The state in which a protein substance exists depends upon the nature of its combination with acids or bases and is changed by change in the protein compound.
The nature of the compound of a protein that exists at any hydrogen ion concentration can be ascertained if the isoelectric point of the protein is known.
Accordingly information regarding the isoelectric points of vegetable proteins is of importance for operations in which it may be desirable to change the state of protein substances, as in the dehydration of vegetables.
The Protein in Potato Juice.—The hydrogen ion concentration of the filtered juice of the potato is in the neighborhood of 10–7N. Such juice contains the globulin tuberin to the extent of from 1 to 2 per cent.
The character of the compound of tuberin that exists in nature was suggested by its anodic migration in an electric field.
The addition of acid to potato juice dissociated this compound and liberated tuberin at its isoelectric point. The isoelectric point of tuberin coincided with a slightly lower hydrogen ion concentration than 10–4N. At that reaction it existed most nearly uncombined.
The flow of current during cataphoresis was greatest in the neighborhood of the isoelectric point. This evidence supplements that of the direction of the migration of tuberin, since it also suggests the existence of the greatest number of uncombined ions near this point.
At acidities greater than the isoelectric point tuberin combined with acid. The compound that was formed contained nearly three times as much acid as was needed to dissociate the tuberin compound that existed in nature. At such acidities tuberin migrated to the cathode.
Though never completely precipitated tuberin was least soluble in the juice of the potato in the neighborhood of its isoelectric point. Both the compounds of tuberin with acids and with bases were more soluble in the juice than was uncombined tuberin.
The nature of the slight precipitate that separated when potato juice was made slightly alkaline was not determined.
The Protein in Carrot Juice.—The isoelectric point of the protein in carrot juice coincided with that of tuberin. Remarkably similar also were the properties of carrot juice and the juice of the potato. Existing in nature at nearly the same reaction they combined with acids and bases to nearly the same extent and showed minima in solubility at the same hydrogen ion concentrations. The greatest difference in behavior concerned the alkaline precipitate which, in the carrot, was nearly as great as the acid precipitate.
The Protein in Tomato Juice.—The protein of the tomato existed in a precipitated form near its isoelectric point. Accordingly it was not present to any extent in filtered tomato juice. If, however, the considerable acidity at which the tomato exists was neutralized the protein dissolved and was filterable. It then migrated to the anode in an electric field. The addition of sufficient acid to make the hydrogen ion concentration slightly greater than 10–5N again precipitated the protein at its isoelectric point. At greater acidities migration was cathodic.