Mulder has recently obtained a third oxide of protein, represented by the formula I-13, N, or protein plus eight equiva lents of oxygen. As it has not, however, been found to exist naturally in the animal body, it is inferior in point of interest to the other two. Like the tritoxide it is soluble, and is obtained by boiling glutin or yeast for a length of time in water.
By the action of chemical reagents on pro tein a multitude of new compounds are formed, most of which have been only imperfectly examined, and indeed possess but little real interest. I will describe a few of the most important.
Protein and chlorine. — When a current of chlorine is passed through a solution of albu men, or any of the other modifications of pro tein, a substance is produced, containing C,„ H3, N5 0, 5 Cl„ which Mulder considers to be a chlorite of protein, (C4a H3, N, 0„ CI 03). It appears to be formed at the ex pense of three equivalents of water ; three equivalents of hydrochloric acid and one of chlorous acid being simultaneously produced, the latter uniting with the protein. It sepa rates as a snow-white flaky precipitate, and 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 C34 H„ N, 0 1.2 -I- 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 culphoproteic acid, containing C40, 113 „ N,, 0,2, + S03. 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,. H.2 Ni. 0 2 HO -I- S03. 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 hydrochlonc 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 ulrnic 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 off, 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.