Shah Md. Masum

Low-pH-Induced Transformation of Bilayer Membrane into Bicontinuous Cubic Phase in Dioleoylphosphatidylserine/Monoolein Membranes

Cubic biomembranes, nonbilayer membranes with connections in three-dimensional space that have a cubic symmetry, have been observed in various cells. Interconversion between the bilayer liquid-crystalline (L(alpha)) phase and cubic phases attracted much attention in terms of both biological and physicochemical aspects. Herein we report the pH effect on the phase and structure of dioleoylphosphatidylserine (DOPS)/monoolein (MO) membranes under a physiological ion concentration condition, which was revealed by small-angle X-ray scattering (SAXS) measurement. At neutral pH, DOPS/MO membranes containing high concentrations of DOPS were in the L(alpha) phase. First, the pH effect on the phase and structure of the multilamellar vesicles (MLVs) of the DOPS/MO membranes preformed at neutral pH was investigated by adding various low-pH buffers into the MLV suspension. For 20%-DOPS/80%-MO MLVs, at and below pH 2.9, a transition from the L(alpha) to cubic (Q(224)) phase occurred within 1 h. This phase transition was reversible; a subsequent increase in pH to a neutral one in the membrane suspension transformed the cubic phase into the original L(alpha) phase. Second, we found that a decrease in pH transformed large unilamellar vesicles of DOPS/MO membranes into the cubic phase under similar conditions. We have proposed the mechanism of the low-pH-induced phase transition and also made a quantitative analysis on the critical pH of the phase transition. This finding is the first demonstration that a change in pH can induce a reversible phase transition between the L(alpha) and cubic phases of lipid membranes within 1 h.

Design and facile synthesis of neoglycolipids as lactosylceramide mimetics and their transformation into glycoliposomes.

Neoglycolipids composed of disaccharide glycoside and phospholipid were designed and prepared as mimetics of lactosylceramide. The lactosyl- and N-acetyllactosaminyl-phospholipids (Lac-DPPA and LacNAc-DPPA) were enzymatically synthesized from lactose and LacNAc respectively by cellulase-mediated condensation with 1,6-hexanediol, followed by conjugation of the resulting glycosides and dipalmitoylphosphatidyl choline (DPPC) mediated by Streptomyces phospholipase D. Alternatively, allyl β-lactoside was ozonolyzed to give an aldehyde, which was condensed with dipalmytoyl phosphatidyl ethanolamine to afford a second type of glycolipid (Lac-DPPE). NMR spectroscopy indicated that the neoglycolipids behave differently in different solvent systems. X-ray diffraction clearly showed that multilamellar vesicles (MLVs) of Lac-DPPE and Lac-DPPA-MLV are in the bilayer gel phase at 20 °C, whereas those of Lac-DPPE-MLV were in the lamellar liquid-crystalline phase at 50 °C. Differential scanning calorimetry showed that Lac-DPPE-MLV had complex thermotropic behavior depending on the incubation conditions. After a long incubation at 10 °C, endothermic transitions are observed at 39.6, 42.3 °C, and 42.9 °C. These neoglycolipids have the ability to trap calcein, a chelating derivative of fluorescein, in MLVs and showed specific binding to lectin in plate assays using fluorescently labeled compounds.

The effect of peptides and ions interacting with an electrically neutral membrane interface on the structure and stability of lipid membranes in the liquid-crystalline phase and in the liquid-ordered phase

We investigated the effects of a de novo designed peptide, WLFLLKKK (peptide-1) and La3+, which can bind with the electrically neutral lipid membrane interface, on the stability of the phosphatidylcholine (PC) membrane in the Lα phase and that of the liquid-ordered (lo) phase membranes. The results of spacing of the multilamellar vesicle and shape changes of the giant unilamellar vesicle (GUV) indicate that the peptide-1 can be partitioned into the membrane interface in the Lα phase but not into that in the lo phase. La3+ induced shape changes of GUVs of the lo phase membrane, which are the same as those of GUVs in the Lα phase. This indicates that the binding of La3+ induced an increase in the lateral compression pressure of the membrane, which decreased the surface area of the membrane in the lo phase. The difference of the membrane interface between the Lα phase and the lo phase is discussed.

