Thermodynamics of the interactions of some chloro-substituted alcohols with hen egg-white lysozyme
High-sensitivity differential scanning calorimetry (DSC) has been used to study the thermal denaturation of lysozyme in aqueous mixtures of 3-chloropropane-1-ol, 3-chloropropane-1,2-diol, n-propanol, 4-chlorobutan-1-ol and n-butanol at pH 2.4, 4.5 and 6.0. The chloro-substituted alcohols are more effective destabilizers at all pH values compared with their normal alcohols in the order: 4-chlorobutan-1-ol > n-butanol > 3-chloropropan-1-ol > n-propanol > 3-chloropropane-1,2-diol. Comparison of calorimetric and van't Hoff enthalpies in the presence of these alcohols at all pH values shows the validity of the two-state approximation for the unfolding of lysozyme in these co-solute systems. The reversibility of the thermal denaturation has also been demonstrated by the reversibility of the DSC curves in the presence of above-mentioned alcohols and diol. The quantitative thermodynamic parameters accompanying the thermal denaturation have been evaluated. The thermal denaturation in the presence of 4-chlorobutan-1-ol shows unusual behaviour particularly at pH 4.5: the data shows partial irreversibility in the calorimetric transitions and the cooperativity ratio suggests intermolecular cooperation which increases as the concentration of the alcohol decreases in the mixture. The observed thermal destabilization of the protein in the presence of these systems is explained on the basis of patterns of interactions of the co-solutes with the native and unfolded state of the protein involving a combination of exclusion and binding that change during the course of the native ⇌ denatured reaction.