Ryosuke Shimozawa

Interaction of surfactants with vesicle membrane of dipalmitoylphosphatidylcholine. Effect on gel-to-liquid-crystalline phase transition of lipid bilayer.

The gel-to-liquid-crystalline phase transition of dipalmitoylphosphatidylcholine (DPPC) vesicle membrane was observed in the presence of various types of surfactants; sodium alkylsulfates, alkyltrimethylammonium bromides, alkanoyl-N-methylglucamides, and hexaethyleneglycol mono n-dodecyl ether. The phase transition was monitored by a change in scattered light intensity of the lipid suspension. For all the surfactants examined, the phase transition temperature was depressed linearly with the surfactant concentration in the measured concentration range, from which the partition coefficient, K, of the surfactant between bulk solution and lipid membrane was estimated. Except alkyltrimethylammonium bromides, log K and log CMC showed a linear relationship, which indicates that the driving force to transfer the surfactant from bulk solution to lipid membrane is a hydrophobic interaction. The addition of surfactants increased the transition width. The extent of widening the transition width was in the order of sodium alkylsulfate greater than alkyltrimethylammonium bromides greater than hexaethyleneglycol mono n-dodecyl ether; in the case of alkanoyl-N-methylglucamides, the transition width was not affected by the addition. These effects on the transition width was interpreted qualitatively in terms of the cooperativity of the transition.

Systematic study on the solubilization of phospholipid vesicles by various surfactants

Abstract The solubilization of phospholipid vesicles by surfactants were investigated for systematically selected lipid—surfactant combinations. The solubilization process was followed by the change in 90° scattered-light intensity resulting from the stepwise addition of surfactant solution to the vesicle preparation. Two break points appeared in the turbidity curve, which allowed a determination of the surfactant concentrations corresponding to the onset and completion of the solubilization. From the lipid concentration dependence of these critical surfactant concentrations, the effective surfactant to lipid molar ratios in the surfactant-saturated lipid bilayer, Rsat, and in the lipid-saturated mixed micelle, Rsol, were estimated according to the “three-stage” model proposed for the vesicle solubilization. For phosphatidylcholine vesicles, both Rsol and Rsat exhibited no significant dependence on the alkyl chain length and head group type of the surfactants. When the acyl chain length of the lipids was shortened, the Rsol value decreased, while Rsat remained unaltered. For negatively charged phosphatidic acid vesicles, Rsol depended strongly on the head group charge of the surfactants. The evaluation of the solubilizing power of surfactants in terms of Rsol values leads to the conclusion that the solubilizing power for electrically neutral phosphatidylcholine vesicles is primarily determined by the lipid acyl chain length rather than the alkyl chain length and head group type of the surfactants. The viscosity measurements demonstrated that the rod-like or disc-like mixed micelles initially formed by the lipid solubilization transform to spherical micelles at higher concentration of the surfactants.

Chemical relaxation studies in micellar solutions of sodium alkyl sulfates

Abstract Pressure-jump-induced chemical relaxation measurements were carried out with solutions of sodium decyl sulfate (SDeS), sodium dodecyl sulfate (SDS), and sodium tetradecyl sulfate (STS). In pure SDeS solution no relaxation effect could be observed in the time range of the pressure-jump technique, whereas by adding a small amount of decyl alcohol, typical relaxations were observed which gave τ −1 increasing linearly with SDeS concentration. In pure SDS solution only one relaxation was observed which yielded τ −1 almost independent of SDS concentration. In pure STS solution, two well-separated relaxation processes were observed. The slow relaxation time ( τ 2 ) was almost independent of the concentration, while the reciprocal of the fast relaxation time ( τ 1 −1 ) increased linearly with the concentration. τ in pure SDS and in SDeS with added decyl alcohol and τ 2 in pure STS were interpreted as those associated with the micelle-forming process, while τ 1 in pure STS was interpreted as that associated with the so-called exchange process. The effects of added long chain alcohol on τ were also studied for SDS and STS. A discussion of the present results on STS was attempted according to several models which were proposed previously.

