Emergent Glycerophospholipid Fluorescent Probes: Synthesis and Applications.

Fluorescent labeling through bioconjugation is the preferred tool for the investigation of biological functions involving lipids, namely for clarifying metabolic pathways and molecular mechanisms of diseases. The lack of functionalized lipid probes with biological and physicochemical properties suitable for these studies is still a major limitation. Moreover, the synthesis of these probes is challenging and strongly dependent on the application envisioned. The objective of this Review is to highlight advances in the application of fluorescent glycerophospholipid probes through innovative approaches in the synthesis reported in the past decade. The reaction pathways, choice of fluorophore, and location of fluorophore in the glycerophospholipid structure are critically addressed. The relevance of these bioconjugates is exemplified with applications using advanced analysis by fluorescence enhancement or quenching to unravel biomembrane structure features and phospholipase activity. Finally, this Review reinforces the need for innovative and more efficient routes for the synthesis of tailored glycerophospholipids fluorescent conjugates.

Headgroup-Inversed Liposomes: Biointerfaces, Supported Bilayers and Applications.

Phospholipids are a major component of the cell membrane. In most natural phospholipids, the phosphate acts as a bridge, connecting the other portion of the polar headgroup with the hydrophobic tails. Such bridging phosphate is chemically quite inert. Synthetic lipids inversing the headgroup polarity of phosphocholine (PC) have been recently reported, and these are named CP lipids with a terminal phosphate, or CPe with the terminal phosphate capped by an ethyl group. This Feature Article summarizes the properties and applications of such inversed lipids. First, CPe liposomes were found to be highly resistant to protein adsorption with an even longer blood circulation time than PC liposomes, allowing for enhanced accumulation in tumor sites. CPe liposomes do not interact with PC liposomes either, and this observation was different from that reported using CP polymers, which adhere strongly to cells. Second, CP liposomes interact strongly with many metal oxide nanoparticles (but not silica) forming supported lipid bilayers, while PC liposomes only form supported bilayers on silica. Finally, CP liposomes are good metal ligands based on their exposed terminal phosphate. Zn2+ binds to CP liposomes so strongly that Zn2+ sandwiched multilayered lipid structures were observed. Aside from these fundamental aspects, the potential applications of these headgroup-inversed lipids in drug delivery and biosensor development have also been described, which in turn has promoted fundamental biointerface insights.

IKMTSL-PTE, a Phospholipid-Based EPR Probe for Surface Electrostatic Potential of Biological Interfaces at Neutral pH: Effects of Temperature and Effective Dielectric Constant of the Solvent.

The synthesis and characterization of a lipidlike electrostatic spin probe, (S)-2,3-bis(palmitoyloxy)propyl 2-((4-(4-(dimethylamino)-2-ethyl-1-oxyl-5,5-dimethyl-2,5-dihydro-1H-imidazol-2-yl)benzyl)disulfanyl)ethyl phosphate (IKMTSL-PTE), are being reported. The intrinsic pKa0 of IKMTSL-PTE was determined by X-band (9.5 GHz) electron paramagnetic resonance (EPR) titration of a water-soluble model compound, 4-(dimethylamino)-2-ethyl-2-(4-(((2-hydroxyethyl)disulfanyl)methyl)phenyl)-5,5-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl (IKMTSL-ME), an adduct of methanethiosulfonate spin label IKMTSL and 2-mercaptoethanol. The pKa0 of IKMTSL-ME in bulk aqueous solutions was found to be significantly higher than that of 4-(((2-hydroxyethyl)disulfanyl)methyl)-2,2,3,5,5-pentamethylimidazolidin-1-oxyl (IMTSL-ME), an adduct of the corresponding methanethiosulfonate spin label IMTSL and 2-mercaptoethanol (17 °C, pKa0 = 6.16 ± 0.03 vs 20 °C, pKa0 = 3.33 ± 0.03, respectively). A series of EPR titration experiments with IKMTSL-ME in aqueous solutions containing 0-60% v/v isopropanol have been carried out at 17 and 48 °C to determine the effects of temperature and bulk dielectric permittivity constant, ε, on the probe pKa. A linear relationship between the probe pKa and ε has been established and found to be essentially the same at 17 and 48 °C. The polarity term contributing to the pKa of IKMTSL-PTE at an uncharged lipidlike interface was determined by incorporating the probe into electrically neutral micelles formed from nonionic detergent Triton X-100, and it was found, similar to IMTSL-PTE, to be negative. In negatively charged DMPG lipid bilayers, IKMTSL-PTE exhibits ionization transitions with significantly higher pKa values than those previously reported for IMTSL-PTE (e.g., at 17 °C, pKai = 7.80 ± 0.03 vs pKa0 = 5.70 ± 0.05). The surface electrostatic potentials of DMPG lipid bilayers calculated using IKMTSL-PTE titration data were found to be somewhat lower than those calculated using IMTSL-PTE. The lower values measured by IKMTSL-PTE are the likely consequences of the structure of the linker that positions the reporter nitroxide further away from the bilayer plane into aqueous phase. Overall, the ionization transitions of IKMTSL-PTE with pKa values close to the neutral pH range make this lipidlike molecule a valuable spectroscopic EPR probe for studying the electrostatic phenomena at biological interfaces, including lipid bilayer/membrane protein systems, that could be unstable in the acidic pH range accessible by the previously available probes.

