David F. O'Brien

Quasi-elastic light scattering determination of the size distribution of extruded vesicles

The size distribution of phospholipid vesicles prepared by the freeze thaw-extrusion method were determined by the non-perturbing technique of quasi-elastic light scattering (QELS) and compared to latex particles of known size. Multiangle QELS experiments were performed to avoid errors due to the angular dependence of the scattering function of the particles. The experimentally determined autocorrelation function was analyzed by multiple mathematical procedures, i.e. single exponential, CUMULANT, exponential sampling, non-negatively constrained least square and CONTIN, in order to select suitable models for vesicle characterization. The most consistent results were obtained with CUMULANT, non-negatively constrained least square and CONTIN. In many instances single exponential analysis gave comparable results to these procedures, which indicates the vesicles have a narrow distribution of sizes. The influence of filter pore size, extrusion pressure and lipid concentration on the size and size distribution of extruded vesicles was determined. Extrusion through 100-, 200- and 400-nm pore size filters produced a unimodal distribution of vesicles, with somewhat smaller diameters as the extrusion pressure increased. The larger the filter pore size, the more dependent the vesicle size was on applied pressure. The observed vesicle size was independent of the lipid concentration between 0.1 and 10 mg ml-1.

Visible light-induced destabilization of endocytosed liposomes

The potential biomedical utility of the photoinduced destabilization of liposomes depends in part on the use of green to near infrared light with its inherent therapeutic advantages. The polymerization of bilayers can be sensitized to green light by associating selected amphiphilic cyanine dyes, i.e. the cationic 1,1′‐dioctadecyl‐3,3,3′,3′‐tetramethylindocarbocyanine (DiI), or the corresponding anionic disulfonated DiI (DiI‐DS), with the lipid bilayer. The DiI sensitization of the polymerization of 1,2‐dioleoyl‐sn‐glycero‐3‐phosphoethanolamine/1,2‐bis[10‐(2′,4′‐hexadienoyloxy)‐decanoyl]‐sn‐glycero‐3‐phosphocholine liposomes caused liposome destabilization with release of encapsulated aqueous markers. In separate experiments, similar photosensitive liposomes were endocytosed by cultured HeLa cells. Exposure of the cells and liposomes to 550 nm light caused a net movement of the liposome‐encapsulated 8‐hydroxypyrene‐1,3,6‐trisulfonic acid (HPTS) from low pH compartment(s) to higher pH compartment(s). This suggests that photolysis of DiI‐labelled liposomes results in delivery of the contents of the endocytosed liposomes to the cytoplasm. The release of HPTS into the cytoplasm appears to require the photoactivated fusion of the labelled liposomes with the endosomal membrane. These studies aid in the design of visible light sensitive liposomes for the delivery of liposome‐encapsulated reagents to the cytoplasm.

Polymerization and domain formation in lipid assemblies

Lipid assemblies are arrays of noncovalently associated amphiphiles, i.e. supramolecular assemblies. They may be classified as supported or self-supported assmeblies. The advent of methods to polymerize these supramolecular assemblies has opened up opportunities for the creation of new materials. This review emphasizes the interaction of polymerization and lipid domain formation within supramolecular assemblies. The polymerization of amphiphilic assemblies can “lock in” preexisting lipid domains or create lipid domains from random mixtures depending on the nature of the polymerizable amphiphile. Lipid diacetylenes or fluorinated lipids provide a convenient means to form an unpolymerized immiscible mixture of reactive and nonreactive lipids in monolayers or bilayers. In contrast the polymerization of dienoyl-, sorbyl-, or acryloyl-substituted lipids can effectively induce the phase separation of unreactive lipids from the growing polymeric domains. Polymerization-induced lipid domains can endow bilayer vesicles with latent instability sites or can be used to concentrate membrane-associated electron or energy transfer cofactors. These polymeric materials suggest new approaches to the delivery of reagents as well as the transduction of light energy.

Anisotropies in the electrical properties of rod-like aggregates of liquid crystalline phthalocyanines: Direct current conductivities and field-effect mobilities

The direct current (dc) conductivities and organic field-effect transistor (OFET) characteristics of a class of octa-substituted liquid crystalline (discotic mesophase) phthalocyanines (Pcs) are discussed. These molecules self-organize into columnar aggregates with large coherence lengths (up to approximately 300 nm). Langmuir–Blodgett films of these molecules were horizontally transferred to either interdigitated microelectrodes (IME) or OFET substrates, so that current flow could be measured either parallel or perpendicular to the column axis. Twenty-eight bilayer films of these Pcs on the IME substrates showed anisotropies in dc conductivity up to 50:1, whereas similar Pc films showed anisotropies in field effect mobilities of approximately 10:1, for a variety of W/L ratios (source/drain dimensions and spacing). Field-effect mobilities of 1 to 5 × 10 -6 cm 2 ·V -1 ·s -1 were determined from OFET measurements, along the Pc column axis, whereas charge mobilities measured from the space charge limited current regime on the IME substrates were in the range of 10 -4 cm 2 ·V -1 ·s -1 . Conductive tip atomic force microscopy measurements on the apprximately 500-nm length scale showed that the conductivity anisotropy can be as high as 1000:1 when the Pc columns are intimately contacted to an adjacent Au bond pad.

