Complexation of Lysozyme with Sodium Poly(styrenesulfonate) via the Two-State and Non-Two-State Unfoldings of Lysozyme

To provide an in-depth understanding of the complexation mechanism of protein and polyelectrolyte, a heating–cooling–reheating protocol was employed to study the unfolding and refolding behaviors of a model protein, lysozyme, in the presence of a negatively charged polyelectrolyte, sodium poly­(styrenesulfonate) (PSS). It was found that, with elevated PSS concentration, a new state (state I) was first formed via a “two-state” conversion process and this state could further convert to a completely unfolded state (state II) via a “non-two-state” conversion. This non-two-state conversion process occurs without the coexistence of states I and II but involves the formation of various intermediate unfolded protein structures. Different from the pure lysozyme that exhibited refolding upon cooling from its heat-denatured state, lysozyme in state I could undergo unfolding upon heating but no refolding upon cooling, while lysozyme in state II did not undergo unfolding or refolding upon thermal treatments. In addition, the effects of ionic strength and molecular weight of polyelectrolyte on the unfolding and refolding behaviors of lysozyme were also investigated. The present work provides a better understanding of the principles governing protein–polyelectrolyte interactions and may have implications for the fabrication of biocolloids and biofilms

Stepwise Ordering of Imidazolium-Based Cationic Surfactants during Cooling-Induced Crystallization

Surfactants bearing imidazolium cations represent a new class of building blocks in molecular self-assembly. These imidazolium-based cationic surfactants can exhibit various morphologies during phase transformations. In this work, we studied the self-assembly and phase behavior of 1-hexadecyl-3-methylimidazolium chloride (C₁₆mimCl) aqueous dispersions (0.5–10 wt %) by using isothermal titration calorimetry, differential scanning calorimetry, synchrotron small- and wide-angle X-ray scattering, freeze–fracture electron microscopy, optical microscopy, electrical conductance, and Fourier transform infrared spectroscopy. It was found that C₁₆mimCl in aqueous solutions can form two different crystalline phases. At higher C₁₆mimCl concentrations (>6 wt %), the initial spherical micelles convert directly to the stable crystalline phase upon cooling. At lower concentrations (0.5 or 1 wt %), the micelles first convert to a metastable crystalline phase upon cooling and then transform to the stable crystalline phase upon further incubation at low temperature. The electrical conductance measurement reveals that the two crystalline phases have similar surface charge densities and surface curvatures. Besides, the microscopic and spectroscopic investigations of the two crystalline phases suggest that the metastable crystalline phase has preassembled morphology and a preordered submolecular packing state that contribute to the final stable crystalline structure. The formation of a preordered structure prior to the final crystalline state deepens our understanding of the crystallization mechanisms of common surfactants and amphiphilic ionic liquids and should thus be widely recognized and explored