Diacetylenic lipid microstructures: structural characterization by X-ray diffraction and comparison with the saturated phosphatidylcholine analogue

The concept of strongly interacting groups in self-assembly of soft matter

Amphiphilic molecules in solution typically produce structures coming from cooperative interactions of many synergetically acting functional units. If all essential interactions are weak, the structure can be treated theoretically based on a free energy expansion for small interaction parameters. However, most self-assembling soft matter systems involve strong specific interactions of functional units leading to qualitatively new structures of highly soluble micellar or fibrillar aggregates. Here we focus on the systems with the so-called strongly interacting groups (SIGs) incorporated into unimer molecules and discuss the effects of packing frustrations and unimer chirality as well as the origins of spontaneous morphological chirality in the case of achiral unimers. We describe several theoretical approaches (overcoming the limitations of weak interaction models) including the concepts of super-strong segregation, geometrical mismatch and orientational frustration. We also review some recently developed phenomenological theories of surfactant membranes and multiscale hierarchical approaches based on all-atomic modeling of packing structures of amphiphilic molecules with SIGs. In particular, we discuss self-assembling structures in systems possessing simultaneously several distinct types of SIGs: solutions of beta-sheet oligopeptides (showing different fibrillar morphologies), aromatic diamide-ester molecules (forming membranes, helical ribbons and tubules), and triarylamine amide derivatives (producing light-controlled supramolecular nanowires).

Polyethylene glycol as a hydration agent in oriented membrane bilayer samples

Techniques such as NMR, ESR, fluorescence depolarization, and neutron scattering are commonly used to investigate the physical properties of membranes. Oriented membrane bilayer systems (single crystals) are often employed in these investigations. It is important to know and be able to control the level of hydration in these samples. In particular, one must have confidence that a sample is in fact "fully hydrated" and remains so during the course of the experiment. Full hydration is difficult to obtain by hydrating oriented samples using water-saturated vapor. An alternative method for hydrating oriented samples is to surround the oriented sample by a polymer solution. Higher hydration levels are achieved using this method. Three nuclear magnetic resonance studies using headgroup deuterated 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) were done to compare the hydration level of oriented headgroup samples surrounded by a polymer/water solution and fully hydrated multibilayer dispersions. Transition temperatures, quadrupolar splittings (at 50 degrees C) and spin-lattice relaxation times (at 50 degrees C) were measured. The simple tests of the transition temperature and quadrupolar splitting to determine full hydration, as my results show, are not sufficient. In this paper I demonstrate that more fully hydrated samples can easily be achieved by surrounding the oriented sample with a 5 wt% polyethylene glycol/water solution than by hydrating in water saturated vapor.

Synthesis, properties, and mechanistic insight into the self-assembly of a lamellar fibrous superstructure from a synthetically simple discotic molecule

The discotic molecule 4-chloro-2,6-bis(octadecylamino)-pyrimidine-5-carbaldehyde, displays gelation behavior in dodecane, heptane, chloroform, and dichloromethane. The aggregation behavior of this material was studied by dynamic light scattering, differential scanning calorimetry, scanning electron microscopy, polarized optical microscopy, small-angle X-ray diffraction, and wide-angle X-ray diffraction techniques. Combined with molecular modeling calculations, Fourier transform infrared, and 1H NMR studies, we propose a mechanism for the self-assembly of this fibrous lamellar architecture. Notably, we have shown that the fibers grow via stacking interactions along their main axis, via hydrogen bonding along their short axis, and via van der Waals interactions (lamellae) along the third axis. This type of morphology is desirable since it provides an opportunity to synthetically control and optimize mechanical, electrical, optical, and transport properties along the length of the fiber.

Buckling instability of lipid tubules with multibilayer walls under local radial indentation

The mechanical behavior of self-assembled lipid tubules is an important property which determines their suitability for technological applications. We study the instability of multibilayer lipid tubules (with wall thickness t and external radius R(ext)) beyond elastic response under local radial atomic force microscopy indentations. A discontinuity in force-distance curves associated with the buckling instability of lipid tubules is observed. The critical force at which lipid tubules undergo a buckling transition linearly scales as t/R(ext). In addition, a reduced critical buckling force is found to extend a distance of approximately 1 microm from the end of lipid tubules.

