Membranes of Cationic Gemini Lipids based on Cholesterol with Hydroxyl Headgroups and their Interactions with DNA and Phospholipid

Two series of cholesterol-based cationic gemini lipids with and without hydroxyl functions at the headgroups possessing different lengths of polymethylene -(CH2)(n)-] (n = 3, 4, 5, 6, 12) spacer have been synthesized. Each gemini lipid formed stable suspension in water. The suspensions of these gemini lipids in water were investigated using transmission electron microscopy, dynamic light scattering, zeta potential measurements and X-ray diffraction to characterize the nature of the individual aggregates formed therein. The aggregation properties of these gemini lipids in water were found to strongly depend upon the length of the spacer and the presence of hydroxyl group at the headgroup region. Lipoplex formation (DNA binding) and the release of the DNA from such lipoplexes were performed to understand the nature of interactions that prevail between these cationic cholesterol aggregates and duplex DNA. The interactions between such gemini lipids and DNA depend both on the presence of OH on the headgroups and the spacer length between the headgroups. Finally, we studied the effect of incorporation of each cationic gemini lipid into dipalmitoyl phosphatidylcholine vesicles using differential scanning calorimetry. The properties of the resulting mixed membranes were found again to depend upon the nature of the headgroup and the spacer chain length.

Diacylglycerol Kinase Is Stimulated by Arachidonic Acid in Neural Membranes.

The effect of arachidonic acid (AA) on the activity of diacylglycerol (DG) kinase in neural membranes was investigated. When rat brain cortical membranes were incubated with 0.5 mM dipalmitin and [gamma-P-32]ATP, formation of phosphatidic acid (PA) was observed. It was linear up to 5 min, and the initial rate was similar to 1.0 nmol/min/mg of protein. The DG kinase activity was stimulated twofold by 0.25 mM AA. The stimulation was apparent at the earliest time point measured (1 min) and with the lowest concentration of AA tested (62.5 mu M). The stimulation was proportional to the concentration of AA up to 250 mu M. AA was the most potent stimulator of DG kinase, and linolenic acid showed similar to 40% stimulation. Oleic acid showed no effect, whereas linoleic and the saturated fatty acids tested were inhibitory. AA stimulation of DG kinase was observed only with membranes of cerebrum, cerebellum, and myelin and not with brain cytosol or liver membranes. AA also stimulated the formation of PA in the absence of added dipalmitin (endogenous activity) with membranes prepared from whole brain. DG kinase of neural membranes was extracted with 2 M NaCl, which on dialysis yielded a precipitate. Both the precipitate and the supernatant showed DG kinase activity, but only the enzyme in the precipitate was stimulated by AA at concentrations as low as 25 mu M. It is suggested that AA, through its effect on DG kinase, regulates the level of DG in neural membranes, which in turn regulates protein kinase C activity.

Regulation of diacylglycerol kinase in rat brain membranes by docosahexaenoic acid

The effect of docosahexaenoic acid (DHA) on the diacylglycerol kinase (DG kinase) activity in rat brain membranes was investigated. DHA at 500 mu M concentration, stimulated the enzyme activity by about 2 fold. This effect was concentration-and time-dependent and was observed after very short periods of incubation (one min). DHA stimulation of DG kinase was observed only with rat brain membranes, and not with rat brain cytosol or rat liver membranes. Treating the rat brain membranes with phospholipase A(2) which released free fatty acids including DHA, significantly stimulated the DG kinase activity. It is concluded that DHA through its stimulatory effect on DG kinase may regulate the signalling events in growth-related situations in the brain such as synaptogenesis.

Verifying scalings for bending rigidity of bilayer membranes using mesoscale models

The bending rigidity kappa of bilayer membranes was studied with coarse grained soft repulsive potentials using dissipative particle dynamics (DPD) simulations. Using a modified Andersen barostat to maintain the bilayers in a tensionless state, the bending rigidity was obtained from a Fourier analysis of the height fluctuations. From simulations carried out over a wide range of membrane thickness, the continuum scaling relation kappa proportional to d(2) was captured for both the L-alpha and L-beta phases. For membranes with 4 to 6 tail beads, the bending rigidity in the L-beta phase was found to be 10-15 times higher than that observed for the L-alpha phase. From the quadratic scalings obtained, a six fold increase in the area stretch modulus, k(A) was observed across the transition. The magnitude of increase in both kappa and k(A) from the L-alpha to the L-beta phase is consistent with current experimental observations in lipid bilayers and to our knowledge provides for the first time a direct evaluation of the mechanical properties in the L-beta phase.