Two-dimensional nuclear magnetic resonance structure determination module for introductory biochemistry: synthesis and structural characterization of lyso-glycerophospholipids.

In this laboratory module, introductory biochemistry students are exposed to two-dimensional (1) H-nuclear magnetic resonance of glycerophospholipids (GPLs). Working in groups of three, students enzymatically synthesized and purified a variety of 2-acyl lyso GPLs. The structure of the 2-acyl lyso GPL was verified using (1) H-correlation spectroscopy. Students scored significantly higher on an assessment of NMR knowledge after having participated in this lab module and in comparison to a similar cohort who did not participate. Inaddition, student confidence in their NMR knowledge and abilities increased 62% following the module and correlated with their ability to apply their NMR knowledge. Based on these results, the laboratory module was very effective at providing students with a more extensive understanding of the underlying concepts of NMR as a tool for structural determination.

Asymmetric synthesis of cyclo-archaeol and ß-glucosyl cyclo-archaeol.

An efficient asymmetric synthesis of cyclo-archaeol and ß-glucosyl cyclo-archaeol is presented employing catalytic asymmetric conjugate addition and catalytic epoxide ring opening as the key steps. Their occurrence in deep sea hydrothermal vents has been confirmed by chromatographic comparison with natural samples.

Asymmetric synthesis and structure elucidation of a glycerophospholipid from Mycobacterium tuberculosis.

A glycerophospholipid (1-O-tuberculostearoyl-2-O-palmitoyl-sn-glycero-3-phosphoethanolamine) from Mycobacterium tuberculosis was isolated from the reference strain H37Rv. The molecular structure of this tuberculostearoyl [(R)-10-methyloctadecyl] and palmitoyl containing phosphatidylethanolamine (PE) has been resolved. The substitution pattern on the glycerol backbone could be determined by comparison of the isolate to the two synthetically prepared regioisomers. MS/MS analysis was used to determine its molecular structure. Production of this synthetic version of mycobacterial PE in high yield, with a stereochemically correct and pathogen-specific fatty acyl group, can be used as a standard in LC-MS based lipidomic analyses to detect trace amounts of mycobacterial PE in human blood, sputum, or tissues as a marker of infection by mycobacteria.

Synthesis and evaluation of lysophosphatidylserine analogues as inducers of mast cell degranulation. Potent activities of lysophosphatidylthreonine and its 2-deoxy derivative.

