Adsorption dynamics of alpha-lactalbumin and beta-lactoglobulin at air-water interfaces

Dynamics of adsorption of C-14 radiolabeled beta-lactoglobulin and alpha-la ctalbumin at the air-water interface was investigated through the measureme nt of surface pressure (pi pi) and surface concentration (Gamma) via a radi otracer technique. Adsorption was diffusion controlled at short times, the rates of increase of pi and Gamma being lower at longer times because of an energy barrier. At low concentrations, an apparent time lag was observed i n the evolution of pi for beta-lactoglobulin but not for alpha-lactalbumin which was shown to be due to the nonlinear nature of the pi-Gamma relations hip for the former. The area per molecule of an adsorbed beta-lactoglobulin during the dynamics of adsorption was smaller than that for spread monolay er since beta-lactoglobulin was not fully unfolded during adsorption. For a lpha-lactalbumin, however, no such difference in the molecular areas for ad sorbed and spread monolayer was observed indicating thereby that alpha-lact albumin unfolded much more rapidly than beta-lactoglobulin. Evolution of Ga mma for alpha-lactalbumin was found to occur in two steps possibly due to t he change in the orientation of the adsorbed protein from a side-on to an e nd-on orientation. A previously developed mechanistic model (G. Narsimhan a nd F. Uraizee, Biotechnology Prog. 8, 187 (1992)) was improved to account f or the presence of hydrophobic patches on the surface of the protein molecu le as well as an adsorbed protein layer at the air-water interface. The mod el predictions agreed quite well with the experimental evolution of Gamma f or beta-lactoglobulin and alpha-lactalbumin. The model calculations seem to indicate that alpha-lactalbumin changes its orientation at the air-water i nterface from side-on to other orientations at higher surface concentration s. (C) 1999 Academic Press.