CLASSIFICATION OF PROTEMS. The following classification of proteids, which is an expansion of that given above, is largely that proposed by llalliburton, and is believed to be applicable to both animal and vegetable proteids. It is based chiefly upon differences in solubility of the vari ous bodies: The ln•oteids posses$ a number of general char acteristics. All are insoluble in alcohol and ether. Water will dissolve some, but not all.
Nally of those insoluble in water arc• soluble in weak saline solutions, and some are soluble and others insoluble in concentrated saline solutions.
With the aid of heat all proteids are soluble in concentrated mineral acids and alkalis, but are transformed as well as dissolved. Proteids are soluble by the aid of ferments, as those in gas tric and pancreatic juices, but in this case they also undergo a change, adding water and breaking down into protcids of snmllcr molecular weight known as peptones, intermediate bodies formed in the process being called 'proteoses' or `allm moses.' proteids soluble in water or saline solutions, especially the albumins and globulins.
are rendered insoluble or are coagulated when heated. A familiar example of this is the coagu lation of egg white in cooking. The temperature required for coagulation differs with different proteids. With the exception of peptones, pro teids are colloids, and pass with difficulty, if at all, through animal mend:rallies. Some animal proteids like luemoglobins are crystallizable. A considerable number of crystallizable proteids have been prepared from seeds, nuts, etc., by Osborne and his associates and others. As re gards the action of proteids on polarized light, all the proteids are hevorotatory, the amount of rotation varying with the different kinds.
There are several color reactions characteristic of proteids. Some of the principal ones follow: With strong nitric acid, the proteids when heated to boiling give yellow flakes or a yellow solution, this being known as the xanthoproteic reaction. • When a proteid in the solid state or in solution is boiled with a solution of mercuric nitrate in nitric acid containing sonic nitrous acid (Mil lon's reagent) a red coloration is produced.
This and the former reaction depend on the pres ence of an aromatic radicle in the proteid mole cule. On treating a proteid substance with a little sugar and some strong sulphuric acid a red coloration is produced. On adding to a solution of a proteid body some caustic potash and then a few drops of a very dilute solution of copper sulphate a violet blue color is obtained (biuret reaction). Peptones and albumoses behave some what differently, giving a rose-red color instead of violet if only a trace of copper sulphate is used. After being boiled with alcohol and washed with ether, and dissolved in strong hydro chloric acid, proteids give a blue coloration.
Proteids are precipitated by a great many re agents, the peptones and albumoses being ex cepted in a number of eases. Thus solutions of proteids are precipitated ( 1 ) by strong acids, as nitric; (2) by picric acid: (3) by acetic acid and potassium ferrocyanide; (4) by acetic acid and excess of neutral salts like sodium phos phate; (5) by salts of heavy metals. as copper sulphate; (6) by tannin; (7) by alcohol; (S) and by saturation with some neutral salts, as ammonium sulphate. When proteids unite with mineral substances the resulting bodies are called albuminates; thus the precipitate formed when copper sulphate is added to a solution of albu min is copper albuminate.
Precipitation should be distinguished from coagulation. The latter term applies when an in soluble or coagulated proteid as formed from a soluble proteid, which may be brought about (1) by heat; (2) by ferments, as for instance when milk is coagulated by rennet ; (3) and when an insoluble proteid is produced by certain reagents, as nitric acid. There' are precipitants of pro teids which give a precipitate readily soluble in suitable reagents, a, saline solution. in such cases the dissolved proteid continues to show its typical reactions.