Mafumi Hishida

Bending modulus of lipid bilayers in a liquid-crystalline phase including an anomalous swelling regime estimated by neutron spin echo experiments

Abstract.Membrane fluctuations of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were investigated by neutron spin echo spectroscopy. The intermediate structure factor was analyzed in terms of the model proposed by Zilman and Granek (Phys. Rev. Lett. 77, 4788 (1996)), and the bending modulus of lipid bilayers was derived. The hardening of a lipid bilayer upon approaching the main transition point in the anomalous swelling regime was observed, which naturally connects the bending modulus in the gel phase below the main transition temperature.

Unbinding of lipid bilayers induced by osmotic pressure in relation to unilamellar vesicle formation

Small-angle X-ray scattering and phase-contrast microscopy experiments were performed to investigate the effect of osmotic pressure on vesicle formation in a dioleoylphosphatidylcholine (DOPC)/water/NaI system. The multi-lamellar structure of lipid bilayers is unstabilized when a lipid film with a sufficient amount of NaI is hydrated by pure water. It has been confirmed that this phenomenon is due to the effect of osmotic pressure induced by a heterogeneous distribution of NaI molecules. This could be the origin of the unbinding of lipid bilayers to conform large uni-lamellar vesicles.

Reentrant nematic phase in 4-alkyl-4′-cyanobiphenyl (nCB) binary mixtures

The appearance of reentrant nematic phase was identified in nCB/8CB (n = 1–6) binary mixtures using differential scanning calorimetry, small-angle X-ray diffraction and polarising microscopy with photomonitor. Phase diagrams can roughly be unified if plotted against the average number of alkyl chain length. Distinction was recognised for the phase boundaries between nematic and smectic A phases of even and odd n of the minor component nCB. The character of 8CB as ‘SmA former’ is briefly discussed.

Comprehensive characterisation of the E phase of 6-octyl-2-phenylazulene

Molecular dynamics in the soft crystal, E phase of 6-octyl-2-phenylazulene was studied by X-ray diffraction, and dielectric and Fourier transform infrared spectroscopies. Clear dielectric dispersion is observed due to the head-to-tail disordering in a kHz range around 400 K. The CH stretching frequencies exhibit more pronounced shifts at the phase transition point from the high-temperature ordered crystal to the E phase than that at the melting from the E phase to the isotropic liquid. The results are discussed in comparison with a typical E series, 4-n-alkyl-4′-isothiocyanatobiphenyl (n being the number of carbon atoms in the alkyl group).

Transition of the hydration state of a surfactant accompanying structural transitions of self-assembled aggregates

What role does water play in the self-assembly of soft materials? To understand the correlation between the hydration state and the various self-assembled structures of a nonionic surfactant, terahertz time-domain spectroscopy has been performed for a C(12)E(5) solution with complementary use of small-angle x-ray scattering. Precise observations of the hydration state show clearly that transitions of the hydration state are accompanied by structural phase transitions of the surfactant from hexagonal to micelle to lamellae. These transitions of hydration state suggest that water is not a homogeneous solvent, and the interaction between water and the soft material is important for self-assembly.

Communication: Salt-induced water orientation at a surface of non-ionic surfactant in relation to a mechanism of Hofmeister effect

The behavior of water molecules at the surface of nonionic surfactant (monomyristolein) and effects of monovalent ions on the behavior are investigated using the heterodyne-detected vibrational sum frequency generation spectroscopy. It is found that water molecules at the surface are oriented with their hydrogen atoms pointing to the bulk, and that the degree of orientation depends on the anion strongly but weakly on the cation. With measured surface potentials in those saline solutions, it is concluded that the heterogeneous distribution of anions and cations in combination with the nonionic surfactant causes the water orientation. This heterogeneous distribution well explains the contrasting order of anions and cations with respect to the ion size in the Hofmeister series.

Salt effects on lamellar repeat distance depending on head groups of neutrally charged lipids.

Change in lamellar repeat distances of neutrally charged lipids upon addition of monovalent salts was measured with small-angle X-ray scattering for combinations of two lipids (PC and PE lipids) and six salts. Large dependence on lipid head group is observed in addition to those on added cation and anion. The ion and lipid dependences have little correlation with measured surface potentials of lipid membranes. These results indicate that the lamellar swelling by salt is not explained through balance among interactions considered previously (van der Waals interaction, electrostatic repulsion emerged by ion binding, etc.). It is suggested that effect of water structure, which is affected by not only ions but also lipid itself, should be taken into account for understanding membrane-membrane interactions, as in the Hofmeister effect.

Cooperativity between Water and Lipids in Lamellar to Inverted-Hexagonal Phase Transition

It has been unclear whether the role of water in the self-assembly of soft materials and biomolecules is influential or water is just a background medium. Here we investigate the correlation betwee...

Strucutural Changes of Dipalmitoyl Phosphatidylcholine Aqueous Solution Induced by Temperature, Pressure, and Adding Ethanol

A swollen phase, in which the mean repeat distance of lipid bilayers is larger than the other phases, is found between the liquid-crystalline phase and the interdigitated gel phase in DPPC aqueous solution. Temperature, pressure and ethanol concentration dependences of the structure were investigated by small-angle neutron scattering, and a bending rigidity of lipid bilayers was by neutron spin echo. The nature of the swollen phase is similar to the anomalous swelling reported previously. However, the temperature dependence of the mean repeat distance and the bending rigidity of lipid bilayers are different. This phase could be a precursor to the interdigitated gel phase induced by pressure and/or adding ethanol.Temperature, pressure, and ethanol concentration dependences of the structure and dynamics of dipalmitoyl phosphatidylcholine (DPPC) aqueous solution were investigated by small-angle neutron scattering and neutron spin echo experiments. The effect of ethanol is almost the same as that of applying pressure to induce the interdigitated gel phase. A swollen phase, in which the mean repeat distance of lipid bilayers is larger than these of the other phases, is found between the liquid-crystalline phase and the interdigitated gel phase. The nature of the swollen phase is similar to the anomalous swelling observed above the transition point between the liquid-crystalline phase and the ripple gel phase. However, the temperature dependence of the mean repeat distance and the bending rigidity of lipid bilayers in the swollen phase are different from those of the anomalous swelling.

Effect of n-alkanes on lipid bilayers depending on headgroups.

Phase behavior and structural properties were examined for phospholipid bilayers having different headgroups (DMPC, DMPS and DMPE) with added n-alkanes to study effect of flexible additives. Change in the temperatures of main transition of the lipid/alkane mixtures against the length of added alkanes depends largely on the headgroup. Theoretical analysis of the change of the temperature of transition indicates that the headgroup dependence is dominantly originated in the strong dependence of total enthalpy on the headgroups. The results of X-ray diffraction show that the enthalpic stabilization due to enhanced packing of acyl chains of the lipid by alkanes in the gel phase causes the headgroup-dependent change in the phase transition behavior. The enhanced packing in the gel phase also leads to easy emergence of the subgel phase with very short relaxation time at room temperature in the DMPE-based bilayers.