Surfactant lipids containing aromatic units produce vesicular membranes with high thermal stability

Six new vesicle-forming, cationic surfactant lipids are synthesized. Four of them contain 'flat' aromatic units at different locations of hydrophobic segments. In order to estimate the influence of aromatic units in the lipid monomer two other surfactant lipids of related structure with n-butyloxy units in the places of aromatic groups were also prepared. Transmission electron microscopy confirmed the vesicular membrane formation from these newly synthesized lipids. DSC or temperature-dependent keto-enol tautomerism of benzoylacetanilide-doped vesicles reveal a remarkable increase in the thermal stability of the membranes formed from aromatic surfactant lipids in contradistinction to their counterparts that contain n-butyloxy units. The enhanced thermal stability originates presumably as a consequence of inter-monomer stacking.

Synthesis, Thermotropic Behavior, and Permeability Properties of Vesicular Membranes Composed of Cationic Mixed-Chain Surfactants

In order to elucidate the role of the linkage region that connects polar headgroups with hydrophobic segments in a lipid monomer, cationic mixed-chain amphiphiles containing acyl and alkyl hydrophobic segments connected at the level of Me(2)N(+) headgroups 2a-d were synthesized. Related dialkyldimethyl-ammonium ion surfactants 1a-e and diacyl systems 3a-c were also synthesized. Despite mismatch in the connector region, amphiphiles 2a-d form bilayer vesicles like their dialkyl and diacyl counterparts, as revealed by electron microscopy. Introduction of an ester connector function between the polar and hydrophobic parts raises the phase transition temperature (T-m), transition enthalpies, and resistance to ion permeation. Consideration of energy minimized conformations points toward the importance of differences in the depth of chain penetration into the putative bilayer.

The physics of active membranes

We review recent work on the physical properties of model fluid membranes in nonequilibrium situations resembling those encountered in the living cell and contrast their properties with those of the more familiar membranes at thermal equilibrium. We survey models for the effect of (i) active pumps and (ii) active fission–fusion processes encountered in intracellular trafficking on the stability and fluctuations of fluid membranes. Our purpose is twofold: to highlight the exciting nonequilibrium phenomena that arise in biological systems, and to show how some crucial features of living systems, namely dissipative energy uptake and directed motion, can fruitfully be incorporated into physical models of biological interest.

Thermal Lipid Order−Disorder Transitions in Mixtures of Cationic Cholesteryl Lipid Analogues and Dipalmitoyl Phosphatidylcholine Membranes

Two types of cationic cholesteryl amphiphiles, one where the headgroup is attached to the steroid by an ester linkage and the second by an ether linkage, were synthesized. A third type of cholesteryl lipid bearing an oligoethylene glycol segment was also prepared. Each of these synthetic lipids generated vesicle-like aggregates with closed inner aqueous compartments from their aqueous suspensions. We examined their interaction with L-α-dipalmitoyl phosphatidylcholine (DPPC) membranes using fluorescence anisotropy, transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). When included in membranes, the synthetic cholesteryl lipids were found to quench the chain motion of the acyl chains of DPPC. This suggests that these cationic cholesteryl derivatives act as filler molecules despite modification at the headgroup level from the molecular structure of natural cholesterol. Careful analyses of DSC and fluorescence anisotropy data suggest that the nature of perturbation induced by each of these cationic cholesterol derivatives is dependent on the details of their molecular structure and provides significant information on the nature of interaction of these derivatives with phospholipid molecules. In general, amphiphiles that support structured water at the interfacial region tend to rigidify the fluid phase more than others. Importantly, these cholesteryl amphiphiles behave less like cholesterol in that their incorporation in DPPC not only abolishes the phase transition but also depresses the phase transition temperature.

