Lipid bicontinuous cubic phases have attracted enormous interest as bio-compatible scaffolds for use in a wide range of applications including membrane protein crystallisation, drug delivery and biosensing. One of the major bottlenecks that has hindered exploitation of these structures is an inability to create targeted highly swollen bicontinuous cubic structures with large and tunable pore sizes. In contrast, cubic structures found in vivo have periodicities approaching the micron scale. We have been able to engineer and control highly swollen bicontinuous cubic phases of spacegroup Im3m containing only lipids by (a) increasing the bilayer stiffness by adding cholesterol and (b) inducing electrostatic repulsion across the water channels by addition of anionic lipids to monoolein. By controlling the composition of the ternary mixtures we have been able to achieve lattice parameters up to 470 Å, which is 5 times that observed in pure monoolein and nearly twice the size of any lipidic cubic phase reported previously. These lattice parameters significantly exceed the predicted maximum swelling for bicontinuous cubic lipid structures, which suggest that thermal fluctuations should destroy such phases for lattice parameters larger than 300 Å.

We attempt to mimic cellular biomembrane structures using a mixture of two important biochemical components – a membrane protein and a lipid, in the presence of water. Protein loaded lipid structures were subjected to a wide range of pressures to examine their morphological reorganizations using synchrotron X-ray radiation under stepwise pressure variation and rapid pressure jumps. Here we report the first evidence of a highly swollen gyroid cubic phase under high pressure (gyroid type structures have been seen in ER and Golgi membranes) and the close resemblance of lipid nanostructural behavior with literature studies on compression–decompression of piezophilic biomembranes. These studies are promising for understanding complex biomembrane restructuring and hence functioning, such as how cells cope with extreme conditions of high pressures, and protect their delicate internal organelles.

Synthetic branched-chain glycolipids are suitable as model systems in understanding biological cell membranes, particularly because certain natural lipids possess chain branching. Herein, four branched-chain glycopyranosides, namely, 2-hexyl-decyl-α-d-glucopyranoside (α-Glc–OC10C6), 2-hexyl-decyl-β-d-glucopyranoside (β-Glc–OC10C6), 2-hexyl-decyl-α-d-galactopyranoside (α-Gal–OC10C6), and 2-hexyl-decyl-β-d-galactopyranoside (β-Gal–OC10C6), with a total alkyl chain length of 16 carbon atoms have been synthesized, and their phase behavior has been studied. The partial binary phase diagrams of these nonionic surfactants in water were investigated by optical polarizing microscopy (OPM) and small-angle X-ray scattering (SAXS). The introduction of chain branching in the hydrocarbon chain region is shown to result in the formation of inverse structures such as inverse hexagonal and inverse bicontinuous cubic phases. A comparison of the four compounds showed that they exhibited different polymorphism, especially in the thermotropic state, as a result of contributions from anomeric and epimeric effects according to their stereochemistry. The neat α-Glc–OC10C6 compound exhibited a lamellar (Lα) phase whereas dry α-Gal–OC10C6 formed an inverse bicontinuous cubic Ia3d (QIIG) phase. Both β-anomers of glucoside and galactoside adopted the inverse hexagonal phase (HII) in the dry state. Generally, in the presence of water, all four glycolipids formed inverse bicontinuous cubic Ia3d (QIIG) and Pn3m (QIID) phases over wide temperature and concentration ranges. The formation of inverse nonlamellar phases by these Guerbet branched-chain glycosides confirms their potential as materials for novel biotechnological applications such as drug delivery and crystallization of membrane proteins.

Sphingomyelin is the only sphingolipid occurring naturally in mammalian cells and can form up to 50% of the total phospholipid content of the myelin sheath which surrounds nerves. Having predominantly long, saturated acyl chains, it has a relatively high chain melting temperature and has been strongly associated with formation of lipid microdomains. Here, the lyotropic phase behaviour of sphingomyelin from three different natural sources (bovine brain, egg yolk and milk) in excess water is studied as a function of temperature and pressure by small- and wide-angle X-ray scattering, and solid state NMR. The different hydrocarbon chain length distributions of the three lipid extracts results in significant differences in their gel phase structure; both the bovine brain and egg yolk sphingomyelins can form a ripple gel phase but milk sphingomyelin forms an interdigitated gel phase due to the high degree of chain mismatch in its longer hydrocarbon chain components.

A centred-rectangular lyotropic ribbon phase, rarely observed in inverse (type II) systems, has been found in the branched-chain polyoxyethylene surfactant tetradecyloctadecyl-tetraoxyethylene ether (C14C16EO4) in excess water. This phase is stabilised by the application of hydrostatic pressure. The ratio of the 2-D cell parameters, b/a, is observed to be less than √3 (1.732) over the range of temperatures and pressures studied. The constructed pressure–temperature phase diagram shows that, at high temperatures or low pressures, the inverse ribbon phase converts into an inverse micellar cubic phase of spacegroup Fd3m, and at the opposite extreme, a lamellar gel phase was formed. The lattice parameters of the inverse ribbon phase were found to vary with pressure, with the structure becoming increasingly distorted away from 2-D hexagonal symmetry (b/a = √3) with increasing pressure.

