Hai-Yuan Sun

Comparative Study of Halogen‐ and Hydrogen‐Bond Interactions between Benzene Derivatives and Dimethyl Sulfoxide

The halogen bond, similar to the hydrogen bond, is an important noncovalent interaction and plays important roles in diverse chemistry-related fields. Herein, bromine- and iodine-based halogen-bonding interactions between two benzene derivatives (C6 F5 Br and C6 F5 I) and dimethyl sulfoxide (DMSO) are investigated by using IR and NMR spectroscopy and ab initio calculations. The results are compared with those of interactions between C6 F5 Cl/C6 F5 H and DMSO. First, the interaction energy of the hydrogen bond is stronger than those of bromine- and chlorine-based halogen bonds, but weaker than iodine-based halogen bond. Second, attractive energies depend on 1/r(n) , in which n is between three and four for both hydrogen and halogen bonds, whereas all repulsive energies are found to depend on 1/r(8.5) . Third, the directionality of halogen bonds is greater than that of the hydrogen bond. The bromine- and iodine-based halogen bonds are strict in this regard and the chlorine-based halogen bond only slightly deviates from 180°. The directional order is iodine-based halogen bond>bromine-based halogen bond>chlorine-based halogen bond>hydrogen bond. Fourth, upon the formation of hydrogen and halogen bonds, charge transfers from DMSO to the hydrogen- and halogen-bond donors. The CH3 group contributes positively to stabilization of the complexes.

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 (C(16)mimCl) 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 C(16)mimCl in aqueous solutions can form two different crystalline phases. At higher C(16)mimCl 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.

Two-State or Non-Two-State? An Excess Spectroscopy-based Approach to Differentiate the Existing Forms of Molecules in Liquids Mixtures

Characterization/identification of the clusters/associates in liquids has long been a challenging topic. In this paper, we report a method to identify molecules with two different existing forms in a binary liquid solution. In this so-called two-state situation, the excess infrared spectra of a vibration mode of the respective molecule will show identical band shape if the other component is transparent in the region. More conveniently, the positions of the positive peak, negative peak, and zero-value will be seen to be fixed with varying compositions of the binary system. In the case of non-two-state mixtures, for example the mere solvation of solute by solvent, those positions will be variable. The conclusions are supported/demonstrated by computational simulation and experiments on two binary systems, D2O−H2O and C6F5I−cyclo-C6H12.

Molecular-level pictures of the phase transitions of saturated and unsaturated phospholipid binary mixtures

Binary lipid mixtures consisting of saturated and unsaturated lipids are important models for natural cell membranes. However, a detailed molecular picture for the phase transition process of such lipid binary mixtures remains unclear. Herein, by using deuterated dipalmitoylphosphatidylcholine (DPPC-d62) and hydrogenated dioleoylphosphatidic acid (DOPA), we expect to separately analyze the changes of the two lipid components during thermotropic phase transitions by temperature-dependent Fourier transform infrared (FTIR) spectroscopy, and uncover the hidden secrets of a seemingly single endothermic peak observed in differential scanning calorimetry (DSC) experiments. We found that at low DOPA concentrations (10–20 mol%), a gel to fluid conformational transition of DPPC-d62 and a fluidization transition of DOPA by DPPC-d62 were observed. The two lipids were found to have nonsynchronous conformational rearrangements in the tail regions upon thermotropic phase transitions, with the change of DOPA earlier than DPPC-d62. Besides, in the mixed fluid phase at 40 °C, the unsaturated DOPA can be fluidized (loosened) by the saturated DPPC-d62, and such an isothermal fluidization effect is more pronounced at elevated DPPC-d62 concentrations. At higher DOPA contents (30–50 mol%), only DPPC-d62 molecules have conformational transitions upon heating. The present work demonstrates for the first time that the unsaturated lipid component can have significant conformational reorganizations in the phase transition process of a saturated–unsaturated binary lipid mixture.

Full picture of the thermotropic phase behavior of cardiolipin bilayer in water: identification of a metastable subgel phase

The formation of a stable crystalline phase from a fluid phase usually involves the passage through a gel or metastable subgel phase during cooling. Herein, we found for the first time that for the four-tailed lipid, tetramyristoyl cardiolipin (TMCL), a metastable subgel phase (“subgel II”-type phase, or SGII) was formed upon cooling from the gel phase, and such a metastable subgel phase served as the precursor towards the formation of a stable crystalline phase. A detailed thermotropic, X-ray diffraction and infrared spectroscopic study was carried out to study the formation of the subgel and crystalline phases, along with a comparison of the gel and fluid phases. All the four phases (crystalline, subgel, gel and fluid) are lamellar structured and the interlamellar spacings of the gel and fluid phases swell as the water content increases in pure water. A full picture of the thermotropic phase behavior of TMCL in water was obtained and it was found that the transformations between fluid–gel and gel–subgel are reversible, while the phase transformation processes involving the crystalline phase are irreversible. The gel to subgel transition mainly involves the reorganization of the tail packing state (from hexagonal to orthorhombic), and this phase transition was found to be independent of solvent. Changing from subgel to crystalline phase needs a long incubation time at relatively low temperatures (0–5 °C), and this crystalline phase converts to a gel phase upon heating. The present work sheds light on the crystallization process of amphiphiles and has profound implications on the stability of cardiolipin-based model systems or supramolecular self-assembly systems.

Phase behavior of a binary lipid system containing long- and short-chain phosphatidylcholines

Bilayered micelles, or so-called bicelles, are generally made of long- and short-chain lipids. They are extensively used as model membranes to study the structure of membrane-associated peptides or proteins and their interactions with membranes. However, the phase behavior of lipid mixtures composed of long- and short-chain lipids, especially at low temperatures, is still not very clear. In this work, the most commonly used long-chain lipid, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and a short-chain lipid, 1,2-dioctanoyl-sn-glycero-3-phosphocholine (diC8PC), were selected as a bicellar model to study their phase behavior. Over the whole range of DPPC/diC8PC molar ratios (q) studied in this work, a lamellar crystalline phase (Lc′) enriched in DPPC was found to be the most stable phase at 5 °C, together with a diC8PC-enriched micelle phase. Interestingly, a metastable phase, named the U phase in this work, was observed in the mixtures with a DPPC/diC8PC molar ratio between 1 and 4. The metastable U phase was found to be lacking in long-range order in the direction of the bilayer surface normal, but bearing a different “crystalline phase-like” hydrocarbon chain packing mode, in comparison with the lamellar crystalline phase. The kinetic properties of the U phase were also studied in detail, and it was found that the phase acts as a precursor phase in the process of forming the most stable crystalline phase. This work deepens our understanding of lipid crystallization behavior, and is also a step forward towards a more detailed picture of the phase behavior of lipid mixtures composed of long- and short-chain lipids.

Structural properties of paeonol encapsulated liposomes at physiological temperature: Synchrotron small-angle and wide-angle X-ray diffraction studies

Structural properties of paeonol-encapsulated liposomes containing cholesterol or stigmasterol at 37°C have been investigated by synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. We compared the structural properties of pure dipalmitoylphosphatidylcholine (DPPC) liposomes, sterol–DPPC liposomes, and those of paeonol–sterol–DPPC liposomes at different molar ratios. Three conclusions can be drawn: First, phase separation occurs in both sterol–DPPC and paeonol–sterol–DPPC liposomes. Second, the incorporation of paeonol molecules into sterol– DPPC liposomes weakens the membrane order. Third, cholesterol has a stronger tendency to interact with DPPC as compared to its counterpart in plant, stigmasterol.