Liquid-crystal imaging of molecular-tilt ordering in self-assembled lipid tubules

Self-assembled cylindrical tubules of chiral phospholipids are interesting supramolecular structures. Understanding the molecular-tilt order is a key step in controlling the size and shape of the tubules and designing new functional materials. The current theories based on the chiral interactions, coupled with molecular tilt, have predicted that the tubules could have both uniform and modulated tilt states. Here, we image the molecular-tilt order in the self-assembled tubules of a chiral phospholipid by using liquid crystals as an optical amplification probe. We demonstrate that the organization of the molecular-tilt azimuth in the lipid tubules can induce an azimuthal orientation in the liquid crystals. Both uniform and modulated tilt states of the lipid tubules are observed after liquid-crystal optical amplification.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Formation of high axial ratio microstructures from peptides modified with glutamic acid dialkyl amides

A growing number of amphiphiles are known to form high axial ratio microstructures (HARMs) such as the hollow cylindrical microstructures called lipid tubules. As a prelude to exploring the potential of HARMs formed from lipopeptides in controlled release drug delivery, several microstructure formation conditions were investigated. We report the preparation of several glutamic acid dialkyl amides with varying alkyl chain lengths bearing a verity of peptides (1-4 amino acids) [peptide-Glu-(NHCnH2n+1)2, n=12, 14, 16]. These surfactants have been rapidly and efficiently converted into HARMs in aqueous buffer at physiological pH and ionic strength, or in buffer containing MeOH or EtOH. Helical ribbons and tubular HARMs were produced that were stable for as long as 6 months below the phase transition temperatures of the compounds. To estimate the stability of HARMs in vivo, HARMs formed from (Pro)3-Glu(NHC16H33)2 were incubated with DOPC liposomes or fetal calf serum at 40 degreesC. HARM size and shape did not change significantly, suggesting that such lipopeptide particles can retain their morphology long enough in vivo to be useful as drug delivery vehicles.

Zero-order interfacial enzymatic degradation of phospholipid tubules

The first study of enzymatic hydrolysis of phospholipid tubules is reported. Phosphatidylcholines with acyl chains containing diacetylene groups are known to form tubular microstructures in which the lipids are tightly packed and crystalline. These tubules can be used to probe the role of microstructural form in the mechanics of interfacial enzymatic degradation by such enzymes as phospholipase A2 (PLA2). Hydrolysis by PLA2 may occur most rapidly in regions having the greatest number of bilayer packing defects, such as those that must be found at tubule ends. A microstructure that degrades primarily from its ends should exhibit zero-order kinetics, because the area of the degrading tubule and remains constant as the length of the microstructure decreases. Free fatty acid concentration was measured to follow the generation of PLA2 hydrolysis products in suspensions of diacetylenic phospholipid tubules. The kinetics of tubule hydrolysis were essentially zero-order until conversion was complete, as predicted. However, microscopy of partially hydrolyzed tubules revealed the formation of multiple discrete anionic product domains along the length of degrading tubules as well as in insoluble reaction product microstructures. Furthermore, the rate of tubule hydrolysis was only moderately enhanced by increasing the number of tubule ends, which is consistent with the conclusion that tubule ends are not the only sites of hydrolysis. A model that reconciles the overall kinetics with the morphological evidence is proposed.

Chiral molecular self-assembly of phospholipid tubules: a circular dichroism study

We report on spectroscopic studies of the chiral structure in phospholipid tubules formed in mixtures of alcohol and water. Synthetic phospholipids containing diacetylenic moieties in the acyl chains self-assemble into hollow, cylindrical tubules in appropriate conditions. Circular dichroism provides a direct measure of chirality of the molecular structure. We find that the CD spectra of tubules formed in mixtures of alcohol and water depends strongly on the alcohol used and the lipid concentration. The relative spectral intensity of different circular dichroism bands correlates with the number of bilayers observed using microscopy. The results provide experimental evidence that tubule formation is based on chiral packing of the lipid molecules and that interbilayer interactions are important to the tubule structure.