In response to various exogenous stimuli, mast cells (MCs) release a wide variety of inflammatory mediators stored in their cytoplasmic granules and this release initiates subsequent allergic reactions. Lysophosphatidylserine (lysoPS) has been known as an exogenous inducer to potentiate histamine release from MCs, though even at submicromolar concentrations. In this study, through SAR studies on lysoPS against MC degranulation, we identified lysoPT, a threonine-containing lysophospholipid and its 2-deoxy derivative as novel strong agonists. LysoPT and its 2-deoxy derivative induced histamine release from MCs both in vitro and in vivo at a concentration less than one-tenth that of lysoPS. Notably, lysoPT did not activate a recently proposed lysoPS receptor on MCs, GPR34, demonstrating the presence of another undefined receptor reactive to both lysoPS and lysoPT that is involved in MC degranulation. Thus, the present strong agonists, lysoPT and its 2-deoxy derivative, will be useful tools to understand the mechanisms of lysoPS-induced activation of degranulation of MCs.

Chemical synthesis of fluorescent glycero- and sphingolipids.

Glycero- and sphingolipids have been shown to be building blocks of membranes and lipoproteins, metabolites and important intermediaries in the signalling cascades involved in stress responses, proliferation of cells and also apoptosis. Investigations into the exact functions of these lipids have found that they are fundamentally more important than previously thought and that they are intricately involved in the processes of many significant metabolic pathways and diseases. Investigation of these functions requires the detection of the lipids in their natural environment within membranes. To this end, fluorescent labelling has become one of the preferred means in which to study these essential components due to the relative ease of detection. This review will look at the novel compounds that have been synthesised recently through various methodologies including classical lipid synthesis as well as the innovative application of organometallic chemistry. This field has expanded with the advancements in fluorescence detection and these lipids are being used as specific probes for an extensive range of applications in order to ascertain the mechanisms and signalling capabilities of this very important class of biological compounds.

A new approach to phospholipid synthesis using tetrahydropyranyl glycerol: rapid access to phosphatidic acid and phosphatidylcholine, including mixed-chain glycerophospholipid derivatives.

A new synthesis of phosphatidic acid and phosphatidylcholine is reported, relying on the preparation of 3-tetrahydropyranyl-sn-glycerol as the key intermediate for sequential introduction of the primary and secondary acyl functions to produce chiral diglycerides that are phosphorylated to obtain the target phospholipid compounds.

Glucosamine-glycerophospholipids that activate cell-matrix adhesion and migration.

Two new analogues derived from the platelet activating factor (PAF), containing glucosamine instead of the acetyl group, were synthesized, and their effect on the human keratinocyte cell line HaCaT was evaluated with respect to cytotoxicity, proliferation, adhesion, and migration. Starting with (R)-1,2-isopropylideneglycerol (3), the glycosylation acceptor 1-O-octadecyl-3-O-tert-butyldimethylsilyl-sn-glycerol (6) was synthesized in three steps. Glycosylation of 6 with the already known O-(3,4,6-tri-O-acetyl-2-deoxy-2-dimethylmaleimido-beta-D-glycopyranosyl)trichloracetimidate gave 1-O-octadecyl-2-O-(3',4',6'-tri-O-acetyl-2'-deoxy-2'-dimethylmaleimido-beta-D-glucopyranosyl)-3-O-tert-butyldimethylsilyl-sn-glycerol (7). After removing the (tert-butyldimethyl)silyl (TBDMS) group with FeCl3x6H2O, phosphoryl choline was introduced, yielding [1-O-octadecyl-2-O-(2'-deoxy-2'-dimethylmaleimido-beta-D-glucopyranosyl)-sn-glycero(3)]phosphorylcholine (2) (glucosimide-PAF). pH controlled cleavage of the amino protection group gave [1-O-octadecyl-2-O-(2'-deoxy-2'-amino-beta-D-glucopyranosyl)-sn-glycero(3)]phosphorylcholine hydrochloride (1) (glucosamine-PAF). 2 inhibited proliferation of HaCaT cells by 26% at nontoxic concentrations, while 1 increased the proliferation rate by 30% at low concentrations. At higher concentrations, both compounds showed cytotoxic properties with LD50 = 30 micromol/L (1) and LD50 = 5-6 micromol/L (2). Both 1 and 2 were potent promoters of cell adhesion and migration of HaCaT cells.

[Transdermal delivery of insulin]. / Transdermal'nye sistemy vvedeniia insulina.

