THE PHYSICAL PROPERTIES OF THE PROTEINS 21 



crystals obtained are, according to Inagaki, compounds of protein 

 with sulphuric acid. 



It is possible that the other crystalline proteins are also compounds 

 with acids. This subject will be reconsidered later, in discussing 

 the action of acids and bases on proteins. 



A special case of protein crystallisation is that of haemoglobin. 

 This is a conjugated protein^ i.e., it is a compound of a protein and a 

 chromatogenic group, and its power of crystallisation depends on the 

 presence of this group. Crystals can be obtained by the following 

 method, due chiefly to Zinoffsky and modified by Abderhalden. 



The paste of red blood corpuscles (from a horse), after separating 

 from the serum and washing, is mixed with twice its volume of 

 water and the mixture is then warmed to 35. The corpuscles are 

 by this treatment laked. A very small known quantity of ammonia 

 is then added to dissolve the stromata, and then hydrochloric acid 

 in very dilute solution is added in such quantity as to exactly 

 neutralise the ammonia. The mixture is then cooled to o, and one- 

 quarter the volume of absolute alcohol is added. The whole is kept 

 on ice, and crystals slowly separate out, and settle at the bottom of the 

 vessel. The supernatant liquid is then poured off, and the crystals 

 are washed by decantation with a mixture of one part alcohol and 

 four parts of water previously cooled to o. They can be recrys- 

 tallised by dissolving the crystalline paste in twice its volume of 

 water at 35, then cooling to o, adding one-quarter the volume of 

 alcohol, and allowing the crystals to form slowly at a low temperature. 



It must be remembered that only a relatively small number of 

 proteins have as yet been obtained in crystalline form, and recrystalli- 

 sation as a mode of purification has, so far, not obtained a very wide 

 application. It is not known whether any chemical change takes 

 place during the process, and there is a certain amount of evidence 

 that in the case of crystallised egg-albumin, for instance, the cry- 

 stallised product is different from the original protein existing in 

 the egg-white (see " gold number " of crystallised and non -crystallised 

 egg-albumin, p. 27). 



SECTION VI. THE TEMPERATURE OF HEAT COAGULATION OF 

 PROTEIN SOLUTIONS. 



In 1854 Kiihne noticed that there were two proteins present in 

 muscle-plasma which differed in the temperature of heat coagulation, 

 and since that time the coagulation point has been regarded as an 

 important factor for distinguishing between proteins, and the process 

 of separating proteins in mixtures by means of a fractional heat 

 coagulation has been repeatedly employed. In this operation a 

 solution of the mixture is heated to a definite temperature until 

 a coagulum is produced ; this is filtered off, and the filtrate heated 

 to a still higher temperature, when there is the formation of 

 another coagulum ; further fractions can be taken until no more 

 coagulable protein remains in solution. Thus Halliburton, by such a 

 process, separated in 1887 several proteins from muscle-plasma, which 

 he designated as follows : paramyosinogen, coagulating at 47 ; myo- 

 sinogen^ coagulating at 56 ; myoglobin, coagulating at 63 ; all of which 

 can be precipitated from solution on saturation with sodium chloride 



