINS 73 



crystals contain 1-4 p.c. MgO. A better method of obtaining such crystals has been 

 devised by Osborne. The ground seeds are extracted with 10 per cent, sodium chloride 

 solution, and filtered. The filtrate is diluted with water heated to 50 or 60 C. until a 

 slight turbidity forms. After warming the diluted solution until this turbidity dis- 

 appears, and then allowing it to cool slowly, the protein separates in well-developed 

 crystals. It is possible also to obtain crystals of animal proteins. Haemoglobin, the 

 oxygen-carrying protein of the red blood corpuscles, can be made to crystallise with 

 extreme ease. With some animals, such as the rat, it is only necessary to bring the 

 haemoglobin into solution, by the addition of a little distilled water and ether to. the* 

 blood, to cause the crystallisation of the liberated haemoglobin. 



Egg albumin and serum albumin may also be crystallised with ease by a method 

 devised by Hofmeister and improved by Hopkins. If, for instance, we wish to crystallise 

 egg albumin, white of eggs is treated with an equal bulk of saturated solution of ammo- 

 nium sulphate in order to precipitate the globulin. It is then filtered, and the filtrate 

 is treated with saturated ammonium solution until a slight permanent precipitate 

 is produced. This precipitate is then just redissolved by the cautious addition of water, 

 and dilute acetic acid (10 per cent.) is added drop by drop until a slight precipitate is 

 produced. The flask is now corked and allowed to stand for twenty-four hours, when 

 the precipitate, which will have increased in quantity, will be found to consist entirely 

 of acicular crystals. A similar method may be used for serum albumin. In each case 

 the crystals contain a considerable proportion of ammonium sulphate. This may be 

 replaced by sodium chloride by washing the crystals with a saturated solution of 

 this salt. By absolute alcohol the crystals may be coagulated and may be then washed 

 practically free from salt, but it is not possible to obtain crystals of coagulable protein 

 free from the presence of some salt. 



Although by repeated crystallisation of egg albumin a product may be obtained 

 which is absolutely constant in both its physical and chemical characters, we cannot 

 ascribe to crystallisation the same importance in securing purity and homogeneity of 

 the substance that we can when we are dealing with inorganic salts. This is due to the 

 fact that these crystals take up other colloids with great ease. When haemoglobin, for 

 instance, is crystallised from blood, the first crop of crystals, although thoroughly 

 washed from their mother liquor, always contain a considerable proportion of serum 

 albumin. Indeed, the presence of colloidal material seems to render the production 

 of the so-called mixed crystals much more easy. Thus Schultz has shown that in 

 urine mixed inorganic crystals can be obtained. Human urine is allowed to stand 

 twenty-four to forty-eight hours with dicalcium phosphate and then filtered. On 

 allowing the filtrate to evaporate slowly, a crystalline precipitate is produced consist- 

 ing of whetstone-shaped crystals which are doubly refracting. On treating these 

 crystals with dilute acetic acid this acid extracts calcium phosphate from the crystals. 

 The original shape of the crystals is however retained. The only difference under 

 the microscope consists in the fact that they have now lost their doubly refracting 

 power on polarised light. They consisted of a mixture of calcium sulphate and calcium 

 phosphate from which, on treatment with acid, only the calcium phosphate was dis- 

 solved out. 



The Molecular Weight of Proteins. We may arrive at an approximate idea 

 of the minimum size of the protein molecule in various ways, though in all 

 cases our calculations are apt to be vitiated by the difficulty of obtaining a 

 preparation which is homogeneous, i.e. chemically pure, and by the ease 

 with which molecules of the size which we must assume for proteins form 

 adsorption combinations in varying proportions with other substances. 

 If we assume that each molecule of the respective protein contains only one 

 atom of sulphur, we can calculate its molecular weight. It is evident that 

 the protein which contains 1 per cent, of sulphur will, have a molecular weight 