Synthesis and characterization of novel cationic lipid and cholesterol-coated gold nanoparticles and their interactions with dipalmitoylphosphatidylcholine membranes

Novel gold nanoparticles bearing cationic single-chain, double-chain, and cholesterol based amphiphilic units have been synthesized. These nanoparticles represent size-stable entities in which various cationic lipids have been immobilized through their thiol group onto the gold nanoparticle core. The resulting colloids have been characterized by UV-vis, (1)H NMR, FT-IR spectroscopy, and transmission electron microscopy. The average size of the resultant nanoparticles could be controlled by the relative bulkiness of the capping agent. Thus, the average diameters of the nanoparticles formed from the cationic single-chain, double-chain, and cholesterol based thiolate-coated materials were 5.9,2.9, and 2.04 nm, respectively. We also examined the interaction of these cationic gold nanoparticles with vesicular membranes generated from dipalmitoylphosphatidylcholine (DPPC) lipid suspensions. Nanoparticle doped DPPC vesicular suspensions displayed a characteristic surface plasmon band in their UV-vis spectra. Inclusion of nanoparticles in vesicular suspensions led to increases in the aggregate diameters, as evidenced from dynamic light scattering. Differential scanning calorimetric examination indicated that incorporation of single-chain, double-chain, and cholesteryl-linked cationic nanoparticles exert variable effects on the DPPC melting transitions. While increased doping of single-chain nanoparticles in DPPC resulted in the phases that melt at higher temperatures, inclusion of an incremental amount of double-chain nanoparticles caused the lowering of the melting temperature of DPPC. On the other hand, the cationic cholesteryl nanoparticle interacted with DPPC in membranes in a manner somewhat analogous to that of cholesterol itself and caused broadening of the DPPC melting transition.

Melting and mechanical properties of polymer grafted lipid bilayer membranes

The influence of polymer grafting on the phase behavior and elastic properties of two tail lipid bilayers have been investigated using dissipative particle dynamics simulations. For the range of polymer lengths studied, the L(c) to L(alpha) transition temperature is not significantly affected for grafting fractions, G(f) between 0.16 and 0.25. A decrease in the transition temperature is observed at a relatively high grafting fraction, G(f) = 0.36. At low temperatures, a small increase in the area per head group, a(h), at high G(f) leads to an increase in the chain tilt, inducing order in the bilayer and the solvent. The onset of the phase transition occurs with the nucleation of small patches of thinned membrane which grow and form continuous domains as the temperature increases. This region is the co-existence region between the L(beta)(thick) and the L(alpha)(thin) phases. The simulation results for the membrane area expansion as a function of the grafting density conform extremely well to the scalings predicted by self-consistent mean field theories. We find that the bending modulus shows a small decrease for short polymers (number of beads, N(p) = 10) and low G(f), where the influence of polymer is reduced when compared to the effect of the increased a(h). For longer polymers (N(p) > 15), the bending modulus increases monotonically with increase in grafted polymer. Using the results from mean field theory, we partition the contributions to the bending modulus from the membrane and the polymer and show that the dominant contribution to the increased bending modulus arises from the grafted polymer. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3631940]

Polymer blend based photovoltaic devices.

Solar cells on thin conformable substrates require conventional plastics such asPS and PMMA that provide better mechanical and environmental stability with cost reduction. We can also tune charge transfer between PPV derivatives and fullerene derivatives via morphology control of the plastics in the solar cells. Our group has conducted morphology evolution studies in nano- and microscale light emitting domains in poly (2-methoxy, 5-(2'-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) and poly (methyl methacrylate) (PMMA) blends. Our current research has been focused on tricomponent-photoactive solar cells which comprise MEH-PPV, PMMA, and [6,6]-phenyl C61-butyric acid methyl ester (PCBM, Figure 1) in the photoactive layer. Morphology control of the photoactive materials and fine tuning of photovoltaic properties for the solar cells are our primary interest. Similar work has been done by the Sariciftci research group. Additionally, a study on inter- and intramolecular photoinduced charge transfer using MEH-PPV derivatives that have different conjugation lengths (Figure 1, n=1 and 0.85) has been performed.

Adherent Nanoporous Anatase TiO2 Membranes on Stainless Steel Substrates

Nanostructured TiO2 is one of the most commonly used materials in photocatalytic applications and photochemical solar cells. This article describes a method to synthesize nanoporous anatase TiO2 membranes directly on stainless steel (SS), an easily available substrate by anodization to form amorphous TiO2 and a subsequent heat treatment to convert it into anatase, the photoactive phase. To obtain adherent membranes with interfaces that are resistant to peeling, both anodization and heat treatment parameters need to be optimized to obtain a heterostructure that contains a Ti film between the TiO2 membrane and the substrate.