Thermodynamics of the interaction of some chloro- and fluoro-substituted alcohols with bovine α-lactalbumin

The temperature dependence of the excess heat capacity and evaluation of thermodynamic parameters of bovine α-lactalbumin has been studied by high-sensitivity micro differential scanning calorimetry in the presence of 4-chlorobutan-1-ol, n-butanol, 3-chloropropan-1-ol, 3-chloropropan-1,2-diol, propan-1,2-diol, n-propanol, 2,2,2-trifluoroethanol, and ethanol at neutral pH. The chlorosubstituted alcohols are observed to be more effective destabilisers of α-lactalbumin compared with their normal alcohols in the order: 4-chlorobutan-1-ol > n-butanol > 3-chloropropan-1-ol > n-propanol > 2,2,2-trifluoroethanol > 3-chloropropan-1,2-diol > ethanol > propan-1,2-diol. A reversible two-state approximation for the unfolding of the protein has been shown in the absence and presence of these alcohols. A second, smaller, endothermic transition, 23°C beyond the main endotherm was observed only in the presence of 0.125 to 0.250 mol dm-3 3-chloropropan-1-ol. Concentrations higher than 25 mmol dm-3 4-chlorobutan-1-ol resulted in total loss of an observable endotherm. Thermal destabilisation of the protein in these solvent systems is explained on the basis of competing patterns of interactions of the cosolutes with the native versus unfolded states of the protein during the nativedenatured reaction. These results are supported by intrinsic fluorescence, energy transfer and UV difference measurements.