Effect of Aqueous Alcohol Solutions on the Thermal Transition of Lysozyme: A Calorimetric Study

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Abstract

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 x2 < 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 x2 = typical for each alcohol ( ≅ 0.06 for ethanol and ≅ 0.02 for tert-butyl alcohol) and then decrease with increasing x2. In addition, two enthalpy−entropy compensation patterns each having its own compensation temperature (Tc) clearly appear from the data: a compensation data line obtained with rising x2 in the 0− range (with Tc = 281 ± 6 K) followed by a compensation line with Tc = 403 ± 14 K after x2 passes the value. The value of 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.

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