Geng Deng

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.

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.

Evidence that Acetonitrile is Sensitive to Different Interaction Sites of Ionic Liquids as Revealed by Excess Spectroscopy

By studying the interactions between an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and a co-solvent acetonitrile, the C≡N stretching vibration is found to be sensitive to different interaction sites as shown by excess infrared spectroscopy. Four existing forms of acetonitrile molecules are identified and a detailed transformation process of the ionic liquid upon dilution is obtained. Such characteristics of the nitrile group are discussed from the viewpoint of its ability to form hydrogen bonds with proton donors. It is believed that this is due to the intermediate charge donating ability of the C≡N group as compared with other groups such as S=O, CH3 , and aromatic C-H.

Evidences for Cooperative Resonance-Assisted Hydrogen Bonds in Protein Secondary Structure Analogs

Cooperative behaviors of the hydrogen bonding networks in proteins have been discovered for a long time. The structural origin of this cooperativity, however, is still under debate. Here we report a new investigation combining excess infrared spectroscopy and density functional theory calculation on peptide analogs, represented by N-methylformamide (NMF) and N-methylacetamide (NMA). Interestingly, addition of the strong hydrogen bond acceptor, dimethyl sulfoxide, to the pure analogs caused opposite effects, namely red- and blue-shift of the N−H stretching infrared absorption in NMF and NMA, respectively. The contradiction can be reconciled by the marked lowering of the energy levels of the self-associates between NMA molecules due to a cooperative effect of the hydrogen bonds. On the contrary, NMF molecules cannot form long-chain cooperative hydrogen bonds because they tend to form dimers. Even more interestingly, we found excellent linear relationships between changes on bond orders of N−H/N−C/C = O and the hydrogen bond energy gains upon the formation of hydrogen bonding multimers in NMA, suggesting strongly that the cooperativity originates from resonance-assisted hydrogen bonds. Our findings provide insights on the structures of proteins and may also shed lights on the rational design of novel molecular recognition 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.