Kinetics of lipid phase changes

In the past two years, the kinetics of transitions involving the assorted lamellar and inverted hexagonal and cubic phases in bulk hydrated lipid systems have been established using a variety of physical techniques. In several cases, the kinetic data have lead to a transition mechanisms being deciphered.

Lipid Tubule Self-Assembly: Length Dependence on Cooling Rate Through a First-Order Phase Transition

The formation kinetics and self-assembly of multilamellar tubules of the diacetylenic phospholipid 1,2-bis(tricosa-10,12-diynoyl)-sn-glycerol-3-phosphocholine formed under controlled cooling rates were studied by x-ray diffraction and optical, atomic force, and scanning electron microscopy. Tubule formation was driven by a reversible first-order phase transition from an intralamellar, chain-melted L(alpha) phase to a chain-frozen L(beta), phase. These observations are the basis of a highly efficient method of tubule production in which tubule lengths can be controlled, between 1 and 100 micrometers, by varying the cooling rate. These tubules can be made in suspensions with 10 percent lipid by mass, far exceeding the lipid solubility limit.

The temperature-composition phase diagram and mesophase structure characterization of monopentadecenoin in water

The temperature-composition phase diagram of monopentadecenoin, a monoacylglycerol with a cis monounsaturated fatty acid 15 carbon atoms long (C15:1c10) in water was constructed using x-ray diffraction. Low- and wide-angle diffraction patterns were collected from samples of fixed hydration as a function of temperature in the heating direction on x-ray-sensitive film. The temperature and hydration ranges investigated were 0-104 degrees C and 0-60% (w/w) water, respectively. The phases identified in the system include the lamellar crystalline phase, the lamellar liquid crystalline phase, the fluid isotropic phase, and two inverted cubic phases belonging to space groups la3d (Q230) and Pn3m (Q244). Particular attention has been devoted to the issues of phase equilibrium, phase boundary verification, and structure characterization. The phase diagrams of monopentadecenoin, monomyristolein (C14:1c9), and monoolein (C18:1c9) are compared, and the impact of molecular structure on mesophase stability and structure is discussed.

A temperature study of diacetylenic phosphatidylcholine vesicles

Dispersions of the diacetylenic phosphatidylcholine, 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine, DC8,9PC, undergo a change from vesicles to hollow tubes on cooling. We report here a light scattering and multinuclear NMR study of the lipid vesicles over the temperature range 0-50 degrees C. The 'splitting' of the N+(CH3)3 resonance increases with decreasing temperature, consistent with the light scattering measurements which show a decrease in vesicle size with decreasing temperature. The NMR spectrum remains well-resolved over this temperature range, even at temperatures as low as 3 degrees C. Phosphorus NMR also indicates that the 'bilayer structure' is maintained over this temperature range. The various proton resonances and the phosphorous signal from the lipid vesicles broaden as the temperature is lowered. These results will be helpful in developing a model for the tubule-forming ability of DC8,9PC.

Lipid Tubules: A Paradigm for Molecularly Engineered Structures

The use of molecular self-assembly to fabricate microstructures suitable for advanced material development is described. Templating techniques that transform biomolecular self-assemblies into rugged and stable nano- and microstructures are described. By using a lipid-based microcylinder (tubule) as a paradigm, the path followed from research and development to emerging technological applications is detailed. This process includes modification of the lipid molecular structure, the formation and subsequent characterization of cylindrical microstructures, the use of these structures as templates for metallization, and the characterization and assessment of these hollow metal microcylinders for several potential applications.

