Interfacial Behavior of Wheat Puroindolines: Study of Adsorption at the Air – Water Interface from Surface Tension Measurement Using Wilhelmy Plate Method

Adsorption of puroindolines (PIN-a and PIN-b), lipid binding proteins isolated from wheat endosperm at the air–water interface, was studied from the measurement of surface pressure using the Wilhelmy plate method under different physicochemical conditions like pH, ionic strength, and concentration of protein in the bulk medium. The adsorption process was highly cooperative and the initial step of adsorption was diffusion controlled. At relatively low concentrations, an induction time (ti) was observed in the time evolution of surface pressure for adsorption of PIN-a and PIN-b at the air–water interface. The induction time disappeared upon increasing the concentration of protein in the bulk medium. From the variation in surface pressure with time, the diffusion coefficients for the transfer of puroindolines from the bulk aqueous phase to the air–water interface were calculated from a modified form of the Ward and Tordai equation. The values of diffusion coefficients for PIN-a and PIN-b were found to be of the order of 10−6 cm2/s. Beyond diffusion, the adsorption was found to be nonlinear in nature indicating the existence of energy barriers in the adsorption process. The experimental data were analyzed using a first-order rate equation and it was observed that the adsorption took place involving a two-step process with which two distinct first-order rate constants (k1 and k2) were associated. In most cases, the values of k1 were higher than the values of k2. The adsorption process is dependent on the initial bulk concentration of the protein, the pH, and the ionic strength of the bulk medium. The presence of inorganic salts in the bulk medium had significant effects on the adsorption of PIN-a and PIN-b at the air–water interface. The experimental results obtained for PIN-a and PIN-b have been critically compared with each other and with previously obtained data for other globular proteins.

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