Synthesis of a double-phospholipid

Self-assembly and biophysical properties of archaeal lipids

Archaea constitute one of the three fundamental domains of life. Archaea possess unique lipids in their cell membranes which distinguish them from bacteria and eukaryotes. This difference in lipid composition is referred to as 'Lipid Divide' and its origins remain elusive. Chemical inertness and the highly branched nature of the archaeal lipids afford the membranes stability against extremes of temperature, pH, and salinity. Based on the molecular architecture, archaeal polar lipids are of two types - monopolar and bipolar. Both monopolar and bipolar lipids have been shown to form vesicles and other well-defined membrane architectures. Bipolar archaeal lipids are among the most unique lipids found in nature because of their membrane-spanning nature and mechanical stability. The majority of the self-assembly studies on archaeal lipids have been carried out using crude polar lipid extracts or molecular mimics. The complexity of the archaeal lipids makes them challenging to synthesize chemically, and as a result, studies on pure lipids are few. There is an ongoing effort to develop simplified routes to synthesize complex archaeal lipids to facilitate diverse biophysical studies and pharmaceutical applications. Investigation on archaeal lipids may help us understand how life survives in extreme conditions and therefore unlock some of the mysteries surrounding the origins of cellular life.

Archaeal tetraether bipolar lipids: Structures, functions and applications

Archaea have developed specific tools permitting life under harsh conditions and archaeal lipids are one of these tools. This microreview describes the particular features of tetraether-type archaeal lipids and their potential applications in biotechnology. Natural and synthetic tetraether lipid structures as well as their applications in drug/gene delivery, vaccines and proteoliposomes or as lipid films are reviewed.

Structure of synthetic transmembrane lipid membranes at the solid/liquid interface studied by specular X-ray reflectivity

We fabricated a new class of supported membranes based on monolayers of artificial bola (transmembrane) lipids. The lipids used in this study are symmetric bola lipids with two phosphocholine head groups, which resemble natural archaea lipids. To prevent bending of the hydrocarbon chains, stiff triple bonds are inserted in the middle of the hydrocarbon cores. The formation of homogeneous "monolayers" of transmembrane lipids over macroscopic areas can be monitored with fluorescence microscopy. Structures of such supported monolayers in bulk water were characterized with specular X-ray reflectivity using high energy X-ray radiation, which guarantees a high transmission through bulk water. Here, the vertical structure of single monolayers could be resolved from reconstructed electron density profiles. To verify the structural model suggested by the specular reflectivity, we also performed small- and wide-angle X-ray scattering of transmembrane lipid suspensions. The wide-angle patterns reflect a distorted chain-chain correlation, while the small-angle scattering allowed us to model an electron density profile which is consistent with the profile calculated from specular reflectivity.

Phosphatidylcholine-derived bolaamphiphiles via click chemistry

The copper-catalyzed azide alkyne cycloaddition is employed to modify phosphatidylcholine precursors with sn-2 acyl chains containing terminal alkyne or azide groups. Although the reactions are conducted as biphasic dispersions, the yields are essentially quantitative. Bolaamphiphiles are formed by simply clicking together two phosphatidylcholine alkyne precursors to a central bisazide scaffold. The chemistry introduces polar 1,4-triazole units into the lipophilic region of the bilayer membrane, and the bolaamphiphiles do not form stable vesicles. [structure: see text].

Nucleoside-Based Phospholipids and Their Liposomes Formed in Water

Phospholipids and liposomes have been the subjects of considerable attention because of their importance in biological systems. We have efficiently synthesized novel nucleoside-based phospholipids in six-step sequences starting from their corresponding nucleosides. These nucleoside-based phospholipids self-assemble into liposome-like structures in aqueous solutions. We have analyzed the structures of these liposomes by dynamic light scattering, transmission electron microscopy, and confocal microscopy.

Short Synthesis of Polyoxygenated Macrocyclic Rings Using Acetal Linkages. Application to the Preparation of a New Lipidic Polyamine

A short preparation of polyoxygenated macrocycles can be carried out by combining the formation of an acetal linkage, to introduce long alkyl chains, with a ring closure metathesis. As an example, this methodology was used to synthesize a new polyamino lipid.

Synthesis and characterization of carbohydrate-based phospholipids

Novel carbohydrate-based phospholipids containing two saturated C(12) (dilauroyl ribo-phosphocholine) (DLRPC), C(14) (dimyristoyl ribo-phosphocholine) (DMRPC), and C(20) (diarachadonyl ribo-phosphocholine) (DARPC) carboxylic acid chains were synthesized. The physical properties of the supramolecular structures formed by these compounds were compared to those formed by their direct glycerol analogues dilauroyl phosphocholine (DLPC), dimyristoyl phosphocholine (DMPC), and diarachadonyl phosphocholine (DAPC). Modulated differential scanning calorimetry (MDSC) and X-ray diffraction data indicated that with chain lengths < or =14 carbons, the carbohydrate backbone increased the thermal stability of the bilayer below the phase-transition temperature (T(m)) as compared to the glycerol-based lipids. With longer chains (C(20)), the bilayer structure was destabilized as compared to glycerol-based lipids. NMR studies of a DMRPC vesicle dispersion reveal split choline headgroup signals and distinct magnetization transfer effects arising from the "inner" and "outer" surfaces of the bilayer vesicle. Modulated differential scanning calorimetry also demonstrated that glycerol- and carbohydrate-based lipids mix, as evidenced by a single intermediate T(m). In addition, carbohydrate-based lipid/cholesterol mixtures exhibited a decrease in enthalpy with an increase in cholesterol concentration. Unlike glycerol phospholipids, carbohydrate lipids were resistant to enzymatic degradation by phospholipase A(2) (PLA(2)).

