The proteins are precipitated by certain complex acids called the alkaloidal reagents because they were first used for the pre cipitation of alkaloids (q.v.). The more important of these acids are tannic, picric, phosphotungstic, phosphomolybdic, metaphos phoric, sulphosalicylic, tri-iodo-hydriodic and ferrocyanic. These reagents only precipitate in acid solutions, the precipitates being soluble in alkalis. The peptones and peptides are less readily precipitated than the more complex proteins. The proteins are also precipitated in neutral or slightly alkaline solution by the salts of certain metals, especially those of copper, lead, mercury, iron and gold.
Of considerable importance in connection with the general physical properties of the proteins is the fact that the solutions are usually in the colloidal state (see COLLOIDS). The feature of this condition is that the material is suspended in the solvent (water, salt solution, etc.) in particles of a certain size. If they are larger than o•000i mm. (o• rµ) they can be' retained by filter paper or porcelain and are visible with an ordinary high-power microscope, and constitute a "coarse suspension" or precipitate; if they are smaller than IX mm. (I,u,u) they form a "molecular solution," and the substance will diffuse through parchment or collodion membranes. Between these limits we have the colloids, in which the particles are too large to pass through parchment membranes and too small to be removed by filter paper, or be seen with an ordinary microscope, but are large enough to interfere with the passage of light rays, being thus rendered "visible" by the ultramicroscope. The proteins, and particularly the complex natural proteins, exist in the colloidal state owing to the great size of the molecules. The length of the molecule of egg-albumin has been found to be 4.17/2p., that of the molecule of serum al bumin 4.41,u,u, whilst the molecules of serum globulin or of gelatin may be as large as 50—roo,up.
The particles in suspension may in some cases exist in the form of single molecules, but a slight change in the conditions may cause these to coalesce to form larger clumps, even to the extent of providing visible aggregates, or a precipitate. This can some times be effected by altering the acidity or alkalinity of the fluid, sometimes by the addition of a small amount of a particular neutral salt. When the aggregates formed cannot be dispersed by reverting to the original conditions, the phenomenon is known as coagulation. The best known example is that of the coagula tion of the albumins and globulins of the egg by boiling, an altera tion ("denaturation") taking place in the native proteins. The
altered protein is precipitated at a certain reaction, which is generally on the acid side of strict neutrality. At the coagulating temperature the particles of protein in the precipitate adhere together to form an irreversible coagulum. To produce this coagulation, a given temperature and reaction and the presence of certain inorganic substances are necessary. The first two conditions vary somewhat with different proteins and have been used for the characterization of some of the albumins and globulins.
The use of strong solutions of neutral salts as a method for the precipitation of certain proteins is of considerable importance. The chief salts employed are magnesium sulphate, sodium sul phate and particularly ammonium sulphate. When a solution is fully saturated with the last substance all proteins with the exception of the peptones and peptides are completely precipitated. The addition of one part of a saturated aqueous solution of am monium sulphate to one part of a fluid is called "half-saturation" and is used for the separation of globulins from albumins. The methods for the complete removal of proteins from a solution depend on the nature of the proteins present. In the case of the albumins and globulins, heat coagulation at the correct reaction may suffice. The most important precipitants are tungstic acid, metaphosphoric acid and colloidal iron, though tannic and mercuric and lead salts are sometimes employed.
Uses of Proteins.—The most important use of protein is that of a food-stuff for man and the domestic animals, since protein is the only form in which they can obtain a supply of the amino acids that are essential for the building-up and maintenance of the protoplasmic tissues of the body. Some of the amino-acids are used for the formation in the tissues of certain derivatives (gener ally known as "internal secretions") without which it is impossible for the body as a whole to function properly. It will be seen therefore that the nature and balance of the various amino-acids in a particular protein will have a marked effect on what is known as the "biological value" of the protein as a food. Thus the chief protein of Indian corn is zein, which does not contain any tryptophane—one of the essential amino-acids—and con sequently persons whose sole protein is that of Indian corn are apt to suffer from the disease known as Pellagra, which may be due to the deficiency of tryptophane.