Effect of Electrostatic Interactions on Phase Stability of Cubic Phases of Biomembranes

We investigated effect of electrostatic interactions due to surfacecharges on structures and stability of cubic phases of monoolein (MO)membrane using the small-angle X-ray scattering method. Firstly, wechanged the surface charge density of the membrane by usingdioleoylphosphatidic acid (DOPA). As increasing DOPA concentration in themembrane at 30 wt % lipid concentration, a Q224 to Q229 phasetransition occurred at 0.6 mol % DOPA, and at and above 25 mol %, DOPA/MOmembranes were in the Lα phase. NaCl in the bulk phase reduced theeffect of DOPA. These results indicate that as the electrostaticinteractions increase, the most stable phase changes as follows: Q224⇒ Q229 ⇒ Lα. The increase in DOPAconcentration reduced the absolute value of spontaneous curvature of themembrane, | H0 |. Secondly, we changed the surface charge of themembrane by adding a de novo designed peptide, which has netpositive charges and a binding site on the electrically neutral membraneinterface. The peptide-1 (WLFLLKKK) induced a Q224 to Q229phase transition in the MO membrane at low peptide concentration. As NaClconcentration increases, the MO/peptide-1 membrane changed from Q229to Q224 phase. The increase in peptide-1 concentration reduced |H0 |. Based on these results, the stability of the cubic phases and themechanism of phase transition between cubic phase and Lα phase arediscussed.

Effects of Surface Charge Density of Lipid Membranes on the Pore Formation Induced by Magainin 2

Interactions of antimicrobial peptides with lipid membranes have been investigated using a suspension of small liposomes, and their details remain unclear. Using the single GUV method, we have succeeded in revealing elementary processes of the pore formation in lipid membranes induced by magainin 2 in our previous paper (Biochemistry, 44, 15823, 2005). In this report, to elucidate the mechanism of the magainin 2-induced pore formation, we investigated the effect of surface charge density of membranes on the pore formation. To change the surface charge density, we controlled negatively charged DOPG concentration in DOPG/DOPC membrane from 30 to 60 mol%. We found that, in all kinds of GUVs, magainin 2 induced a rapid leakage of calcein from single GUVs, showing that magainin 2 formed pores in the membrane. For GUVs with the same charge density, the fraction of leaked GUV, PLS, increased with time, and PLS at a fixed time increased with magainin 2 concentration. The magainin 2 concentration at PLS = 0.5 at a fixed time increased with a decrease in the surface charge density, indicating that higher concentrations of magainin 2 in a buffer were required to induce the pore formation in GUVs with lower surface charge density. Using the Gouy-Chapman theory, we obtained magainin 2 concentration in the membrane interface, assuming the intrinsic binding constant of magainin 2 with DOPG/DOPC membranes. On the basis of these results, we discuss the role of magainin 2 concentration in the membrane interface in the magainin 2-induced pore formation.

Electrostatic Effects in Phase Transitions of Biomembranes between Cubic Phases and Lamellar Liquid‐Crystalline (Lα) phase

Elucidation of the mechanisms of transitions between cubic phase and liquid‐crystalline (Lα) phase, and between different IPMS cubic phases, are essential for understanding of dynamics of biomembranes and topological transformation of lipid membranes. Recently, we found that electrostatic interactions due to surface charges of lipid membranes induce transition between cubic phase and Lα phase, and between different IPMS cubic phases. As electrostatic interactions increase, the most stable phase of a monoolein (MO) membrane changes: Q224 ⇒ Q229 ⇒ Lα. We also found that a de novo designed peptide partitioning into electrically neutral lipid membrane changed the phase stability of the MO membranes. As peptide‐1 concentration increased, the most stable phase of a MO membrane changes: Q224 ⇒ Q229 ⇒Lα. In both cases, the increase in the electrostatic repulsive interaction greatly reduced the absolute value of spontaneous curvature of the MO monolayer membrane. We also investigated factors such as poly (L‐lysine...