The transition of α-helix to β-structure of poly(d-lysine) induced by phosphatidic acid vesicles and its kinetics at alkaline pH

Abstract Static and dynamic circular dichroism (CD) measurements were carried out for poly( l -lysine) in suspensions of dilauroylphosphatidic acid (DLPA) vesicles at alkaline pH (8–11.5). The static experiments demonstrated that the α-helix of poly( l -lysine) induced by deprotonation in alkaline solutions is transformed to β-structure by the addition of DLPA vesicles. Stopped-flow CD measurements for such order-to-order transition revealed that the rate determining step is the unfolding process of α-helix to random coil. Previously, we have reported the conformational change of poly( l -lysine) induced by DLPA vesicles at neutral pH, where the β-structure transition from random coil was observed. Thus two types of transition of poly( l -lysine) are observed depending on bulk pH, i.e., from random coil to β-structure and from α-helix to β-structure. So far the phospholipid-induced conformations of poly( l -lysine) were interpreted in terms of counterbalance between the positively charged terminals of the lysyl chains and the negative headgroups of the phospholipid in vesicle. However, present work indicates the direct interaction other than electrostatic interaction between the lysyl chain and phosphate groups of the lipid.

Effect of sodium octanoate and sodium perfluorooctanoate on gel-to-liquid-crystalline phase transition of dipalmitoylphosphatidylcholine vesicle membrane

The gel-to-liquid-crystalline phase transition of dipalmitoylphosphatidylcholine (DPPC) vesicle membrane was measured in the presence of sodium octanoate (SO) (pH 3 and 10) and sodium perfluorooctanoate (SPFO) (pH uncontrolled) by monitoring the scattered light intensity of the vesicle suspension. The phase transition temperature, Tm, decreased linearly with the concentration of added SO within the measured concentration range; the uncharged form of SO (pH 3) was much more effective for the depression of Tm than the charged form (pH 10). On the other hand, with increasing SPFO concentration, levelling off of Tm was observed after depression at an initial stage. From the depression of Tm, the partition coefficients, K, of these surfactants between bulk solution and DPPC vesicle membrane were estimated and compared with those obtained previously for other surfactant systems. The value of K for charged SO fell on the straight line of log K vs. Nc plot for anionic surfactants, where Nc is the carbon number of the hydrocarbon chain of surfactants, whereas K for uncharged SO showed a large positive deviation from the straight line of the plot for non-ionic surfactants. The latter suggested that some specific interaction, presumably hydrogen bond formation, may act between the protonated carboxyl group of SO and the lipid head group. The K value estimated for SPFO was much larger than that for charged SO. This difference in the affinity for the lipid bilayer between fluorocarbon surfactant and hydrocarbon surfactant may be attributed to the difference in their hydrophobicity.

Miscibility of binary phospholipid mixtures under hydrated and unhydrated conditions. I. Phosphatidic acids with different acyl chain length

Abstract Phase diagrams for binary mixtures of diacylphosphatidylethanolamines (PE) with different acyl chain length (n = 14, 16, and 18, where n represents the number of carbon atoms per chain) were constructed by differential scanning calorimetry in the presence of and absence of added water, in order to examine the miscibility of these phospholipid species under fully hydrated and non-hydrated conditions. The phase diagrams were analyzed according to a thermodynamic relationship derived by applying the Bragg-Williams approximation to the non-ideality of mixing. The non-ideality parameters of mixing, ϱ0 were estimated for binary mixtures of PE species in hydrated liquid-crystalline (ϱ0(L)) and gel (ϱ0(S)) bilayers, and in non-hydrated liquid (ϱ0(L)) and solid (ϱ0(S)) phases, based on which the mixing behavior of PE species in various phases was discussed. For non-hydrated mixtures, ϱ0(L) = 0 was obtained regardless of the chain length difference between the two components, demonstrating that the two components form an ideal solution in non-hydrated liquid phase. With other phases, positive values were obtained for ϱ0 Both ϱ0(L) and ϱ0(S) for hydrated mixtures were larger than the corresponding values for non-hydrated mixtures, which indicates that the miscibility of PE with different chain length is poorer in hydrated bilayer than non-hydrated bulk phase. This difference in the mixing behavior between hydrated and non-hydrated PE mixtures was interpreted qualitatively in terms of the difference in pair-interactions between the hydrated lipid bilayer and the bulk lipid phase.

