164 



PROTEIN. 



when dried, is hard, semitransparent, and nearly 

 colourless. This substance is sometimes called 

 chloroproteic acid, since it is found to combine 

 without decomposition with several metallic 

 oxides. When treated with ammonia,however, 

 it is decomposed, nitrogen gas is given off, and 

 tritoxide of protein is formed, together with 

 hydrochloric acid, which combines with the 

 excess of ammonia. This is the most conve- 

 nient way of preparing the tritoxide, as it is 

 easily separated from the muriate of ammonia 

 by washing with alcohol, in which it is in- 

 soluble. 



Protein and nitric acid. By the action of 

 nitric acid on protein compounds, oxalic acid, 

 ammonia, nitrogen, nitric oxide, together with 

 a new compound called Xanthoproteic acid, 

 are obtained ; which latter, being insoluble, is 

 readily purified by washing with water. Xan- 

 thoproteic acid is of a bright yellow colour, 

 from which circumstance it derives its name : 

 it reddens litmus, is uncrystallizable, tasteless, 

 and, when strongly heated, does not melt, but 

 is decomposed, giving off the smell of burnt 

 feathers. It is soluble in strong acids, and 

 when water is added to the solution, a precipi- 

 tate, containing both the acids in a loose state 

 of combination, is thrown down. It forms 

 with metallic oxides true salts, most of which 

 are of a deep orange-colour, and insoluble in 

 water ; the alkaline xanthoproteates, however, 

 are soluble. It is bibasic, and consists of C 34 

 H 24 N 4 O ia + 2 HO. The troublesome and 

 indelible stain which nitric acid causes when 

 dropped on the skin is owing to the formation 

 of this substance. 



Protein and sulphuric acid. When protein 

 is treated with strong sulphuric acid it forms a 

 white insoluble compound, called by Mulder 

 sulphoproteic acid, containing C 40 , H 31 , N 5 , 

 O ia ,+ SO 3 . To purify it, it should be 

 washed with cold water as long as the wash- 

 ings give a precipitate with baryta water: when 

 dry, it is hard, tough, semitransparent, and 

 nearly colourless ; it forms with alkalies, so- 

 luble, and with the other oxides, insoluble, 

 sulphoproteates. 



There is another compound of protein and 

 sulphuric acid, called by Mulder sulphobiproteic 

 acid, which is formed when dilute sulphuric 

 acid is gradually added to a solution of protein 

 in acetic acid : it appears to consist of two 

 equivalents of protein, two of water, and one 

 of sulphuric acid, and is represented by the 

 formula C 80 H 62 N 10 O 24 + 2 HO f SO 3 . 

 If a protein compound be heated with sul- 

 phuric acid it becomes purple, but the colour 

 disappears on dilution with water. 



Protein and hydrochloric acid. Concen- 

 trated hydrochloric acid slowly dissolves pro- 

 tein even at common temperatures, and still 

 more readily when gently warmed : the solu- 

 tion is at first yellowish, but if the air is not 

 excluded, the colour soon changes to a deep 

 blue or purple. The appearance of this blue 

 colour is one of the most striking tests for 

 protein and its modifications, fibrin, albumen, 

 and casein, as it is produced in them all by 

 hydrochloric acid. When allowed to boil, if 



the acid is strong, a black substance similar 

 to ulmic acid is formed, together with muriate 

 of ammonia. 



Protein and potash. The action of potash 

 on protein possesses considerable interest. 

 When treated with a dilute solution of the 

 alkali, in the cold, it readily dissolves, and, ac- 

 cording to Mulder, a little ammonia is always 

 given off, however dilute the alkaline solution 

 may be. When boiled in a strong solution of 

 potash it is completely decomposed ; ammonia, 

 carbonic, and formic acids are formed, together 

 with three new compounds, which have been 

 called leucin, protid, erythroprotid. To obtain 

 these substances in a state of purity, the fol- 

 lowing process may be adopted. The protein 

 compound is boiled with solution of potash as 

 long as any ammonia is given ofl^ and then 

 neutralized with sulphuric acid, which disen- 

 gages the carbonic acid and combines with the 

 excess of potash : the solution is then eva- 

 porated to dryness on a water-bath, by which 

 means the greater part of the formic acid is 

 volatilized. The organic compounds are then 

 separated from the sulphate of potash by re- 

 peated boiling in alcohol, in which they are all 

 more or less soluble. On cooling, the alco- 

 holic solution deposits the erythroprotid, which 

 is of a reddish-brown colour, and nearly in- 

 soluble in cold alcohol. When left for a short 

 time to spontaneous evaporation, the leucin 

 crystallizes out, and the liquid then contains 

 only protid, with a trace of erythroprotid, and 

 a little formiate of potash. 



Erythroprotid, when pure, is of a fine red 

 colour ; it is soluble in boiling alcohol and in 

 water, and is precipitated from its solutions, of 

 a rose red colour, by many of the metallic 

 salts, as those of silver, mercury, and lead : 

 it is thrown down also by tannic acid. When 

 a current of sulphuretted hydrogen is passed 

 through its aqueous solution, it gradually be- 

 comes colourless ; but if the solution, thus 

 treated, be kept in vacuo a short time, the 

 colour returns. The formula of erythroprotid 

 isC 13 H 8 N0 6 . 



Protid (C 13 H 9 NO 4 ) may be separated 

 from the impure alcoholic solution by diluting 

 with water, and precipitating with subacetate 

 of lead, which throws down protid, but not 

 erythroprotid, which latter is also present in 

 small quantity. The precipitate is washed 

 with water, and decomposed by sulphuretted 

 hydrogen ; the solution is filtered and evapo- 

 rated, after which the protid is left in a state of 

 purity. It is of a pale yellow colour, amor- 

 phous, and, when dry, very brittle. It differs 

 from erythroprotid in not being precipitated 

 from its solutions by any of the metallic salts 

 except basic acetate of lead; while erythro- 

 protid is not affected by that reagent : conse- 

 quently if the two exist together in solution, 

 the erythroprotid may be thrown down by the 

 neutral acetate, and the protid by the basic 

 salt. 



Leucin, which gradually separates when the 

 alcoholic solution is concentrated, is a crystal- 

 line substance closely resembling chloresterine 

 in appearance : it consists of C ja H 12 NO 4 . 



