Molecular Interactions between Lecithin and Bile Salts/Acids in Oils and Their Effects on Reverse Micellization

It has been known that the addition of bile salts to lecithin organosols induces the formation of reverse wormlike micelles and that the worms are similar to long polymer chains that entangle each other to form viscoelastic solutions. In this study, we further investigated the effects of different bile salts and bile acids on the growth of lecithin reverse worms in cyclohexane and <i>n</i>-decane. We utilized rheological and small-angle scattering techniques to analyze the properties and structures of the reverse micelles. All of the bile salts can transform the originally spherical lecithin reverse micelles into wormlike micelles and their rheological behaviors can be described by the single-relaxation-time Maxwell model. However, their efficiencies to induce the worms are different. In contrast, before phase separation, bile acids can induce only short cylindrical micelles that are not long enough to impart viscoelasticity. We used Fourier transform infrared spectroscopy to investigate the interactions between lecithin and bile salts/acids and found that different bile salts/acids employ different functional groups to form hydrogen bonds with lecithin. Such effects determine the relative positions of the bile salts/acids in the headgroups of lecithin, thus resulting in varying efficiencies to alter the effective critical packing parameter for the formation of wormlike micelles. This work highlights the importance of intermolecular interactions in molecular self-assembly

Impact of Hydrophobic Sequence Patterning on the Coil-to-Globule Transition of Protein-like Polymers

Understanding the driving forces for the collapse of a polymer chain from a random coil to a globule would be invaluable in enabling scientists to predict the folding of polypeptide sequences into defined tertiary structures. The HP model considers hydrophobic collapse to be the major driving force for protein folding. However, due to the inherent presence of chirality and hydrogen bonding in polypeptides, it has been difficult to experimentally test the ability of hydrophobic forces to independently drive structural transitions. In this work, we use polypeptoids, which lack backbone hydrogen bonding and chirality, to probe the exclusive effect of hydrophobicity on the coil-to-globule collapse. Two sequences containing the same composition of only hydrophobic “H” <i>N</i>-methylglycine and polar “P” <i>N</i>-(2-carboxyethyl)­glycine monomers are shown to have very different globule collapse behaviors due only to the difference in their monomer sequence. As compared to a repeating sequence with an even distribution of H and P monomers, a designed protein-like sequence collapses into a more compact globule in aqueous solution as evidenced by small-angle X-ray scattering, dynamic light scattering, and probing with environmentally sensitive fluorophores. The free energy change for the coil-to-globule transition was determined by equilibrium denaturant titration with acetonitrile. Using a two-state model, the protein-like sequence is shown to have a much greater driving force for globule formation, as well as a higher <i>m</i> value, indicating increased cooperativity for the collapse transition. This difference in globule collapse behavior validates the ability of the HP model to describe structural transitions based solely on hydrophobic forces

Thermo-Switchable Pressure-Sensitive Adhesives Based on Poly(<i>N</i>‑vinyl caprolactam) Non-Covalently Cross-Linked by Poly(ethylene glycol)

The properties of new hydrophilic pressure-sensitive adhesives (PSA) obtained by blending poly­(<i>N</i>-vinyl caprolactam) (PVCL) with short-molecular weight poly­(ethylene glycol) (PEG) were studied in aqueous media by a combination of several calorimetric and adhesion testing techniques. We found that the adhesive properties of the blends are the result of an extensive hydrogen bonding network formed between PVCL and PEG similar to the poly­(<i>N</i>-vinylpyrrolidone) (PVP)/PEG blends, except the extent of cross-linking is nearly 3 times higher in PVCL–PEG networks. Accordingly, we observed substantially higher peel adhesion in PVCL–PEG blends, which depends strongly on the amount of adsorbed water and the temperature. The adhesive properties of PVCL–PEG gels are considerably diminished when the amount of absorbed water exceeds 30% or at elevated temperature but can be easily recovered by drying or cooling the sample. The observed responsiveness of PVCL–PEG hydrogels in physiologically relevant temperature range makes them interesting candidates for industrial and biomedical applications as smart PSAs

Thermo-Switchable Pressure-Sensitive Adhesives Based on Poly(<i>N</i>‑vinyl caprolactam) Non-Covalently Cross-Linked by Poly(ethylene glycol)

The properties of new hydrophilic pressure-sensitive adhesives (PSA) obtained by blending poly­(<i>N</i>-vinyl caprolactam) (PVCL) with short-molecular weight poly­(ethylene glycol) (PEG) were studied in aqueous media by a combination of several calorimetric and adhesion testing techniques. We found that the adhesive properties of the blends are the result of an extensive hydrogen bonding network formed between PVCL and PEG similar to the poly­(<i>N</i>-vinylpyrrolidone) (PVP)/PEG blends, except the extent of cross-linking is nearly 3 times higher in PVCL–PEG networks. Accordingly, we observed substantially higher peel adhesion in PVCL–PEG blends, which depends strongly on the amount of adsorbed water and the temperature. The adhesive properties of PVCL–PEG gels are considerably diminished when the amount of absorbed water exceeds 30% or at elevated temperature but can be easily recovered by drying or cooling the sample. The observed responsiveness of PVCL–PEG hydrogels in physiologically relevant temperature range makes them interesting candidates for industrial and biomedical applications as smart PSAs

Effects of Alkali Cations and Halide Anions on the Self-Assembly of Phosphatidylcholine in Oils

The interactions between ions and phospholipids are closely associated with the structures and functions of cell membrane. Instead of conventional aqueous systems, we systematically investigated the effects of inorganic ions on the self-assembly of lecithin, a zwitterionic phosphatidylcholine, in cyclohexane. Previous studies have shown that addition of inorganic salts with specific divalent and trivalent cations can transform lecithin organosols into organogels. In this study, we focused on the effect of monovalent alkali halides. Fourier transform infrared spectroscopy was used to demonstrate that the binding strength of the alkali cations with the phosphate of lecithin is in the order Li⁺ > Na⁺ > K⁺. More importantly, the cation–phosphate interaction is affected by the paired halide anions, and the effect follows the series I^– > Br^– > Cl^–. The salts of stronger interactions with lecithin, including LiCl, LiBr, LiI, and NaI, were found to induce cylindrical micelles sufficiently long to form organogels, while others remain organosols. A mechanism based on the charge density of ions and the enthalpy change of the ion exchange between alkali halides and lecithin headgroup is provided to explain the contrasting interactions and the effectiveness of the salts to induce organogelation