Stereochemistry-Dependent Self-Assembly in Synthetic Glycolipid/Water Systems: The Aqueous Phase Structure of 1,3-Di-O-dodecyl-2-(β-maltoheptaosyl)glycerol

Stereochemical Purity Can Induce a New Crystalline Mesophase in Phytantriol Lipids

The impact of stereochemical purity of lipids on their self-assembly behavior is critical for establishing their true phase behavior from their commercial counterparts, which often contains stereoisomeric mixtures and other impurities. Here, stereochemically pure phytantriol (PT), (3,7,11,15-tetramethylhexadecane-1,2,3-triol) was synthesized from the natural trans-phytol and its thermotropic and lyotropic phase behavior in water investigated by small-angle X-ray scattering (SAXS), polarized optical microscopy (POM), and differential scanning calorimetry (DSC). These chemically pure lipids contain two chiral centers at the hydrophilic head group region and two chiral centers at the lipophilic tail region, allowing us to address the question of whether the molecular stereochemistry is related to the macroscopic phase behavior of phytantriol. In contrast to its commercial stereoisomeric mixtures, which form an isotropic micellar phase, neat (2S,3S,7R,11R)-3,7,11,15-tetramethylhexadecane-1,2,3-triol (S,S-PT) shows a smectic lamellar phase at room temperature, whereas (2R,3R,7R,11R)-3,7,11,15-tetramethylhexadecane-1,2,3-triol (R,R-PT) forms solid crystals. The lyotropic phase behavior of R,R-PT appears to be identical to that of the previously reported commercial stereoisomeric PT mixtures. In contrast, S,S-PT exhibits a different phase behavior. A lamellar crystalline phase (L_c) is formed instead of an isotropic micellar phase at a low water content, which also coexisted with other phases at low temperature. Subtle change in the shape of the diastereomers leads to variable steric interactions and subsequently affects the packing of the lipids at the molecular level, thereby influencing its self-assembling behavior. Finally, lipidic cubic phase crystallization of the membrane protein bacteriorhodopsin yielded a larger number of microcrystals with a higher average crystal length from S,S-PT than from commercial PT, suggesting faster nucleation.

Physicochemical characterization of natural-like branched-chain glycosides toward formation of hexosomes and vesicles

Synthetic branched-chain glycolipids have become of great interest in biomimicking research, since they provide a suitable alternative for natural glycolipids, which are difficult to extract from natural resources. Therefore, branched-chain glycolipids obtained by direct syntheses are of utmost interest. In this work, two new branched-chain glycolipids are presented, namely, 2-hexyldecyl β(α)-D-glucoside (2-HDG) and 2-hexyldecyl β(α)-D-maltoside (2-HDM) based on glucose and maltose, respectively. The self-assembly properties of these glycolipids have been studied, observing the phase behavior under thermotropic and lyotropic conditions. Due to their amphiphilic characteristics, 2-HDG and 2-HDM possess rich phase behavior in dry form and in aqueous dispersions. In the thermotropic study, 2-HDG formed a columnar hexagonal liquid crystalline phase, whereas in a binary aqueous system, 2-HDG formed an inverted hexagonal liquid crystalline phase in equilibrium with excess aqueous solution. Furthermore, aqueous dispersions of the hexagonal liquid crystal could be obtained, dispersions known as hexosomes. On the other hand, 2-HDM formed a lamellar liquid crystalline phase (smectic A) in thermotropic conditions, whereas multilamellar vesicles have been observed in equilibrium with aqueous media. Surprisingly, 2-HDM mixed with sodium dodecyl sulfate or aerosol OT induced the formation of more stable unilamellar vesicles. Thus, the branched-chain glycolipids 2-HDG and 2-HDM not only provided alternative nonionic surfactants with rich phase behavior and versatile nanostructures, but also could be used as new drug carrier systems in the future.

Synthesis and mesomorphic properties of glycosyl dialkyl- and diacyl-glycerols bearing saturated, unsaturated and methyl branched fatty acid and fatty alcohol chains

Glycosyl dialkyl- and diacyl-glycerols bearing saturated, unsaturated or chiral methyl branched chains in the tail and disaccharide and trisaccharide carbohydrate headgroups were synthesised. Standard procedures were used for the preparation of the educts and the glyco lipids: trichloracetimidate procedure for the preparation of long-chained compounds, glycosylation using the beta-peracetate and boron trifluoride etherate was successful for the preparation of lipids with a medium-alkyl chain length. Preparation of the ester was afforded in a multi-step synthesis according to published procedures. Thus, several lipids were synthesised in a few synthetic steps in good yields. The introduction of unsaturated or methyl branched chains lead to liquid crystallinity at ambient temperature, because these compounds will be used as model compounds for biological systems. The biophysical properties of these compounds will be reported in a following paper.