Two-dimensional polymerization of lipid bilayers : Effect of lipid lateral diffusion on the rate and degree of polymerization

Hydrated amphiphiles can yield quite complex lyotropic liquid crystals such as the lamellar (bilayer) and nonlamellar phases. Lamellar structures can be solidlike or liquid crystalline. An important characteristic of these lamellar phases is the lateral diffusion of the lipids which increases by ca. 102 at the main phase transition, Tm. The rate (Rp) and degree (Xn) of polymerization were determined for polymerizable lipids in these two phases. A determination of the effect of temperature between 25 and 45 °C on the Rp of redox-initiated polymerization of mono-SorbPC bilayers showed a discontinuity near the Tm. The calculated activation energy, Ea, and frequency factor, A, for the polymerization at temperatures below Tm are 10 kcal/mol and 107, respectively. A similar calculation for the polymerization at temperatures above the Tm gave an Ea = 24 kcal/mol and A = 1016. The degree of polymerization, relative to poly(methyl methacrylate) standards, for bilayers of mono-SorbPC at temperatures above and below...

Chemical synthesis of a polymerizable bis-substituted phosphoethanolamine

Abstract The first chemical synthesis of a 1,2-bis-substituted polymerizable-sn-glycerol-3-phosphoethanolamine is reported. The synthetic approach utilizes the preparation of 1,2-bis(16-methy 1-2,4-(E,E)-octadecadienoyl)-3-(4-methoxybenzyl)-sn-glycerol via the acylation of 3-(4-methoxybenzyl)-sn-glycerol with 16-methyl-2,4-(E,E)-octadecadienoic acid. The deprotection of the 4-methoxybenzyl ether group is accomplished in high yield via Lewis acid catalyzed hydrolysis at −78°C. The final step is the phosphorylation of this 1,2-bis(dienoyl)-sn-glycerol.

Formation and spectroelectrochemical characterization of multilayer and submonolayer thin films of 2,3,9,10,16,17,23,24-octa(2-benzyloxy-ethoxy) phthalocyninato copper (CuPc(OC2OBz)8)

We review here the self-assembly, electrochemical and spectroelectrochemical properties of 2,3,9,10,16,17,23,24-octa(2-benzyloxyethoxy) phthalocyaninato copper (CuPc(OC2OBz)8), where terminal benzyl groups on the eight ethylene oxide side chains assist in forming unusually ordered, mechanically rigid thin films. New spectroscopic characterization of cast CuPc(OC2OBz)8 films is discussed in comparison with cast films of its metal-free analogue H2Pc(OC2OBz)8 and similar copper and dihydrogen phthalocyanines with benzyl — terminated triethylene oxide substituents, CuPc((OC2O)3Bz)8, and H2Pc((OC2O)3Bz)8, which do not demonstrate the same degree of ordering as CuPc(OC2OBz)8. AFM studies of horizontally transferred LB films of CuPc(OC2OBz)8 show column–column distances of ca 2.8 nm and confirm the high degree of ordering previously surmised from spectroscopic characterization of multilayer thin films. The oxidative electrochemistry of multilayer thin films prepared from these Pcs is strongly dependent on the chemical identity of the supporting electrolyte anion and on annealing of the thin films. Compliance of the films to counter-ion transport limits the extent of electrochemical doping. Preliminary studies of the oxidative electrochemistry of isolated CuPc(OC2OBz)8 aggregates (diluted into an electroinactive methyl arachidate thin film) on an electroactive, integrated optic waveguide (EA-IOW) are also presented. Monitoring the change in absorbance at 633 nm on the waveguide surface allows the determination of the onset potential for oxidation of the isolated aggregates, which appears to be less positive in potential versus that observed for multilayer Pc assemblies.

Organic photovoltaic cells containing discotic liquid crystalline phthalocyanines

We report on the photovoltaic properties of solar cells containing a new discotic liquid crystalline material (DL-CuPc) based on copper phthalocyanine. In addition to being soluble, these materials can self-organize into highly ordered structures which can lead to good transport properties that can potentially be superior to those of amorphous materials. Increase in short-circuit current density and fill factor was obtained by thermal annealing of spin-coated DL-CuPc layer in bi-layer solar cells based on junction between DL-CuPc and C60. These improvements are explained by change in structure and morphology upon thermal annealing.

Polymerization of supramolecular assemblies: Comparison of lamellar and inverted hexagonal phases

Abstract Since the first reports of the polymerization of hydrated bilayers in the early 1980s, a wide variety of polymerizable groups and lipids has been successfully employed. Among the various strategies explored for the polymerization of lamellar phases, a particularly useful method relies on the design of suitable polymerizable amphiphiles, which upon hydration form assemblies that can then be polymerized with retention of structure. We have recently extended this strategy to successfully polymerize the inverted hexagonal (H II ) phase. This report is the first comparison of radical chain polymerizations in lamellar and H II phases. The number average degree of polymerization of polymers obtained in both lamellar and H II phases depended strongly on the initiation chemistry, but were insensitive to the lipid phases. The immediate benefit of these studies is the knowledge that polymer size can be varied extensively in both phases in order to obtain different materials properties.

Effect of lipid structure on the formation of ordered cast films of polydiacetylenes

The efficient formation of polydiacetylenes in cast films demonstrates that a well-ordered multilayer structure can be readily formed by the casting of hydrated bilayer membranes onto solid supports.