title: 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.
authors.affiliation: Department of Chemistry, Indian Institute of Technology, New Delhi, India.
authors.forename: S
authors.initials: S
authors.lastname: Jain
authors.forename: J C
authors.initials: JC
authors.lastname: Ahluwalia
mesh_terms: D000818:Animals; D002152:Calorimetry, Differential Scanning; D002645:Chickens; D004531:Eggs / Q000032:analysis; D006863:Hydrogen-Ion Concentration; D009113:Muramidase / Q000737:chemistry*; D011489:Protein Denaturation; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D000644:Quaternary Ammonium Compounds; D009113:Muramidase