Effect of pH on Thickness of Surface Films Made with Insulin and Other Large Molecules

Abstract

By using suitable adaptations of the methods developed by Langmuir, Schaefer and Blodgett 1 2 films of salmine, insulin, thymus histone, thymus nucleic acid, trypsin, and casein have been built up on chrome plated iron or on stainless steel plates. These plates were first covered with a suitable number of layers of barium stearate by the method of Blodgett. 2 They were then immersed in a 10^-3 M solution of uranyl acetate at pH 6.0 to condition the barium stearate surface, thus rendering it hydrophilic. Each conditioned plate with an adhering layer of water was then immersed in a 0.1% solution of the substance to be adsorbed and shaken in this solution for the period of time cited. The plate was then removed, washed, and dried in air. The insulin and other surface films described below were thus formed by adsorption of molecules or molecular aggregates directly from solution and not by transfer of films previously formed on the surface of a solution. The thickness of such films has been studied as a function of a number of factors of physiological interest, such as the nature of the adsorbing surface, the time of exposure of the plates to the solutions, and the concentration, salt content, and pH of the solutions to which the plates were exposed.

The purpose of the present paper is to point out, in a preliminary way, the great influence of pH in determining the thickness of certain of these films, using typical experiments with salmine, insulin, and casein as examples. The present experiments were made at room temperature, 25°-30° C, with the salt content of the solutions kept at the minimum level.