Synthesis and mesomorphic properties of glycosyl dialkyl- and diacyl-glycerols bearing saturated, unsaturated and methyl branched fatty acid and fatty alcohol chains

The biophysical properties of a series of glycosyl dialkyl- and diacyl-glycerols bearing unsaturated or chiral methyl branched chains in the tail, and di- and trisaccharide carbohydrate headgroups are described. Thermotropism was investigated by polarising microscopy, the lyotropism was investigated by small angle X-ray diffraction and by the contact preparation method, and the gel to liquid crystalline phase transition by FT-IR-spectroscopy. The compounds displayed thermotropic Smectic A (SmA), cubic and columnar phases, whereas in the lyotropic phase diagram lamellar, hexagonal and cubic phases are found. The introduction of unsaturated or methyl branched chains leads to liquid crystallinity at ambient temperature. The difference between the 1,3-oleyl-glycerol maltoside and the corresponding 1,2-oleoyl-glycerol maltoside is small.

Oligomer-to-polymer transition in short ethylene glycol chains connected to mobile hydrophobic anchors

We studied the structure of short ethylene glycol (EG) chains with N repeating units (EGN, N = 3, 6, 9, 12, and 15) connected to hydrophobic dihexadecyl chains by means of a combination of differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (SAXS/WAXS). These synthetic amphiphiles dispersed in water form planar lamellar stacks and hexagonal cylinders confining the EG chains to restricted geometries. Owing to the self-assembly of the anchoring points, the lateral density of EG chains in planar lamella can be quantitatively controlled. Furthermore, the chain-melting phase transition of the anchors enables us to "switch" the intermolecular distance reversibly. SAXS/WAXS results suggest that the shorter EG chains (N = 3, 6, and 9) assume a helical conformation in stacks of planar lamella. When the EG chains are further elongated (N = 12 and 15), the lamellar periodicities cannot be explained by a linear extrapolation of shorter oligomers, but can be interpreted well as polymer brushes following the scaling theorem. Such rich phase behaviors of EGN molecules can be used as a simple model of oligo/poly-saccharide chains on cell surfaces, which act not only as flexible repellers between neighboring cells but also as stable spacers for functional ligands.

Synthesis and formulation of neoglycolipids for the functionalization of liposomes and lipoplexes

Novel carbohydrate-based agents for the stabilization of ternary liposome:mu:DNA (LMD) nonviral vector systems are described. LMD vector systems comprise plasmid DNA (pDNA; D,7.5 kb) expressing a reporter gene (in this instance beta-galactosidase expressing gene) that is precondensed with the adenoviral core peptide mu (mu, M; MRRAHHRRRRASHRRMRGG) and then further packaged by means of DC-Chol:DOPE (3:2; m/m) cationic liposomes. Final optimized lipid:mu:pDNA ratio is typically 12:0.6:1 (w/w/w). We report the synthesis of a series of nine neoglycolipids prepared by coupling completely unprotected sugar monomers or oligomers (mannose, glucose, galactose, glucuronic acid, maltose, lactose, maltotriose, maltotetraose, and maltoheptaose) through their reducing-residue termini to an aminoxy-functionalized cholesterol-based lipid. Characterization of these novel neoglycolipids by (1)H NMR reveals that the coupling reaction has a major configurational preference for the beta-anomer. Unusually, even mannose coupling results in a neoglycolipid product with a predominantly beta-anomeric conformation (>85%). Formulation of neoglycolipids into LMD vector systems by incubation of LMD particles with neoglycolipid micelles results in the formation of a range of potential stabilized-LMD (sLMD) vector systems. Those potential sLMD systems prepared with longer chain neoglycolipids are found to have enhanced stabilities, with respect to aggregation in high ionic strength buffers, and enhanced transfection efficacies in comparison to the transfection properties of the naked first generation LMD vector system (i.e., gene delivery and expression). By contrast, when LMD vector systems are incubated with poly(ethylene glycol) DSPE-PEG micelles, resulting PEG-LMD vector systems are very stable with respect to colloidal instablility and aggregation in high ionic strength buffers and in serum, but are completely refractory to transfection. These data suggest that oligosaccharides could represent an alternative to PEG as a stealth polymer able to stabilize synthetic nonviral vector systems in some fluids but without impairing transfection efficiency. Furthermore, sLMD systems prepared with longer chain neoglycolipids appear to have sufficient useful characteristics to form the basis of viable second-generation LMD vector systems after further development.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Interglycolipid Membrane Interactions: pH-Dependent Aggregation of Nonionic Synthetic Glycolipid Vesicles

Large unilamellar vesicles composed of a nonionic synthetic glycolipid, 1,3-di-O-phytanyl-2-O-(beta-D-maltotriosyl)glycerol exhibited a pH-dependent aggregation-disaggregation process; vesicle aggregation occurred in a lower pH region and vesicle disaggregation occurred in a higher pH region. This process was almost reversible and the aggregation threshold pH increased as NaCl concentration increased. Electrophoretic mobility measurements revealed that the glycolipid vesicles are negatively charged in the range pH 1.6-13. The change in zeta-potentials as functions of pH and NaCl concentration could be well described by the Gouy-Chapman expression of the surface charges with an assumption that the interfacial charges arise from the "adsorption" of OH(-) at the vesicle-water interface and the dissociation of hydroxyl groups of the sugar headgroup in a higher pH regime (>pH 10). The pH-dependent aggregation process was reasonably well described by the classical DLVO theory. Thus, the double-layer repulsive forces appear to be a major factor in stabilizing the glycolipid vesicle suspension. Copyright 2000 Academic Press.