Binding of (1:1)-Metal-Complex Dyes to Proteins, Studied by the Monolayer Method

Abstract

The reactions of monolayers of model proteins (edestin and a wool derivative) on acid solutions of a number of 1:1-chromium-ligand dyes, of both monosulphonated and disulphonated types, and certain nonmetallized sulphonated dyes of similar structure, have been studied. Measurements of film area, pressure, compressibility, and viscosity have been made. All sulphonate groups appear to enter the water. Thus monosulphonated dye molecules penetrate between the protein chains, reducing the film viscosity, whereas disulphonates orient horizontally beneath the protein. A free —OH group present in the dye molecule causes it to bind more firmly to the protein, with increase in film viscosity and in surface compressional modulus (scm). All the dyes appear to act just as do the nonmetallized counterparts: film areas are increased; monosulphonates form mixed films and reduce both viscosity and scm of the film; disulphonates produce no change in viscosity or slope of the π/A curve but slightly increase the scm; the pressure-area changes are reversible. The viscosity of the protein film over a simple chromium salt solution alone, however, is very greatly increased. Evidence is obtained that reactions of the present protein films with dyes can reasonably be used to interpret dye-wool bonding reactions. There is no evidence from the present tests that the chromium atom in the dye molecule forms any bond with the protein. The good wash fastness which these dyes in wool have in technical practice must, therefore, be due solely to their high molecular weight and to mechanical effects of the structure of the fiber.