Efficient synthesis of 40- and 48-membered tetraether macrocyclic bisphosphocholines

An efficient route toward the synthesis of unsaturated (bis-diacetylenic) and saturated 40- and 48-membered macrocyclic biphosphocholines has been developed using 2-phenyl-5-hydroxy-1,3-dioxane as a common glycerol synthon. Ring closure was accomplished using either high-dilution Glaser oxidation of [(Cy3P)2RU==CHPh]Cl2-catalyzed olefin metathesis conditions. Deprotection of benzyl ethers using trimethylsilyl iodide (TMS-I) in the presence of diacetylenic moieties has also been demonstrated for the first time.

Synthetic Approaches to Novel Archaeal Tetraether Glycolipid Analogues

Symmetrical and unsymmetrical archaeal tetraether glycolipid analogues have been prepared. The syntheses are based upon the elaboration of lipid cores from versatile chiral starting materials followed by simultaneous or sequential introduction of polar headgroups. Three pathways (A-C) were elaborated for the synthesis of stereochemically defined lipids 14-16 characterized by a straight bridging spacer and two dihydrocitronellyl chains attached to glycerol units at the sn-3 and sn-2 positions, respectively. Pathway C appeared to be particularly advantageous for the synthesis of tetraether 9, which possesses a cyclopentane unit as found in thermoacidophilic lipids. Diglycosylated lipids 4-6 were produced in 49-53% yields by reaction of diols 14-16 with beta-D-galactofuranosyl donor 31, whereas unsymmetrical lipids possessing either two different carbohydrate units 7 or a saccharidic moiety and a phosphate group 8 were efficiently prepared from monoprotected diol 35. These compounds represent the first examples of tetraether-type analogues containing a phosphate unit and/or glycosyl moieties.

Total Synthesis of Archaeal 72-Membered Macrocyclic Tetraether Lipids

Total synthesis of archaeal 72-membered macrocyclic tetraether lipids 3a and 3b is reported. The synthesis was principally composed of preparation of the functionalized half-sized diether compounds 11 and 15 first followed by appropriate dimerization through Julia coupling and final macrocyclization of the crucial dialdehydes 23 and 31 by McMurry coupling. This strategy appeared to be advantageous for the stereoselective synthesis of both natural 72-membered tetraether lipids 3a and 3b using common synthetic intermediates. In addition, this approach was so designed that its synthetic flexibility would allow construction of unnatural structural variants for physicochemical studies. Also described are the results of differential scanning calorimetric analysis of the synthesized lipids 3a and 3b. Both 3a and 3b showed almost the same phase behavior with the broad endothermic phase transition at -53 degrees C. The enthalpy of the phase transition, DeltaH, was estimated to be 1.8 and 1.9 kcal/mol for 3a and 3b, respectively. The physicochemical as well as polymorphismic properties of 3a and 3b turned out to be indistinguishable despite of their regioisomeric structures. The physical structure of the phases in terms of the chemical structure is also discussed.

Total Synthesis of Archaeal 36-Membered Macrocyclic Diether Lipid

Total synthesis of archaeal 36-membered macrocyclic diether lipid 2 is reported. The synthesis is based upon stereoselective preparation of functionalized isoprenoid chains, ether-linkage formation between the isoprenoid chains with a glycerol derivative, and the ultimate intramolecular dicarbonyl coupling using low-valent titanium known as McMurry coupling. This synthetic method has successfully provided the first practical route to chemically defined archaeal macrocyclic membrane lipids, which were not available because of the lack of synthetic access. Also described is a highly stereoselective and convenient synthesis of stereochemically homogeneous archaeal biphytanyl glycerol lipid 1.

Synthesis of Defective Phospholipids

Phospholipids have been synthesized that possess a normal 16-carbon chain plus a "defective" chain only 8 or 12 carbons long and terminated with methoxyl, hydroxyl, or carboxyl groups. In addition, dimeric phospholipids have been prepared in which two phospholipid units are joined at position-1 with chains of 22 or 32 carbons while unconnected chains at position-2 are, once again, short and functionalized. These phospholipids are potentially useful for constructing membranes that contain cavities or irregularities and, therefore, are capable of serving as self-assembled host systems in which drugs and other guest molecules are retained and, perhaps, eventually released.