THE CHEMICAL IDENTIFICATION OF THYROXIN

Abstract

Thyroxin is a white, highly crystalline substance, odorless, and tasteless.It may be separated from aqueous or alcoholic solutions in microscopic crystals which are not soluble in any organic solvent, except those which are strongly basic or acidic in nature.It is soluble in alcohol in the presence of mineral acid or an alkali metal hydroxide.It is stable toward heat, and its melting point is in the neighborhood of 250°C.Since it is odorless and colorless and is not easily affected by oxidation and reduction, its most important chemical and physical properties are concerned with the acidic and basic groups within the molecule.Thyroxin is a weak acid, but possesses basic properties in the presence of mineral acids.In 1915, it was suggested that the organic nucleus in thyroxin is indole (1).Its solubility in alkali metal hydroxides, but not in carbonates, indicated that it was of phenolic nature, and its salt-forming power with acids was attributed to an imino group.After it was known that thyroxin contained about 60 per cent of iodine, and before the empirical and structural formulas were determined, the chemical properties of the molecule were best expressed by di-iodo-di-hydroxy-indole.The first derivative of thyroxin, which helped to give an insight into its chemical structure, was the sulfate.Thyroxin which was precipitated from alkaline alcohol by acetic acid was found to contain 65 per cent of iodine.Thyroxin, precipitated by boiling an aqueous ammoniacal solution, also contained 65 per cent of iodine.Thyroxin, precipitated by adding sulfuric acid to an aqueous alkaline solution and boiling, was found to contain 60 per cent of iodine.The difference in iodine content was shown to be due to the formation of a salt with sulfuric acid, and by estimating the molecular weight of thyroxin from the molecular weight of