Thermodynamic, thermomechanical, and structural properties of a hydrated asymmetric phosphatidylcholine

1-Behenyl-2-lauryl-sn-glycero-3-phosphocholine (22/12 PC) belongs to a unique group of phospholipids in which the molecule has one acyl chain almost twice as long as the other. The temperature-composition phase diagram for this lipid in the range of 25-65 degrees C, and 0 to 84.3% (w/w) water has been constructed by using the isoplethal method in the heating direction and x-ray diffraction for phase identification and structure characterization. At water contents between 10.3 and 34% (w/w) and at temperatures below 43 degrees C, a single mixed interdigitated lamellar gel phase (Lm beta, [symbol: see text]) of the type described by Hui et al. (1984. Biochemistry. 23:5570-5577) and McIntosh et al. (1984. Biochemistry. 23:4038-4044) was found. A second phase consisting of bulk aqueous solution coexists with the Lm beta phase at hydration levels above 34% (w/w) water in the temperature range between 25 and 43 degrees C. Above 43 degrees C, a partially interdigitated lamellar liquid crystalline (Lp alpha) phase ([symbol: see text]) is seen in the water concentration range extending from 0 to 84.3% (w/w). The pure Lp alpha phase is found below 43% (w/w) water, while coexistence of the Lp alpha phase and the bulk aqueous solution is observed above this water concentration which marks the hydration boundary. Interestingly, the latter boundary for both Lm beta and Lp alpha phases is nearly vertical in the temperature range studied. Furthermore, the lamellar chain-melting transition temperature appears to be relatively insensitive to hydration in the range 0-85% (w/w) water. We have confirmed the identify of the Lm beta phase by constructing a 5.7-A resolution electron density profile on oriented samples by the swelling method. Temperature-induced chain melting effects an increase in lipid bilayer thickness suggesting that the Lp alpha phase has chains packed in the partially as opposed to the mixed interdigitated configuration. Unlike the symmetric phosphatidylcholines a ripple (P beta') phase was not found as an intermediate between the low and high temperature lamellar phases of 22/12 PC. The specific volume of 22/12 PC is 940 (+/- 1) microliter/g and 946 (+/- 1) microliter/g in the hydrated lamellar gel state at 28 (+/- 2) and 40 (+/- 2) degrees C, respectively, from neutral buoyancy experiments. Based on measurements of the temperature dependence of the various lattice parameters of the different phases encountered in this study the corresponding lattice thermal expansion coefficients have been measured. These are discussed and their dependence on lipid hydration is reported.

Effect of alcohol chain length on tubule formation in 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine

Aqueous dispersions of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine, on cooling below the chain melting temperature, form hollow cylindrical structures known as 'tubules'. We have studied the formation of tubules in methanol/water, ethanol/water and n-propanol/water. For each alcohol, there is a defined window of alcohol/water ratios in which the lipid precipitates with the tubule morphology. As the chain length of alcohol is increased, the window shifts towards lower alcohol fraction. Light scattering studies show that at very low lipid concentrations the tubules self-assemble directly from the isotropic phase where as for lipid concentrations greater than 4 mg/ml an intermediate L alpha phase is observed. These results indicate that the mechanism of tubule formation may be dependent on lipid concentration.

Structure of polymerizable lipid bilayers. V. Synthesis, bilayer structure and properties of diacetylenic ether and ester lipids

