Phase Diagram of Androsterol−Dipalmitoylphosphatidylcholine Mixtures Dispersed in Excess Water

Transition Mechanism from the Metastable Two-Dimensional Gel to the Stable Three-Dimensional Crystal of Imidazolium-Based Ionic Liquids

One important quest for making high quality materials with amphiphiles is to understand how a disordered self-assembly changes to a stable crystalline state. Herein, we addressed the basic question by investigating the phase transition mechanism of imidazolium-based ionic liquid (IL) [C16mim]Br, using time-resolved small- and wide-angle X-ray scattering (SAXS-WAXS), differential scanning calorimetry, and Fourier transform infrared spectroscopy techniques. Totally, a hexagonal phase, two lamellar-gel phases, and three lamellar-crystalline phases were observed, showing the special polymorphism of the system. It was demonstrated that at low concentrations the two-dimensional gel phase (Lβ1) transforms into the most stable lamellar-crystal phase (Lc3) through two intermediate crystalline phases Lc1 and Lc2. At high concentrations, the Lβ1 phase changes to a condensed lamellar gel phase (Lβ2) before changing to Lc2 and eventually to Lc3. Comparative studies using [C16mim]Cl and [C16mim]NO3 unveiled that the interactions between the counterions and the headgroups of the IL, as well as the dehydration process, govern the nucleation process of Lc3 and thus the formation of the crystal. The in-depth investigation on the transition mechanism and the phase polymorphism in the present work advances our understanding of the crystallization of amphiphilic ionic liquids in dispersions and would promote future applications.

Phloretin-induced reduction in dipole potential of sterol-containing bilayers

The phloretin-induced reduction in the dipole potential of planar lipid bilayers containing cholesterol, ergosterol, stigmasterol, 7-dehydrocholesterol and 5α-androstan-3β-ol was investigated. It is shown that effects depend on the type and concentration of membrane sterol. It is supposed that the effectiveness of phloretin in reducing the dipole potential of the bilayers that contain cholesterol, ergosterol and 7-dehydrocholesterol correlates with the ordering and condensing effects. The role of the concentration-dependent ability of different sterols to promote lateral heterogeneity in membranes is also discussed.

Regional cooperativity in the phase transitions of dipalmitoylphosphatidylcholine bilayers: the lipid tail triggers the isothermal crystallization process

We have a long-standing interest to explore the answer of the question: Which part of the amphiphilic molecule triggers the phase transition of the self-assembled aggregates consisting of these amphiphiles? This is an important issue regarding the phase transition kinetics of amphiphiles. To this end, we studied the phase transition behaviors of dipalmitoylphosphatidylcholine (DPPC) by differential scanning calorimetry, synchrotron X-ray scattering, Fourier transform infrared spectroscopy, and image analysis. We found that different parts (head, interface, and tail) of DPPC molecules all exhibit nonsynchronous changes during the sub-, pre-, and main transitions. Particular efforts have been devoted to studying the isothermal subgel (L(c')) formation process. It was found that only the lipid interface and tail regions change, and only when the rearrangement of the lipid hydrocarbon chain packing reaches a certain extent can the interfacial C═O groups be induced to undergo vibrational environment changes. The result means that the hydrocarbon tail is the part that triggers the gel (L(β')) to L(c') phase transition. The present work deepens our understanding on the phase transition mechanisms of DPPC and may shed light on those of other phospholipids and other types of amphiphiles.

Nonsynchronous change in the head and tail of dioctadecyldimethylammonium bromide molecules during the liquid crystalline to coagel phase transformation process

Dioctadecyldimethylammonium bromide (DODAB) is known to self-assemble into several lamellar structures in water, existing as either liquid crystalline, gel, or coagel phases. In this work, by using differential scanning calorimetry, Fourier transform infrared spectroscopy, and X-ray diffraction techniques, we have characterized the details of the phase transition mechanisms of the DODAB aqueous dispersions. It was found that the liquid crystalline converts to the coagel phase via a two-step mechanism: first to the gel phase upon cooling and then to the stable coagel phase. Although significant conformational changes in the hydrocarbon tails were observed in both steps, changes in the headgroups of DODAB were only detected in the second step. More interestingly, we found that the lipid tails change prior to the headgroups during the overall liquid crystalline to coagel phase transformation process. This is regarded as a nonsynchronicity phenomenon, which reflects the regional (head/tail) imbalance in molecular interactions. Such a nonsynchronicity phenomenon in the self-assembled aggregates composed of the medium-sized DODAB molecules will shed light on our understanding of the polymorphism and reversibility of amphiphiles including both surfactants and biomembrane phospholipids.

Structural characterization on the gel to liquid-crystal phase transition of fully hydrated DSPC and DSPE bilayers

The structural properties of fully hydrated distearoylphosphatidylcholine (DSPC) and distearoylphosphatidylethanolamine (DSPE) bilayers near the main phase transition were investigated using molecular dynamics simulations on the basis of a united-atom model. Although largely similar in their molecular structures, the two lipids were found with different molecular packing modes at temperatures below the phase transition. For DSPC, three packing modes, namely, cross-tilt, partially interdigitated, and "mixed" gel phases, were observed, while, for DSPE, the lipid tails were almost perpendicular to the lipid surface. Above the main transition temperature, both lipid bilayers transformed into a disordered liquid-crystal phase with marked greater area per lipid and gauche % of the acyl chains and smaller bilayer thickness and order parameter, in comparison with the gel phase. The transformation process of liquid-crystal to gel phase was proved to experience the nucleation and growth stages in a hexagonal manner. The electron density profiles of some major components of both lipid bilayers at various temperatures have been calculated, and the results reveal that both lipid bilayers have less interdigitation around the main transition temperature.