Surface Tension Kinetics of the Wild Type and Four Synthetic Stability Mutants of T4 Phage Lysozyme at the Air–Water Interface

Abstract

Surface tension kinetics exhibited by the wild type and selected stability mutants of T4 lysozyme at the air–water interface were monitored with DuNoüy tensiometry. Mutant lysozymes were produced by substitution of the isoleucine at position 3 with cysteine, leucine, glycine, and tryptophan. Each substitution resulted in an altered structural stability quantified by a change in the free energy of unfolding. Surface pressure kinetics were compared to the kinetic model evolving from a simple model for protein adsorption. This model allowed for parallel, irreversible adsorption into two states directly from solution, where state 2 molecules were more tightly bound to the surface and occupied greater interfacial area than state 1 molecules. Moreover, the model allowed state 2 molecules to increase spreading pressure more than state 1 molecules occupying the same interfacial area. The model indicated that less stable variants of T4 lysozyme have a greater tendency to adsorb in state 2, and state 2 molecules increase spreading pressure more than state 1 molecules occupying the same interfacial area. While agreement between the model and experimental data was very good at low concentration, these results suggest that a more comprehensive two-state model should account for the influence of surface coverage on the adsorption rate constants.