When the precipitated proteids are to be used for the estimation of the nitrogen therein contained, it has been proposed to substitute the corresponding zinc salt for the ammonium sulfate.[372] This reagent has given satisfactory results in this laboratory and while a larger experience is desirable before commending it as an acceptable substitute in all cases, yet its obvious advantage, in being free of nitrogen for the use mentioned, entitles it to careful consideration.
The manipulation, with the exception of the precipitation with ammonium sulfate, is the same as that described in the preceding paragraph. The globulins are completely precipitated when the dialysis is complete and may be separated from the soluble albumins and proteoses by filtration.
399. Separation of the Bodies Soluble in Water.—Albumins.—By the methods of treatment just described, the proteid matters soluble in ten per cent sodium chlorid solution are separated into two classes, viz., globulins insoluble in pure water and albumins and proteoses soluble in pure water. The aqueous solution will also contain any amids or nitrogenous bases soluble in the dilute saline solution and in water. Osborne and Voorhees have found that the best way of separating the albumins in the pure aqueous solution is by the application of heat.[373] By means of a fractional coagulation the albumins are divided into classes, viz., those separating at from 60° to 65° and those remaining in solution at that temperature but separating up to 85°. The respective quantities of these albumins are determined by collecting them in a filter and estimating the nitrogen therein by moist combustion in the usual way. Even a larger number of albumins may be secured, as in the maize kernel, by such a fractional precipitation by means of heat. Chittenden and Osborne find in this instance that the precipitation begins at about 40°.[374]
Proteose.—After the separation of the albumins by heat the filtrate may still contain proteid matter. This matter belongs to the proteose class. It may be partially secured by concentrating the filtrate, after the removal of the albumins, to a small bulk when a part of the proteose body will separate. It may be thrown out entirely by treating the filtrate above mentioned with fine-ground salt until it is saturated or by adding salt until the solution contains about twenty per cent thereof and precipitating the proteose by acetic acid.[375]
400. Separation of the Globulins.—The globulins which are extracted with ten per cent solution of sodium chlorid and precipitated on dialysis may be separated by fractional solution into several bodies of nearly related properties. This solution is conveniently accomplished by saline solvents of increasing strength. In the case of the maize globulins, Chittenden and Osborne employ dilute solutions of common salt for effecting the separation, beginning with a quarter of a per cent and ending with a two per cent mixture.[376]
401. Proteids Soluble in Dilute Alcohol.—Some of the proteid bodies which are soluble in dilute salt solution and in water are also soluble in alcohol. Since these bodies are more easily identified by the processes already described, attention will be given in this paragraph solely to those proteid bodies which are insoluble in water or dilute salt solution and are soluble in dilute alcohol.
For the extraction of these bodies, the residue, left after extraction with a ten per cent solution of sodium chlorid or with water, is mixed with enough strong alcohol to secure by the admixture with the water present in the sample an alcohol of about seventy-five per cent strength. The mixture is well shaken and digested for some time, at a temperature of about 46°, and thrown on a filter which is kept at about the same temperature. The residue is again mixed with alcohol of the same strength (seventy-five per cent) using about four liters for two and a half kilos of the original material. During the second digestion the temperature is kept at about 60°. The latter operation is repeated three times and in each case the filtrate obtained is evaporated separately.[377] This process is especially applicable to the meal from maize kernels, which contains a high relative percentage of an alcohol soluble proteid, zein.
The chief part of the zein is found in the first two extracts, obtained as described above. On evaporation, the zein separates as a tough, leathery, yellow-colored mass on the walls of the containing vessel. It is cut into small pieces and digested for several days in cold, pure alcohol. This is followed by digestion with a mixture of ether and pure alcohol, and finally with pure ether. By this treatment a part of the zein becomes insoluble in seventy-five per cent alcohol. The part soluble in dilute alcohol is precipitated by pouring it into water.
Another method of preparing zein is to extract the meal with seventy-five per cent alcohol after it has been treated with a ten per cent salt solution.
In this case the extraction is continued with seventy-five per cent alcohol in successive portions until no more proteid matter passes into solution. The several extracts are united and the alcohol removed by distillation, by which process the zein is separated. It is washed with distilled water, until the sodium chlorid is removed, dissolved in warm alcohol of about eighty per cent strength and any insoluble matter removed by filtration. On evaporating the filtrate nearly to dryness, the zein is separated and pressed as free of water as possible, yielding a yellow, elastic substance resembling molasses candy. This preparation is purified by digestion with pure alcohol and ether in the manner described. The two zeins which are secured by the treatment, one soluble and the other insoluble in alcohol, are practically identical in composition.[378]