A possibility is studied of the transdermal delivery of insulin by using a mixture of synthetic analogues of phosphoglycerides (SAP), as a potential activator of hormone diffusion, through the skin. Experimentally in vitro, it was proven that the diffusion of insulin through the skin of two types of transdermal therapeutic form (TTF)--matrix-type and matrix-hydrogel-type--is possible only in presence of activator SAP-M-99. The detected optimal composition of insulin matrix TTF with the area of 40 sq cm enabled a trandermal hormone diffusion speed of 0.26 UNITS/h, which is compatible with the secretion of insulin by the pancreas of an adult (0.25-1.5 UNITS/h). A change-over for the matrix-hydrogel system of insulin delivery based on a 40 sq cm collagenous sponge enabled to increase the insulin diffusion up to 0.54 UNITS/h.

Nonenzymatic synthesis of glycerolipids catalyzed by imidazole.

Imidazole catalyzed acylations of lysolipids by acyl-CoAs in water at room temperature and at a pH close to neutrality. In the presence of oleoyl-CoA and either lysophosphatidylcholine, 1-palmitoyl-sn-glycero-3-phosphocholine (LPC); lysophosphatidylglycerol, monoacyl-sn-glycero-3-phosphoglycerol; lysophosphatidyl acid, 1-oleoyl-sn-glycero-3-phosphate; lysophosphatidylserine, monoacyl-sn-glycero-3-phosphoserin; or lysophosphatidylethanolamine, monoacyl-sn-glycero-3-phosphoethanolamine, the corresponding phospholipids were synthesized. Similarly, the use of lyso-platelet activating factor, an ether analog of LPC, yielded the formation of 1-O-alkyl-2-oleoyl-sn-glycero-3-phosphocholine. In the presence of LPC, an imidazole-catalyzed synthesis of phosphatidylcholine (PC) occurred when medium, long, and very long chain acyl-CoAs were added. With hydroxyacyl-CoA, a similar PC synthesis was obtained. The process described in the present paper appears to offer several potential applications of interest for the synthesis of glycerophospholipids and triglycerides with labeled and/or an unusual or fragile fatty acid, or when suitable acyltransferases have not yet been described in the literature and/or are not commercially available. The method described is very safe and simple since lipids can be synthesized in tubes containing 0.7% imidazole in water, and left for a few hours at room temperature on the bench.

Synthesis of enantiomerically pure, sn-1 modified sn-2-deoxy-2-amido-glycero-3-phospholipids.

The synthesis of two types of enantiomerically pure sn-2-deoxy-2-amido-glycero-phospholipids differing in the connection of the alkyl chain at the sn-1-position is described. Both types of lipids were prepared from L-serine-methylester as the chiral starting material.

Nature of linkage between the cationic headgroup and cholesteryl skeleton controls gene transfection efficiency.

Three novel cationic cholesterol derivatives with different modes of linkage between the cationic headgroup and the cholesteryl backbone have been synthesized and used as mixtures with 1, 2-dioleoyl-L-alpha-glycero-3-phosphatidyl ethanolamine (DOPE) for liposome-mediated gene transfection. A pronounced improvement in gene transfer efficiency was observed when the cationic center was appended to the cholesteryl backbone using an ether linkage as opposed to when the linkages were based on either ester or urethane groups. Amphiphiles with ether links such as cholest-5-en-3beta-oxyethane-N,N,N-trimethyl ammonium bromide (2) and cholest-5-en-3beta-oxyethane-N,N-dimethyl-N-2-hydroxyethyl ammonium bromide (3) showed transfection efficiencies considerably greater than commercially available gene transfer reagents. Notably, the transfection ability of 2 with DOPE in the presence of serum was significantly greater than Lipofectamine((R)) and Lipofectin((R)). Interestingly, 3 did not require the helper lipid DOPE for transfection. This suggests that these newly described cholesterol-based amphiphiles should be very promising in liposome-mediated gene transfection. The advantage that the ether linkage possesses would be important in the design of newer, more efficient cholesterol-based delivery reagents.