Hitoshi Endo

Flip-Flop of Phospholipids in Vesicles: Kinetic Analysis with Time-Resolved Small-Angle Neutron Scattering

We applied a time-resolved small-angle neutron scattering technique to vesicle systems to determine interparticle transfer and flip-flop of phospholipids. Measurements were performed for large unilamellar vesicles, consisting of dimyristoylphosphatidylcholine (DMPC), 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), or 1-palmitoyl-2-oleoylphosphatidic acid (POPA), which differ either in their acyl chains or headgroup. POPC, which is analogous to naturally occurring phosphatidylcholines, exhibited no transbilayer transfer and very slow interbilayer migration. POPC on the inner leaflet of vesicles did not flop even when phospholipase D converted all POPC molecules on the outer leaflet into POPA, which was shown to exhibit fast flip-flop. From these results, together with the observation that the flip-flop of DMPC was entirely inhibited in the presence of cholesterol, it is deduced that the flip-flop of phosphatidylcholines does not take place spontaneously in cellular plasma membranes rich in cholesterol and that it requires enzymatic activities of energy-dependent and/or -independent flippases/floppases.

Static and dynamic properties of phospholipid bilayer nanodiscs.

Nanodiscs are phospholipid-protein complexes which are relevant to nascent high-density lipoprotein and are applicable as a drug carrier and a tool to immobilize membrane proteins. We evaluated the structure and dynamics of the nanoparticles consisting of dimyristoylphosphatidylcholine (DMPC) and apolipoprotein A-I (apoA-I) with small-angle neutron scattering (SANS) and fluorescence methods and compared them with static/dynamic properties for large unilamellar vesicles. SANS revealed that the nanodisc includes a lipid bilayer with a thickness of 44 A and a radius of 37 A, in which each lipid occupies a smaller area than the reported molecular area of DMPC in vesicles. Fluorescence measurements suggested that DMPC possesses a lower entropy in nanodiscs than in vesicles, because apoA-I molecules, which surround the bilayer, force closer lipid packing, but allow water penetration to the acyl chain ends. Time-resolved SANS experiments revealed that nanodiscs represent a 20-fold higher lipid transfer via an entropically favorable process. The results put forward a conjunction of static/dynamic properties of nanodiscs, where the entropic constraints are responsible for the accelerated desorption of lipids.

Rheo-SANS Studies on Shear-Thickening/Thinning in Aqueous Rodlike Micellar Solutions

Shear-induced thickening/thinning phenomena of aqueous rodlike micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium p-toluene sulfonate (NapTS) were investigated by means of simultaneous measurements of rheology and small-angle neutron scattering (SANS), the so-called Rheo-SANS. The aqueous CTAB/NapTS solutions were classified into five different categories dependent on their flow behavior and micellar structure. By increasing salt concentration and/or shear rates, the micelles underwent morphological transition from (i) spherical or short rodlike micelles to (ii) long rodlike micelles without entanglements, followed by (iii) those with entanglements. These transitions were recognized as changes in flow behavior from Newtonian to shear-thickening and shear-thinning flow, respectively. In the latter two cases, anisotropic SANS patterns appeared around these critical shear rates. The physical meaning of the anisotropic SANS patterns accompanied by shear-thickening flow behavior is discussed in conjunction with other shear-thickening systems.

Relationship between mesoscale dynamics and shear relaxation of ionic liquids with long alkyl chain

The shear relaxation spectra of three imidazolium-based ionic liquids, 1-methyl-3-octylimidazolium chloride ([C(8)mim][Cl]), 1-methyl-3-octylimidazolium hexafluorophosphate ([C(8)mim][PF(6)]), and 1-dodecyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(12)mim][TFSA]) were measured and compared with the intermediate scattering functions determined with neutron spin echo (NSE) spectroscopy. The shear relaxation is slower than that predicted from the relaxation of the main peak of the structure factor that is common to other molecular liquids, whereas it is faster than that from the relaxation of the pre-peak, that corresponds to the correlation length of about 10 nm specific to ionic liquids with an intermediately long alkyl chain. The role of the pre-peak structure in the mechanism of shear viscosity of ionic liquids is discussed based on the comparison between NSE and shear relaxations.

Collective Structural Changes in Vermiculite Clay Suspensions Induced by Cesium Ions

Following the Fukushima Daiichi nuclear disaster in 2011, Cs radioisotopes have been dispersed over a wide area. Most of the Cs has remained on the surface of the soil because Cs+ is strongly adsorbed in the interlayer spaces of soil clays, particularly vermiculite. We have investigated the microscopic structure of an aqueous suspension of vermiculite clay over a wide length scale (1–1000 Å) by small-angle X-ray scattering. We determined the effect of the adsorption behavior of Cs+ on the structural changes in the clay. It was found that the abruption of the clay sheets was induced by the localization of Cs+ at the interlayer. This work provides important information for predicting the environmental fate of radioactive Cs in polluted areas, and for developing methods to extract Cs from the soil and reduce radioactivity.

2D-Ising-like critical behavior in mixtures of water and 3-methylpyridine including antagonistic salt or ionic surfactant

The effect of an antagonistic salt on the phase behavior and nanoscale structure of a mixture of D2O and 3-methylpyridine was investigated by visual inspection and small-angle neutron scattering (SANS). The addition of the antagonistic salt, namely sodium tetraphenylborate (NaBPh4), induces the shrinking of the two-phase region in contrast to the case in which a normal (hydrophilic) salt is added. Below the phase separation point, the SANS profiles cannot be described by the Ornstein–Zernike function owing to the existence of a long-range periodic structure. With increasing salt concentration, the critical exponents change from the values of 3D-Ising and approach those of 2D-Ising. These results suggest that the concentration fluctuation of the mixture of solvents is limited to a quasi two-dimensional space by the periodic structure induced by the adding the salt. The same behaviors were also observed in mixtures composed of water, 3-methylpyridine, and ionic surfactant.

Analysis of surface structure and hydrogen/deuterium exchange of colloidal silica suspension by contrast-variation small-angle neutron scattering.

The microscopic surface structure and hydrogen/deuterium exchange effect were investigated by contrast-variation small-angle neutron scattering (CV-SANS) for three different-sized amorphous colloidal silica aqueous suspensions. The results show that the fraction of hydrogen/deuterium exchange per nanoparticle, phiH/D, strongly depends on the size of silica nanoparticles. This finding supports that the hydrogen/deuterium exchange occurs exclusively within a finite surface layer of silica nanoparticles, while the inner component remained unchanged. Detailed analyses of the scattering intensity functions led to the estimation of (1) phiH/D and (2) the thickness of the surface layer as functions of the particle radius. The surface layer thickness was found to increase from 18 to 35 A with decreasing the particle radius from 165 to 71.2 A. The surface area per unit weight of silica estimated with the CV-SANS results are comparable to those reported in the literature.

Membrane formation by preferential solvation of ions in mixture of water, 3-methylpyridine, and sodium tetraphenylborate.

The structure and dynamics of a ternary system composed of deuterium oxide (D2O), 3-methylpyridine (3MP), and sodium tetraphenylborate (NaBPh4) are investigated by means of small-angle neutron scattering (SANS) and neutron spin echo (NSE) techniques. In the SANS experiments, a structural phase transition is confirmed between a disordered-phase and an ordered-lamellar-phase upon variation of the composition and/or temperature of the mixture. The characteristic lengths of the structures is on the sub-micrometer scale. A dispersion relation of the structure is measured through NSE experiments, which shows that the relaxation rate follows a cubic relation with momentum transfer. This implies that the dynamics of the system are determined predominantly by membrane fluctuations. The present results indicate that 3MP-rich domains are microscopically separated from bulk water in the presence of NaBPh4, and that the layers behave as membranes. These results are interpreted that preferential solvation of salt in each solvent induces a microphase separation between the solvents, and the periodic structure of 3MP-rich domains is stabilized by the long-range electrostatic interaction arising from Na(+) ions in D2O-rich domains.

Pressure-induced reentrant micellization of amphiphilic block copolymers in dilute aqueous solutions.

The pressure-induced structural changes of a block copolymer, poly(2-ethoxyethoxyethyl vinyl ether)-block-poly(2-hydroxyethyl vinyl ether) (pEOEOVE-b-pHOVE) in aqueous solutions, were studied by means of small-angle neutron scattering (SANS) and dynamic light scattering (DLS) from atmospheric pressure up to 400 MPa. pEOEOVE-b-pHOVE formed a spherical micellar structure above 40 degrees C due to poor solubility of pEOEOVE. Micellization phase diagram was determined by DLS, and a covex-upward pressure-temperature (P-T) phase diagram was obtained having a peak around (P,T)=(150 MPa,48 degrees C). The SANS curves at 50 degrees C were analyzed as a function of P. The micellar core size decreased by pressurizing at low Ps (P<or=150 MPa) and then increased by further pressurizing (150 MPa<P<400 MPa). It was confirmed that the water volume fraction in the micellar core was larger at high Ps than that at low Ps even when the core sizes are the same at both pressure regions. This means that the temperature dependence of hydration is more selective at low pressures than at high pressures, in good accordance with our previous results on concentrated aqueous solutions of block copolymers [Phys. Rev. Lett. 96, 048303 (2006)].

Mesoscopic structures of vermiculite and weathered biotite clays in suspension with and without cesium ions.

The effect of cesium (Cs) adsorption on the mesoscopic structure of the clay minerals vermiculite and weathered biotite (WB) in suspensions was elucidated by small-angle X-ray scattering (SAXS). The clay minerals form multilayered structures, and the Cs cations (Cs(+)) are strongly adsorbed in the interlayer space of the soil clays, in particular vermiculite and WB. SAXS was used to monitor the relationship between Cs(+) adsorption at the clay interlayers and the structural changes at length scales from 1 to 1000 Å. The variation in the distance between the neighboring clay sheets and the spatial arrangement of the clay sheets with and without Cs(+) were clarified. Our quantitative analyses revealed that the number of stacked layers of pure vermiculite was decreased by Cs(+) addition, whereas that of WB increased. Moreover, the average distance between the neighboring layers of vermiculite in suspension was larger than that of WB, which reflects the different conditions of Cs(+) intercalation. These findings provide fundamental insights that are important for predicting the environmental fate of radioactive Cs in contaminated regions and for developing methods for extracting Cs from soil.