Lipid fluorination enables phase separation from fluid phospholipid bilayers

Supramolecular Chemistry in the Biomembrane

The combination of supramolecular functional systems with biomolecular chemistry has been a fruitful exercise for decades, leading to a greater understanding of biomolecules and to a great variety of applications, for example, in drug delivery and sensing. Within these developments, the phospholipid bilayer membrane, surrounding live cells, with all its functions has also intrigued supramolecular chemists. Herein, recent efforts from the supramolecular chemistry community to mimic natural functions of lipid membranes, such as sensing, molecular recognition, membrane fusion, signal transduction, and gated transport, are reviewed.

Thermo-driven self-assembly of a PEG-containing amphiphile in a bilayer membrane

Self-assembly of lipid molecules in a plasma membrane, namely lipid raft formation, is involved in various dynamic functions of cells. Inspired by the raft formation observed in the cells, here we studied thermally induced self-assembly of a synthetic amphiphile, bola-AkDPA, in a bilayer membrane. The synthetic amphiphile consists of a hydrophobic unit including fluorescent aromatic and aliphatic components and hydrophilic tetraethylene glycol chains attached at both ends of the hydrophobic unit. In a polar solvent, bola-AkDPA formed aggregates to show excimer emission. In a lipid bilayer membrane, bola-AkDPA showed intensified excimer emission upon increase of its concentration or elevation of the temperature; bola-type amphiphiles containing oligoethylene glycol chains likely tend to form self-assemblies in a bilayer membrane triggered by thermal stimuli.

A synthetic multi-block amphiphile containing oligoethylene glycol chains formed a self-assembly in a bilayer membrane triggered by thermal stimuli.

‘One-pot’ sequential enzymatic modification of synthetic glycolipids in vesicle membranes

To create vesicles with cell-targeting coatings, two soluble enzymes were used to directly glycosylate vesicle surfaces in a ‘one-pot’ procedure.

Thermally-induced lateral assembly of a PEG-containing amphiphile triggering vesicle budding

A macrocyclic amphiphile consisting of a thermo-responsive octaethylene glycol chain with hydrophobic aromatic and aliphatic units undergoes lateral self-assembly in a liquid-disordered-state phospholipid bilayer membrane upon heating, which further leads to vesicle budding.

Sialylation of lactosyl lipids in membrane microdomains byT. cruzi trans-sialidase

SolubleT. cruzi trans-sialidase transformed a synthetic lactosyl glycolipid in microdomains more slowly than the same substrate dispersed across the bilayer surface, producing phospholipid vesicles with a Neu5Ac(α2-3)Gal(β1-4)Glc “glycocalyx”.

Incorporation of fluorous glycosides to cell membrane and saccharide chain elongation by cellular enzymes

A series of fluorous-tagged glycosides with different number of fluorine atoms are incorporated into the cells, transported to Golgi, elongated by cellular enzymes, and then released to the culture medium. Fluorine content strongly affects on the affinity for cell membrane and glycosylation. Essentially, the fluorocarbon chain in fluorous compound and the hydrocarbon chain are not miscible. However, the fluorous-tagged glycosides have affinity for cell membrane because of its amphiphilicity. The affinity of fluoro-amphiphilic compound for cell membrane is discussed using critical micelle concentration. The separation of glycosylated products by solvent extraction or fluorous solid phase extraction cartridges is also discussed.

X-ray reflectivity study on the structure and phase stability of mixed phospholipid multilayers

Vertically oriented multilayers composed of two saturated phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoserine (DPPS), were deposited on silicon. X-ray reflectivity was used to investigate the structures of the variously mixed phospholipid multilayers as a function of composition. Then, the phase stability was investigated at various annealing temperatures under humid conditions. The results indicated that the lipid spacing of the mixed phospholipid multilayers varied systematically as a function of the DPPC/DPPS ratio and that no macroscopic phase separation occurred during the annealing process under both dry and humid conditions.

Magnetically-controlled release from hydrogel-supported vesicle assemblies

Magnetic nanoparticle-vesicle assemblies embedded within a hydrogel extravesicular matrix have been shown to release their contents in response to a remote magnetic trigger.

Lipid headgroups mediate organization and dynamics in bilayers

We report on the fluorescence lifetime and anisotropy decay dynamics of the tethered chromophore NBD in unilamellar vesicles comprised of phosphoglycerol and phosphocholine lipids with C(12) and C(18) saturated acyl chains, with or without cholesterol and/or sphingomyelin. For the phosphocholine vesicles, we use the chromophore 2-(12-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3-phosphocholine (NBD-PC), and for the phosphoglycerol vesicles, we use the chromophore 2-(12-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (NBD-PG). The addition of cholesterol and/or sphingomyelin to the PC vesicles restricts the chromophore environment, in agreement with the known rigidizing effect of cholesterol on PC membranes. The PG systems do not exhibit an analogous effect with the addition of cholesterol and/or sphingomyelin. The motional freedom of the NBD chromophore is, in general, more restricted in the PC bilayers than it is in the PG bilayers, and we understand this behavior in the context of the role of the lipid headgroups in mediating bilayer organization.

Nanoscale patterning in mixed fluorocarbon-hydrocarbon phospholipid bilayers

A growing body of literature suggests that fluorocarbons can direct self-assembly within hydrocarbon environments. We report here the fabrication and characterization of supported lipid bilayers (SLBs) composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a synthetic, fluorocarbon-functionalized analogue, 1. AFM investigation of these model membranes reveals an intricate, composition-dependent domain structure consisting of approximately 50 nm stripes interspersed between approximately 1 microm sized domains. Although DSC of 1 showed a phase transition near room temperature, DSC of DPPC:1 mixtures exhibited complex phase behavior suggesting domain segregation. Finally, temperature-dependent AFM of DPPC:1 bilayers shows that, while the stripe structures can be melted above the Tm of 1, the stripes and domains result from immiscibility of the hydrocarbon and fluorocarbon lipid gel phases. Fluorination appears to be a promising strategy for chemical self-assembly in two dimensions. In particular, because no modification is made to the lipid headgroups, it may be useful for nanopatterning biologically relevant ligands on bilayers in vitro or in living cells.

Pyrene-sensitized electron transport across vesicle bilayers: Dependence of transport efficiency on pyrene substituents

Endoergic electron transport across vesicle bilayers from ascorbate (Asc-) in the inner waterpool to methylviologen (MV2+) in the outer aqueous solution was driven by the irradiation of pyrene derivatives embedded in the vesicle bilayers. The initial rate of MV2+ reduction is dependent on the substituent group of the pyrenyl ring; a hydrophilic functional group linked with the pyrenyl ring by a short methylene chain acts as a sensitizer for the electron transport. Mechanistic studies using (1-pyrenyl)alkanoic acids (1a-c) as sensitizers suggest that the electron transport is mainly initiated by the reductive quenching of the singlet excited state of the pyrene by Asc- and proceeds by a mechanism involving electron exchange between the pyrenes located at the inner and outer interface across the vesicle bilayer. We designed and synthesized novel unsymmetrically substituted pyrenes having both a hydrophilic group linked by a short methylene chain and a hydrophobic long alkyl group (5a-c), which acted as excellent sensitizers for the electron transport across vesicle bilayers.