A comprehensive Analysis of Bovine αα-Lactalbumin Molten Globule in Presence of Surfactants: Biophysical Correlates

Abstract

The role of different types of interactions and their contribution in the stabilization of bovine α-lactalbumin (α-LA) molten globule in presence of cationic surfactant, hexadecyl trimethyl ammonium bromide (HTAB) and anionic surfactant, sodium dodecyl sulphate (SDS) have been examined using a combination of spectroscopic, light scattering and calorimetric techniques. At lower concentration of the surfactants, the thermodynamic parameters obtained from UV-Visible spectroscopy suggested an increased exposure of non-polar groups in HTAB while a possible restructuring of non-polar groups were indicated in SDS. The fluorescence and circular dichroism spectroscopy showed the formation of an intermediate state at various concentrations in presence of HTAB and SDS while the lifetime measurements supported the assumption of protein-surfactant complex stability in HTAB as compared to SDS. The hydrodynamic diameter and ζ-potential were analyzed by dynamic light scattering (DLS) which also implicated the combined influence of electrostatic and hydrophobic interactions in α-LA unfolding in HTAB and only hydrophobic interactions in SDS. The binding parameters for ANS obtained from isothermal titration calorimetric (ITC) measurements suggested a high stability of α-LA molten globule and the role of enthalpic and entropic contribution in the binding of ANS in HTAB. It also indicated the fragility of α-LA molten globule in SDS. The possible binding sites as well as the interactions of ANS with the molten globule were also investigated from the thermodynamic parameters obtained from ITC. We, thus propose that the molten globule state obtained in HTAB and SDS are very different from one another as well as the conventional molten globule state of α-LA obtained in presence of chemical denaturants.