Ionic strength modulated interactions of sorbitol with lysozyme and amino acids: Quantitative understanding in protein stabilizing effects

Abstract

The mechanistic osmolyte action on protein stabilization has been a subject of extensive investigation. However, experimental evidences for acceptable mechanism need to be strengthened. We have studied interactions of five amino acids and hen egg-white lysozyme with mixture of (sorbitol + NaCl) in order to unravel ionic strength effect on osmolyte induced protein stabilization. Ionic strength has played a significant role in modification of molecular interactions important in several biologically important processes. A combination of densimetry, spectroscopy, and calorimetry has been employed to establish the mechanism of action. Thermodynamic signatures obtained from the density, velocity of sound, and enthalpy of interaction measurements enabled unraveling predominant factors controlling interactions of amino acids with sorbitol + NaCl in aqueous solutions. The results also indicated reduction in the deleterious effect of salt on the stability of a protein by sorbitol. Association between ions of sodium chloride and sorbitol leading to altered water-mediated effects has been quantitatively assessed. This is further supported by lack of direct interactions between the hen egg-white lysozyme and mixture of (sorbitol + NaCl) evidenced by isothermal titration calorimetry. A critical examination of results permitted an in-depth analysis of the molecular mechanism of protein stabilization by the osmolyte sorbitol affected by increased ionic strength.