Reduced lysozyme in solution and its interaction with non-ionic surfactants

Reduced lysozyme at pH 2.5 bound poly(oxyethylene) alkylethers in two steps and the maximum bound amount Qmax of the surfactant reached as large as 0.5-0.7 mole per mole amino acid residue in the cooperative binding step. Binding isotherms were well superimposed when surfactant concentrations were normalized by respective values of the critical micelle concentration, cmc. In terms of the onset concentrations of the cooperative binding C*, hydrophobicity of reduced lysozyme was quantitatively defined as RT In (cmc/C*) which amounted to 670 J per mole surfactant and was unique to the protein irrespective of the kind of surfactant. Qmax could be used as another measure of the hydrophobicity of the protein. The binding isotherms were evaluated by two methods: equilibrium dialysis and surface tension. Their results were consistent with each other and rather complementary. Reduced lysozymes were molecularly dispersed at pH below 2.5 in 0.01 M NaCl but aggregation took place as pH increased. The aggregates could not be dissociated on dilution nor by the addition of nonionic surfactants but by lowering pH. The irreversible nature of the aggregation was reasonably interpreted with a model based on the 'entangled' arrangement of the beta-sheets, which could account for the irreversible aggregation of unfolded proteins in general.