Clxxii. Combination of Proteins with Phthalein Dyes

MANY properties of proteins have been ascribed to adsorption phenomena and surface effects-grouped under the name of colloidal properties. Loeb [1922] and others have shown that many properties of proteins may be explained by the orthodox laws of chemical combination. In this paper an attempt is made to explain the effect of proteins on phthalein dyes as a chemical phenomenon. The interesting observation was made by Rosenthal [1926] that when a small amount of protein has been added to a dilute rose bengal solution the deep pink colour is not discharged as usual when the solution is acidified. This he ascribes'to the fact that the dye behaves as though it were not in solution. It was. thought that this observation deserved further attention and it was found that if the colour of a dilute rose bengal solution were discharged by acidification with hydrochloric acid and then a small amount of protein were added and the solution warmed, a rose-red colour soon developed. The combination of protein and dye resulted in colour change, so that the simple explanation of the dye behaving as though it were not in solution is hardly accurate since'the same colour is obtained when the dye is undoubtedly in solution and already decolorised. The reaction between the protein and acidified dye proceeds very slowly in the cold, probably because an intramolecular change takes place as will be shown later. The question arose as to whether rose bengal (tetraiodotetrachlorofluorescein) was unique in its behaviour or whether all fluorescein and phenolphthalein derivatives behaved similarly. The first dyes examined fell into two classes: