Tsutomu Seimiya

Ionic structure of phospholipid membranes, and binding of calcium ions☆

Abstract Radiotracer measurements of adsorption showed that the binding of Ca 2+ with the phosphate groups of phosphatidylserine, phosphatidylethanolamine, and phosphatidylcholine monolayers depended largely on the molecular conformation of the lipid in the monolayer. The monolayers of phosphatidylethanolamine and phosphatidylcholine bind small amounts of Ca 2+ when the lipid molecules are packed closer than 150 A 2 per molecule. The distribution of bound Ca 2+ on both phosphate and carboxyl groups of phosphatidylserine was analyzed from the differences in accessibility of Ca 2+ to each group. The amount of Ca 2+ binding is larger for the carboxyl than the phosphate groups of phosphatidylserine by a factor of about 3 at molecular areas larger than 150 A 2 /molecule at pH 7.5 and Ca 2+ 0.05 mM. The apparent reactivities of these phospholipids to Ca 2+ varied in the order phosphatidylserine > phosphatidylethanolamine ⪢ phosphatidylcholine. The low reactivity of phosphate groups of phosphatidylethanolamine and phosphatidylcholine may reasonably be understood as a lowered ionic nature of the phospholipid owing to the intra- or intermolecular neutralization of the charge between ammonium cation and phosphate anion groups of the lipids. The Ca 2+ binding is explained by taking into account those ionic equilibria of the acid and base dissociations, the intra- or intermolecular neutralization among phosphate, carboxyl and ammonium groups, and the calcium soap formation at each anionic site. The equilibrium constants, which determine the ionic properties of these phospholipids, are calculated from the adsorption data of Ca 2+ at various pH values, and at various degrees of molecular packing of the phospholipid monolayers.

Corrections for surface tension measured by Wilhelmy method

In the Wilhelmy method of surface tension measurement it is necessary to correct for (1) the lowering of liquid level caused by the meniscus formation when the liquid touches the hanging plate (meniscus correction) and (2) for the difference between the effective and geometrical peripheral length of the plate (peripheral correction). The application of these two corrections is studied for both water and toluene using a number of hanging glass plates and rods and platinum plates differing in the size and shape of their horizontal cross sections. The following empirical equation was obtained: σPM=σ[1−(sS)][1+(l0L)], where πP M denotes the surface tension corrected for the peripheral and meniscus errors, π is the observed surface tension, S is the surface area of a liquid in a vessel, s and L are the cross-sectional area and peripheral length of the hanging plate and rod, and l0 is the constant for the peripheral correction. The values of l0 are from −0.008 to −0.010 cm for plates, −0.005 cm for triangular rods, and zero for cylindrical rods. These values are independent of the peripheral length and material of the plates or rods.

Micellar growth in mixed anionic/cationic surfactant solutions: Sodium dodecyl sulfate/octyltrimethylammonium bromide system

Abstract Static and dynamic light scattering, pulsed-gradient FT-NMR, viscosity, and surface tension are measured for the sodium dodecyl sulfate (SDS)/octyltrimethylammonium bromide (OTAB)/water system (below a total surfactant concentration of 1% by weight). Results in the single-phase micellar region (SDS-rich side) are consistent with the sphere-to-rod transition in micellar shape at a mixing ratio OTAB/(SDS + OTAB) ∼ 0.2. The phase behavior around the critical micelle concentration is also studied. Observed phase diagram in the dilute region is discussed on the basis of the change in the micellar composition with the total surfactant concentration.

Phase transitions of phospholipids in monolayers and surface viscosity

Abstract The surface pressures and the surface viscosities of lecithin, cephalin and its analogs were measured at the air—water and the oil—water interfaces. It was found that the surface viscosity of the phospholipids used in this study was very high, and was comparable to those of some polymer films at the oil—water interface as well as at the air—water interface under the conditions where the monolayers were condensed. The plateaus indicating the phase transitions in monolayers were clearly observed on the pressure-area curves at the oil—water interface in all of the specimens studied. It was found that the phase transitions exactly corresponded to the abrupt increases in surface viscosity. From the results thus obtained, an intermolecular ionic linkage between neighboring molecules in the monolayers is discussed.

Change in size and composition of mixed micelles with concentration in anionic/cationic surfactant solutions

Abstract Light scattering intensities and self-diffusion coefficients of both components have been measured on the sodium dodecyl sulfate (SDS)/octyltrimethylammonium bromide (OTAB)/water system as a function of total surfactant concentration, c, at different mixing ratios (below c of 1% by weight). In the SDS-rich side, as c is decreased, i.e., as the phase boundary between the mixed micelle region and the precipitation region is approached, micelles grow and the mole fraction of OTAB in the mixed micelle, x2m, increases. It is also shown that the mixed micelles grow rapidly when x2m exceeds about 0.25.

Critical structure formation in ampholytic phospholipid monolayers as analyzed by calcium binding characteristics

Abstract The binding of calcium ions was studied by radiotracer method for the monolayer of three ampholytic phospholipids differing in structural separation between phosphate and ammonium groups. The phospholipids used were DL-α-dipalmitoylphosphatidylethanolamine (PE), -n-propanolamine (PNP), and -iso-propanolamine (PIP), all synthesized and purified by thin layer chromatography. The reactivity of these lipids to calcium ions decreased in the following order PE

In situ AFM observation of heterogeneous growth of adsorbed film on cleaved mica surface

Abstract The dynamic process of cationic surfactant molecules to form adsorbed layer on cleaved mica surface was observed in situ for the first time by AFM. At the initial stage of adsorption, the adsorbed molecules are observed as the small island-like aggregates with the size of about 10–30 nm. These islands are replaced by larger patches as time lapses. After the continuous growth of the patches, a close-packed film with uniform appearance is approached. The driving force to increase the size of molecular aggregates may either be by line tension or by two dimensional Laplace pressure that induce coalescence of smaller aggregates into larger patches.

Effect of surface charge on adsorption of bovine serum albumin as studied by ellipsometry 1. Adsorption on cationic monolayer

The adsorption of bovine serum albumin (BSA) onto a cationic monolayer (N,N-dimethyl-N,N-dialkylammonium chloride) spread at the air/water interface was studied by ellipsometry. Both thicknesses and refractive indices of the BSA layer adsorbed at the monolayer/solution interface are estimated from the observed change in phase difference and the ratio of reflection coefficients. The amount of adsorption of BSA resembles a Langmuir type isotherm. The adsorption changes with pH asymmetrically with respect to the pH of maximum adsorption, which was calculated to be 5.06 ± 0.47 mg/m2. The amount of maximum adsorption implies that the BSA molecule adsorbs to the surface in a mode intermediate between “side-on” and “end-on”.

Spreading pressures for fatty-acid crystals at the air/water interface

Published values for the spreading pressures of lipids at the air/water interface are inconsistent. As part of a collaborative programme to establish criteria for surface manometry a detailed study was made of the spreading pressures for tetradecanoic, pentadecanoic and hexadecanoic acid crystals prepared from melts and solutions. The spreading pressures vary considerably for a given acid. The removal of solvents from recrystallised acids and the solubilities of chosen crystal samples in hexane were measured. The spreading pressures were then correlated for the various crystal preparations. Optical and scanning electron microscopy were used to characterise crystal geometries, and the crystallographic forms were determined by X-ray diffraction. It was established that solvent retention in the crystals can lower the spreading pressure, and that crystals with microscopic points and edges show high spreading pressures. The results can be interpreted simply by thermodynamic arguments. Data on the effect of temperature on the spreading pressures and solubilities are also presented, and the enthalpies of spreading and solution are calculated.

Intermicellar interactions and micelle size distribution in aqueous solutions of polyoxyethylene surfactants

Mutual diffusion coefficients (D) and self-diffusion coefficients (D+) in D2O solution of n-dodecylpolyoxyethylene surfactants [C12H25(OC2H4)mOH, m= 5 and 8] have been measured by dynamic light scattering and pulsed-gradient F.t.n.m.r., respectively. On the basis of the ratio D/D+, micelle size distribution and intermicellar interactions have been discussed. The results have been compared with those for the C12E6 system reported previously. Below ca.Tc– 30 °C (Tc is the lower critical solution temperature) size distribution is narrow and intermicellar interactions are weak; this is consistent with the existence of spherical micelles. In the temperature range Tc–30–Tc–15 °C, the excluded volume effects are dominant and increase in magnitude with increasing temperature. At the same time, the size distribution becomes broad. These results originate from elongation of micelles. Above ca.Tc–15 °C, the results can be explained by the decrease in the excluded volume effects and/or rapid increase in attractive interactions with increasing temperature. At still higher temperatures (Tc–4 °C–Tc), attractive interactions become dominant. Proton n.m.r. linewidths and viscosities have also been measured.