Thermodynamic Analysis of Lysozyme Denaturation by Surfactants

Abstract

Spectroscopic techniques (UV absorption, fluorescence and circular dichroism) are applied for probing the conformational stability of lysozyme as a model protein after the impact of surfactants. The investigations allow the equilibrium constant, K, and the free energy change, ΔG, of the transition from the folded (native) to the unfolded (denatured) state to be estimated. ΔG at 25 °C in the absence of additives allows quantifying the conformational stability of the protein. Though the results are based on the validity of several assumptions regarding folding/unfolding mechanism, evaluation procedure, and environmental conditions, the thermodynamics of surfactant-induced unfolding may be estimated. Compared to the unfolding induced by the chaotropic denaturant guanidinium chloride, cationic and zwitterionic surfactants are found to yield lower ΔG values. In the case of lysozyme, anionic and nonionic surfactants did not result in transition curves. The interpretation of the transition curves indicated the existence of a two-state behavior. Quantities which do not significantly depend on the unfolding mechanism, such as the midpoints of denaturant concentrations and thermal unfolding curves, c _1/2 and T _m, may also be applied for comparing conformational stabilities of proteins, even in the case of irreversible transitions. The evaluation of the thermal denaturation allows the derivation of enthalpy and entropy changes, ΔH and ΔS.