Four diacetylenic phosphatidylcholines (PC's) have been synthesized and the structures of bilayers of these lipids have been determined at low resolution by low-angle X-ray diffraction. The PC's all have 18-carbon chains but differ with respect to the ether/ester linkage at the sn-1 and sn-2 positions and the relative position of the diacetylene moiety: diester-PC (1): 1,2-bis(octadeca-4',6'-diynoyl)-sn-glycero-3-phosphocholine diester-PC (2): 1-(octadeca-4',6'-diynoyl)-2-(octadeca-5',7'-diynoy l)-sn- glycero-3-phosphocholine diester-PC (3): 1,2-bis(octadeca-8',10'-diynoyl)-sn-glycerol-3-phosphocholin e diether-PC (4): 1-O-(octadeca-4',6'-diynyl)-2-O-(octadeca-5",7"-din yl)-sn- glycero-3-phosphocholine Only (1) exhibits the typical bilayer profile, whereas (2), (3) and (4) show evidence of interdigitation and/or significant disorder. Only (1) polymerized effectively upon illumination with 254 nm light, turning deep blue in seconds, indicating the formation of long, well-ordered polydiacetylenic structures. Liposomes of these derivatives were tested for permeability by osmotic swelling. Polymerized liposomes of (1) underwent osmotic swelling with urea, glycerol, and acetamide more rapidly than did liposomes of stearoyl-oleoyl-PC, but the initial rates of osmotic swelling of polymerized liposomes of (1) were 3-10-times lower than those of unpolymerized liposomes of (1). Blue polymerized multilayer samples of (1) exhibited an irreversible thermochromic transition to red at approx. 40 degrees C. Differential scanning calorimetry with liposome suspensions of (1) revealed an endotherm at 28.3 degrees C with a transition enthalpy of 40 J/g. PC (1) is a potentially useful diacetylenic lipid which exhibits facile, complete polymerization and a bilayer thickness comparable to that of biomembrane lipids.

Calorimetric studies of lipid tubule formation from ethanol-water solutions

We have used differential scanning calorimetry to systematically investigate the thermal formation of hollow cylindrical crystalline microstructures or 'tubules' upon cooling a diacetylenic phosphatidylcholine (1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine) dispersed in varying volume fractions of ethanol/water. Tubule formation is characterized by a large exothermic event, observed upon cooling the lipid in 60-80% ethanol. The enthalpy of the transition was observed to be highest in this window of tubule formation (128-138 J/g) which is significantly higher than previously reported values for the enthalpy of tubule formation in water (90 -95 J/g). The enthalpy associated with the formation of tubules in 70% ethanol was also found to be strongly dependent on the efficiency of tubule formation and decreased as the number density of tubules decreased. A significant decrease in tubule number density could be brought about by increasing the lipid concentration of the 70% ethanol solution. Tubule number density was maximized at lipid concentrations between 0.5 and 2 mg/ml in 70% ethanol. Examination of the C-H stretch region from infrared spectra of the lipid below the phase transition, indicate that the intramolecular chain order-disorder is similar, regardless of the fraction of ethanol. The higher transition enthalpy for the melting of tubules in 60-80% ethanol (compared to water) implies that the high-temperature phase from which the tubules form in ethanol is more disordered than the lamellar liquid crystalline phase from which tubules form in water.

Biocompatibility of lipid microcylinders: effect on cell growth and antigen presentation in culture

The authors are developing a lipid-based microcylinder for the controlled release of biological response modifiers and as templates for cellular migration and differentiation. These structures are comprised of a photopolymerizable phosphatidylcholine (1,2-ditricosa-10,12-diynoyl-sn-glycero-3-phosphocholine) and form spontaneously as a result of a thermotropic phase transition in aqueous solution or in a cosolvent solution of 70:30 ethanol:water. The hollow cylinders are helically wrapped lipid bilayers, variable in length (50-250 microns, depending on conditions of formation) and are 0.5-1.0 microns in diameter. The interaction has been examined of three types of lipid microcylinders: (1) monomeric, (2) photopolymerized by exposure to 254 nm light, and (3) surface-modified by incorporation of 6 mol% gangliosides, with different human cell lines and peripheral blood leucocytes to evaluate the biocompatibility of these structures. The proliferative status of U937 (a histiocytic monocyte), K562 (an erythroleukaemic cell), and Jurkat's derivative (a T-lymphoblast) as measured by pulsed tritiated thymidine was unaffected by the presence of up to 100 micrograms/ml of lipid microcylinders after 3 d in culture. Adherent human peripheral blood monocytes were shown to form adhesive contacts with the lipid microcylinders. An 'association' index from this interaction shows that after 3 d in culture, the association was much lower for those microcylinders that had incorporated ganglioside compared with monomeric or polymerized structures. The lipid microcylinders do not activate T-cells isolated from human peripheral blood, nor do they inhibit the activation of T-cells by phorbol esters or other mitogens.(ABSTRACT TRUNCATED AT 250 WORDS)