24 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



though crystalline, it is not crystalloid in Graham's sense of 

 that term. Blood pigment is not the only crystallisable proteid.. 

 Long ago crystals of proteid (globulin or vitellin) were observed in 

 the aleurone grains of many seeds, and in the similar proteid occur- 

 ring in the egg-yolk of some fishes and amphibians. By appropriate 

 methods these 'have been separated and re-crystallised. Further, 

 egg-albumin itself has been crystallised. If a solution of white of egg 

 is diluted with half its volume of saturated solution of ammonium 

 sulphate, the globulin present is precipitated and is removed by 

 filtration. The filtrate is now allowed to remain some days at the 

 temperature of the air, and as it becomes more concentrated from 

 evaporation, minute spheroidal globules and finally minute needles, 

 either aggregated or separate, make their appearance (Hofmeister). 

 Crystallisation is much more rapid and perfect if a little acetic or 

 sulphuric acid is added (Hopkins). Serum albumin (from horse and 

 rabbit) has also been similarly crystallised (Giirber). 



Action on Polarised Light. — All the proteids are levo-rotatory, 

 but the amount of rotation they produce varies with the kind of 

 proteid. See Appendix. 



Colour Reactions. — The principal colour reactions : (1) the 

 xanthoproteic ; (2) Millon's ; (3) the "s^iolet colour with copper 

 sulphate and caustic potash, have been already given in the heading 

 to this lesson. The first two colour reactions depend on the presence 

 of an aromatic radicle in the proteid molecule. Peptones behave 

 differently from the native proteids in this last test. They give a 

 rose-red colour instead of a violet, if only a trace of copper sulphate 

 is used. The albumoses act in this respect like the peptones. This 

 rose-red colour is also given by the substance called biuret ; ^ hence 

 the test is called the biuret reaction. 



Precipitants of Proteids. — Proteids are precipitated by a large 

 number of reagents ; the peptones and albumoses are exceptions in 

 many cases, and will be considered separately afterwards (see 

 Lesson VI.). 



Solutions of the proteids are precipitated by — 



1. Strong acids, like nitric acid. 



2. Picric acid. 



3. Acetic acid and potassium ferrocyanide. 



4. Acetic acid and excess of neutral salts like sodium sulphate. 



' Biuret is formed by heating solid urea ; aromonia passes off and leaves biuret, 

 thus : — 



2C0N,H, - NH^ = C,0,N3H5 



[urea] [ammonia] [biuret]. 



