Influence of KCl on the interfacial activity and conformation of hemoglobin studied by Langmuir–Blodgett technique

Abstract

We report here the influence of KCl on the interfacial surface activity and conformation of human adult hemoglobin (Hb) using Langmuir and Langmuir–Blodgett (LB) techniques. The studies were done in absence and in presence of KCl salt in the subphase. We have studied the surface pressure-area (π–A) isotherm and surface pressure-time (π–t) kinetics of Hb with the variation of KCl concentrations (C_KCl). The π–t study shows that the surface activity as well as the magnitude of diffusion and rearrangement of Hb at air/water interface is a function of C_KCl. Conformational study was done by CD spectroscopy and by the FTIR technique. Both the studies show an increasing trend of the α-helix form of Hb in the presence of KCl which may be responsible for the increased surface activity of Hb. The free energy calculations show that the compression of the Hb monolayer is involved in the small free energy change (∼5–25 kcal mol⁻¹) of Hb. The changes in area per molecule and free energy, as well as other results, indicate that the influence of KCl on the Hb monolayer is in line with modified DLVO theory of ion–protein interaction. FE-SEM study shows that the LB films in absence of KCl comprise higher aggregates, whereas in the presence of KCl (0.5 M) it comprises lower aggregates, indicating the structural change of Hb. KCl salt here enhances the α-helix from of Hb, promoting the folded conformation by perturbing the water structure. The overall results show that the intermolecular forces and the surface activity as well as the population of the α-helix of Hb can be tuned by KCl salt.