Conformational study of poly(L-lysine) interacting with acidic phospholipid vesicles

Circular dichroism measurements were carried out on poly(L-lysine) in the presence of vesicles of the negatively charged phospholipids, phosphatidylserine (PS; from bovine brain), phosphatidic acid (PA; prepared from egg yolk lecithin) and dimyristoylphosphatidylglycerol (DMPG). PS vesicles induced a conformational change in poly(L-lysine) from random coil to alpha-helix structure in 5 mM Tes (pH 7.0), whereas PA vesicles gave rise to beta-structure in the same buffer. The fraction of alpha-helix, F alpha (or beta-structure, F beta), increased with increasing PS (or PA) concentration, reaching a saturation value of about 0.7 (or about 1). Mixed vesicles comprising PS and dilauroylphosphatidylcholine (DLPC) also induced alpha-helix conformation, however, the saturation value of F alpha diminished with decreasing PS content in mixed vesicles. On the other hand, the spectral patterns for poly(L-lysine) in DMPG vesicle suspensions exhibited the coexistence of alpha-helix and beta-structure. Both F alpha and F beta increased with DMPG concentration and reached saturation values of about 0.5. Mixed vesicles composed of DMPG and dimyristoylphosphatidylcholine (DMPC) led to a reduction in F beta, while F alpha remained almost constant. The diversity in ordered structure induced by different phospholipid vesicles suggests the participation of lipid head groups in determining the secondary structure of poly(L-lysine) adsorbed on the vesicular surface.

Stability of DLPA/DLPC mixed vesicles against divalent cation-induced aggregation: Importance of the hydration force

Abstract Aggregation kinetics were investigated for DLPA/DLPC mixed vesicles induced by the addition of several divalent cations (Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ). The stability ratio, W , evaluated from the initial velocity of the aggregation increased rapidly above the threshold fraction of DLPC in the mixed vesicle. This trend became more pronounced with the temperature rise. The variation of W with the composition of mixed vesicles was explained quantitatively by introducing hydration respulsion into the interversicular interaction in addition to van der Waals attraction and electrostatic repulsion. The stability ratio enhanced by the temperature rise was also consistent with the picture drawn for the temperature dependence of the hydration interaction. These results demonstrate the importance of the hydration force for the stability of vesicle dispersions.

Kinetic study on micelle formation of tetradecylpyridinium salts

Abstract Pressure-jump and temperature-jump techniques were used to investigate the kinetics of micelle formation in aqueous solutions of tetradecylpyridinium bromide and chloride. In these systems, only the slow relaxation process (relaxation time τ 2 ) was observed. The behavior of τ 2 was similar to that for the homologs, dodecylpyridinium bromide and chloride. A discussion of the present results was given based on the previously proposed model for micelle formation in which a single rate-determining step was assumed in the micellization-dissolution multiple-step process.

Surfactant partition between bulk water and DPPC vesicle membrane : solid-gel vs. liquid-crystalline membrane

The main phase transition temperature, Tm, of dipalmitoylphosphatidylcholine (DPPC) vesicle membrane was measured in the presence of the cationic surfactants tetradecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide. Variation of the perturbing effect of these surfactants on Tm with the lipid concentration was analyzed according to the theory recently proposed by Kaminoh et al. (Y. Kaminoh, C. Tashiro, H. Kamaya and I. Ueda (1988) Biochim. Biophys. Acta 946, 215-220), and the partition coefficients of the surfactant into solid-gel and liquid-crystalline membranes were estimated.