46 A TEXTBOOK OF PHYSIOLOGY 



2. The solution given by them, however, is colloidal, and therefore 

 will not diffuse through animal membranes or parchment paper. In 

 this they are unlike crystalloids, such as inorganic salts, which readily 

 diffuse through such membranes. 



3. Most of the so-called native proteins (albumins and globulins) 

 coagulate when their solutions are heated, different proteins coagu- 

 lating at different temperatures, varying usually from 56 C. to 78 C. 

 A faint degree of acidity and the presence of much neutral salt aids 

 this process. 



Heat coagulation is probably brought about first by the interaction of protein and 

 water : a hydrolytic product, metaprotein, is produced (denaturation); secondly by 

 agglutination and separation. A certain increase of acidity or alkalinity favours 

 denaturation, but opposes agglutination. The reverse is true for neutral salts. 



Protein does not form a true solution; its particles are suspended in the 

 solvent, and carry an electrical charge. Any factor which reduces this charge tends 

 to bring about agglutination, either precipitation, which is reversible on dialyzing 

 away the reagent, or coagulation. The particles are no longer electrically repelled, 

 and run together under the influence of gravity. In the case of proteins, which 

 have amphoteric properties, the charge of the particle is positive when the fluid is 

 acid, and negative when the fluid is alkaline. When a neutral salt is added, the 

 agglutinating ion is that which carries a charge opposite in sign to that of the par- 

 ticle. The agglutinating power increases with the valency. Thus an acid solution of 

 protein is precipitated by negative ions, for the particles of protein have positive 

 charges; the potassium salt of citric aeid (trivalent) is much more effective than 

 the potassium salt of sulphuric acid (divalent), and this more than the potassium salt 

 of hydrochloric acid (univalent). An alkaline solution of protein is agglutinated by 

 positive io.is; cerium chloride (trivalent) is more efficient than barium chloride 

 (divalent), and this more than sodium chloride (monovalent). 



4. Most proteins are uncrystallizable. Some proteins, however, 

 can be fairly easily crystallized, especially certain vegetable proteins, 

 such as those of hemp-seed (edestin) or of the Brazil nut (excelsin). 



5. Almost all proteins turn polarized light to the left. Haemo- 

 globin is an exception. 



6. Certain well-marked colour reactions are given by the majority 

 of proteins, all of which give valuable information as to the constitu- 

 tion of the protein molecule. One such reaction namely, the 

 biuret is characteristic ; a body that does not give this is not classed 

 as a protein. The most important of these reactions are 



(a) The biuret. A violet or rose-pink colour with copper sulphate 

 and caustic potash denotes the presence of two or more CO NH 

 linkages. All proteins give this reaction. 



(6) The xanthoproteic. With nitric acid a white curd turning 

 yellow on heating, and orange on addition of ammonia, denotes the 

 presence of the benzene ring in the protein molecule. Proteins 

 yielding the ringed amino-acids give this reaction. 



(c) Millon's with a mixture of mercuric and mercurous nitrates, 

 a white precipitate turning brick red on heating signifies the presence 

 of a benzene ring with an hydroxyl group attached in other words, 

 the phenolic group. Proteins containing tyrosin give this test. 



(d) Hopkins 's modification of Adamkiewicz's reaction. 



The reaction signifies the presence of tryptophan in the protein 

 molecule. Acetic acid, containing glyoxylic acid as an impurity, is 



