Cation-induced monolayer collapse at lower surface pressure follows specific headgroup percolation

Protein (Lysozyme) Concentration-Dependent Structure, Morphology, and Hysteresis Behavior of a Three-Component (Lysozyme-DMPA-Cholesterol) Protein-Lipid Langmuir Monolayer

Protein (lysozyme)-lipid (DMPA and cholesterol) three-component mixed films (LDC) with varied lysozyme concentration (i.e., LDC_Lx) are investigated at the air-water interface. Elastic modulus-surface pressure (Cs-1-Π) curves derived from Π-A isotherms show that mechanical behavior is strongly dependent on the monolayer composition, and for the same reason, the hysteresis behavior modifies. It is evidenced that the LDC_L0.3 monolayer (lysozyme: 0.3 mg/mL) has significant hysteresis, which is reversible in nature, while the other mixed monolayers do not show such hysteresis behavior. Morphology at the air-water interface via Brewster angle microscopy (BAM) and at the air-solid interface via atomic force microscopy (AFM) shows that the presence of protein in the LDC_Lx monolayer modifies the lateral distribution of molecules, thereby forming a stripe-like pattern at the air-water interface (in optical length scale) with barrier compression or root-like structure on the solid surface at higher Π (in micron length scale), which is not observed in the case of lipid films. Moreover, lysozyme-added LDC_Lx films show an increase in thickness with compression, which is not observed for lipid films, as evidenced from the electron density profiles (EDPs). The morphology modification and thickness variation of LDC_Lx films with compression are most probably due to the reorientation of lysozyme molecules. This structural modification in LDC_Lx films with Π, however, seems to be reversible under expansion, as can be evidenced from the similar in situ morphology observation and similar thickness of the films deposited during both first and second compression. A variation in the strength of interaction forces among film-forming molecules depending on the monolayer composition basically affects the lateral distribution and organizational orientation with surface pressure, thus ultimately influencing macroscopically the monolayer properties such as elastic, hysteresis, morphological, and structural on water and solid surfaces.

Spontaneous and Ion-Specific Formation of Inverted Bilayers at Air/Aqueous Interface

Developing better separation technologies for rare earth metals, an important aspect of a sustainable materials economy, is challenging due to their chemical similarities. Identifying molecular-scale interactions that amplify the subtle differences between the rare earths can be useful in developing new separation technologies. Here, we describe the ion-dependent monolayer to inverted bilayer transformation of extractant molecules at the air/aqueous interface. The inverted bilayers form with Lu³⁺ ions but not with Nd³⁺. By introducing Lu³⁺ ions to preformed monolayers, we extract kinetic parameters corresponding to the monolayer to inverted bilayer conversion. Temperature-dependent studies show Arrhenius behavior with an energy barrier of 40 kcal/mol. The kinetics of monolayer to inverted bilayer conversion is also affected by the character of the background anion, although anions are expected to be repelled from the interface. Our results show the outsized importance of ion-specific effects on interfacial structure and kinetics, pointing to their role in chemical separation methods.

Spontaneous collapse of palmitic acid films on an alkaline buffer containing calcium ions

Understanding the interaction of ions with fatty acids is important to identify their roles in various bioprocesses and to build novel biomimetic systems. In this study, the molecular organization of palmitic acid (PA) films on alkaline buffer solutions (pH 7.4) with and without divalent Ca²⁺ was measured at a constant surface area using Langmuir troughs coupled with microscopy and X-ray interfacial techniques. Without Ca²⁺, PA molecules remained a monolayer organization; however, with Ca²⁺, formation of the inverted bilayers of PA-Ca²⁺ superstructures caused a spontaneous 2D to 3D transformation under no compression due to the strong interaction between PA and the divalent cation. Self-assembly of this highly-organized inverted bilayer superstructure involved a two-step process of nucleation and nuclei growth. During nucleation, densely packed PA and Ca²⁺ monolayer firstly corrugated and some of PA and Ca²⁺ molecules ejected out from the monolayer; the ejected molecules then reorganized and formed the inverted bilayer nuclei. Nucleation was followed by nuclei growth, during which PA and Ca²⁺ in the monolayer kept integrating into the inverted bilayer structure through molecule migration and PA rotation around Ca²⁺.

Lipid-Protein Interactions in Langmuir Monolayers under Dynamically Varied Conditions

In the Langmuir monolayer technique, a single layer of molecules is formed on a water subphase. This approach was used to mimic the antitumoricidal lipid-protein complex of oleic acid and bovine α-lactalbumin called the BAMLET complex. Our previous studies have shown that at the interface, the BAMLET complex is stabilized by the hydrophobic forces supported by the hydrogen bonds. This study provides an insight into the influence of calcium ions and the experimental conditions (temperature and subphase pH) on the stability of the complex at the interface. The Langmuir technique was expanded using a dosing pump to exchange the subphase and deliver additional substances to the system. We investigated the interactions between oleic acid monolayer and α-lactalbumin in the presence of Ca²⁺ in the bulk and the effect of varied experimental conditions on the complex stability. The role of calcium ions in this system is important because (in addition to low pH and relatively high temperature) it affects the conformational changes within the protein molecule and facilitates the transition of α-lactalbumin into the molten globule state. A partially unfolded state is required to form the BAMLET complex. We found that the mixed monolayer spread at the interface is stable despite drastic changes in the process conditions and remains stable even after the subphase exchange. This study of molecular interactions explored by the Langmuir technique with peristaltic pump enabled to understand the role of Ca²⁺ in BAMLET complex formation.

Gopinath C, Bala T. Deposition of Au nanoparticles inside porous CeO2 nanocubes using Langmuir–Blodgett technique

AuCl₄-ions are reduced to Au⁰ inside the pores of CeO₂ nanocubes by oleylamine, which has the dual role of capping and reducing agent.