Effect of aqueous alcohol solutions on the thermal transition of lysozyme: A calorimetric study

Thermal denaturation of lysozyme has been studied at pH = 3 in water/ethanol and water/tert-butyl alcohol mixtures in the water rich region of composition (mole fraction of cosolvent x(2) < 0.12) by high-sensitivity differential scanning calorimetry. The results show that on increasing alcohol concentration, the enthalpy and entropy of denaturation of lysozyme first reach a maximum at an intermediate composition x(2)=x(2)* typical for each alcohol (x(2)*congruent to 0.06 for ethanol and x(2)*congruent to 0.02 for tert-butyl alcohol) and then decrease with increasing x(2). In addition, two enthalpy-entropy compensation patterns each having its own compensation temperature (T-c) clearly appear from the data: a compensation data line obtained with rising x(2) in the 0-x(2)* range (with T-c=281 +/- 6 K) followed by a compensation line with T-c = 403 +/- 14 K after x(2) passes the x(2)* value. The value of x(2)* is close to that at which a change in the nature of solvent component interaction occurs as inferred from compressibility and IR absorption measurements. The data have been interpreted on the basis of the assumption that the addition of short chain alcohols affects the thermal transition of proteins, modifying the extent of enthalpy and entropy contribution associated with structural reorganization of water in the unfolding process.