Differential scanning calorimetric studies on the effect of ammonium and tetraalkylammonium halides on the stability of lysozyme

Abstract

The thermal denaturation of lysozyme was studied at pH 2.50 and 6.00 in aqueous solutions of ammonium (NH₄Cl and NH₄Br) and tetraalkylammonium halides (Me₄NCl, Me₄NBr, Et₄NBr, Pr₄NBr, Bu₄NBr) using high-sensitivity differential scanning calorimetry. The transition temperature, heat capacity, enthalpy, entropy and free energy of denaturation have been determined by a least-squares fit of the excess heat capacity data to the two-state model. Ammonium and tetraalkylammonium halides (except Me₄NCl and NH₄Cl at high concentrations at pH 6.00) are found to destabilize lysozyme and the destabilization increases with increasing concentration and alkyl chain length. However, NH₄Cl and Me₄NCl act as stabilizers at high concentrations at pH 6.00. Results are discussed in terms of electrostatic and hydrophobic interactions. The stabilization of lysozyme by NH₄Cl and Me₄NCl can be attributed to the charge on the quaternary nitrogen atom while destabilization in tetraalkylammonium halides solution is due to the interaction between hydrophobic molecules of the medium and the hydrophobic parts of the protein.