154 COMPOUNDS of THE PROTEINS 



as all our evidence tends to show they are in the proteins. Moreover 

 the results of the formol-titration show that there are not many free 

 carboxyl-groups in the protein molecule, and the same conclusion may 

 be reached from a consideration of the effects of rather concentrated 

 alkali in bringing about " racemization" or optical inactivity of the 

 majority of the amino-acid radicals in proteins. It is found that amino- 

 acid radicals of which the carboyxl-group remains uncombined, are not 

 "racemized" by alkali, while amino-acids of which the carboxyl-groups 

 are neutralized in peptide-linkages are rendered optically inactive by 

 strong alkalies. The great majority of the amino-acids which result 

 from the alkaline hydrolysis of proteins are optically inactive and so we 

 must assume that in the native protein molecule their carboxyl-groups 

 were not unattached. 



Now uncombined Casein is insoluble in water, but when combined 

 with acids or with bases it is soluble. When just sufficient alkali has 

 been employed to carry every particle of casein into solution at least 

 one molecule of the alkali must have combined with each molecule of 

 casein. To carry one gram of casein into solution 11.4X10" 5 equiva- 

 lents of base, or 1.14 c.c. of tenth normal alkali just suffice, indicating 

 a combining-weight for casein of about 8800. The ty rosin and 

 sulphur-contents of casein indicate that the molecular weight of casein 

 must be some multiple of 4400. 



In the presence of excess of alkali, however, the combining capacity 

 of casein for bases is very much greater. We cannot, of course, deter- 

 mine the maximal combining-capacity of casein for bases by titration, 

 because the removal of the uncombined excess of alkali by the acid 

 used in titration simply results in reducing the combining-capacity of 

 the casein for the alkali, just as the running in of acid into a solution 

 of sodium carbonate results in the formation of sodium bicarbonate. 

 Nor is it convenient to determine the maximal combining-capacity 

 of proteins for bases by means of indicators. The method employed 

 is to determine the quantity of uncombined alkali in the protein 

 solution electrometrically by means of the Gas-chain (see Chapter XII), 

 that is by the potential developed at the surface-layer of an electrode 

 of hydrogen dipped into the protein solution. The greater the con- 

 centration of free alkali, i. e., of hydroxyl ions in the solution the less, 

 in proportion, must be the concentration of free hydrogen ions, and 

 the less the concentration of free hydrogen ions in the solution the more 

 hydrogen ions will travel from the superficial layer of the electrode 

 into the superficial layer of solution which is in contact with it. These 

 hydrogen ions carry with them a positive charge, and hence the solution 

 becomes charged positively and the electrode carries a corresponding 

 negative charge. The potential thus created is a measure of the 

 alkalinity (or acidity) of the solution under investigation. For the 

 Hydrogen Electrode we use a piece of platinum foil or platinum gauze 

 coated with platinum-black and saturated with hydrogen gas. 