Over a range of hydration, unsaturated diacylglycerol/phosphatidylcholine mixtures adopt an inverse micellar cubic phase, of crystallographic space group Fd3m. In this study hydrated DOPC:DOG mixtures with a molar ratio close to 1∶2 were examined as a function of hydrostatic pressure, using synchrotron X-ray diffraction. The small-angle diffraction pattern at atmospheric pressure was used to calculate 2-D sections through the electron density map. Pressure initially has very little effect on the structure of the Fd3m cubic phase, in contrast to its effect on hydrated inverse bicontinuous cubic phases. At close to 2 kbar, a sharp transition occurs from the Fd3m phase to a pair of coexisting phases, an inverse hexagonal HII phase plus an (ordered) lamellar phase. Upon increasing the pressure to 3 kbar, a further sharp transition occurs from the HII phase to a (fluid) lamellar phase, in coexistence with the ordered lamellar phase. These transitions are fully reversible, but show hysteresis. Remarkably, the lattice parameter of the Fd3m phase is practically independent of pressure. These results show that these two lipids are miscible at low pressure, adopting a single lyotropic phase (Fd3m); they then become immiscible above a critical pressure, phase separating into DOPC-rich and DOG-rich phases.

The effect of hydrostatic pressure on lipid structure and dynamics is highly important as a tool in biophysics and bio-technology, and in the biology of deep sea organisms. Despite its importance, high hydrostatic pressure remains significantly less utilised than other thermodynamic variables such as temperature and chemical composition. Here, we give an overview of some of the theoretical aspects which determine lipid behaviour under pressure and the techniques and technology available to study these effects. We also summarise several recent experiments which highlight the information available from these approaches.

We have investigated the phase behavior of DPPC (dipalmitoylphosphatidylcholine) monolayers at the water−air interface using molecular dynamics simulations, where the phospholipids and the water molecules are modeled atomistically. We report pressure−area isotherms in the interval of 273−310 K. Our results show evidence for a liquid condensed (LC) to liquid expanded (LE) phase transition and indicate that ordered condensed phases can nucleate from a starting disordered phase on a time scale of approximately 50 ns. The existence of the phase transition is confirmed with structural analyses of the phospholipid pair correlation functions and of the monolayer thickness. We find that the change in the monolayer thickness associated with the LC−LE transition is largely due to a shortening of the hydrocarbon chains, with little modification in the average tilt angle of the choline head group. This result is compatible with recent sum frequency spectroscopy experiments, which concluded that the transition occurs without major changes in the orientation of the head group with respect to the monolayer plane. The dependence of the simulated pressure−area isotherms on temperature, in particular, the reduction in width of the coexistence plateau with increasing temperature, is consistent with published experimental pressure−area isotherms.

A homologous series of three octakis-alkyl-substituted porphyrin (OAP) derivatives with alkyl chain lengths of C8, C10 and C12 has been synthesised. For each compound, both the free-base (i.e. two H-atoms) and the fourfold coordinated zinc complex were investigated, where the latter for each chain length was found to exhibit liquid-crystalline mesophase behaviour. At low temperatures all three metal coordinated compounds formed a solid S phase with a structure based on a square packing of columns with a stacking periodicity of approx. 4.5 Å. At increased temperatures the square packed solid S phases undergo phase transitions to liquid-crystalline columnar phases based on centred rectangular lattices of C2/m symmetry, with the ratio of the lattice parameters, a/b, ranging between 1 and 1.732. The a/b ratio exhibited two discontinuous jumps with increasing temperature prior to melting to the isotropic phase for Zn-OAP-C8, implying the existence of three columnar phases denoted Colr1, Colr2 and Colr3. For the longer chain length compounds, the number of mesophases formed was reduced, first to two for Zn-OAP-C10, then to one for Zn-OAP-C12.

Mesostructured silica can be prepared by surfactant templating either under conditions of low surfactant template concentration (so-called liquid crystal templating) or from a pre-formed mesophase at higher surfactant concentrations (true liquid crystal templating). In this study, the structural properties of such silica products prepared using a surfactant bipyridine complex of Ru(II) are compared with similar products templated with a conventional cationic surfactant, CTAB (cetyltrimethylammonium bromide). Templating with this ruthenium metallosurfactant leads to well-ordered mesoporous silica with catalytically active RuO2nanoparticles distributed uniformly within the silica pores.

The role of cholesterol (Chol) in promoting lamellar phase formation in mixtures with 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (Lyso-PPC) in excess water was investigated using multinuclear solid-state NMR and X-ray scattering. It was found that mixtures containing Chol and Lyso-PPC form a liquid-ordered (Lo) lamellar phase over a range of temperatures and concentrations, as previously observed in mixtures of Chol with various diacylphospholipids. The maximum quadrupolar splitting of the 2H-NMR powder patterns for samples containing per-deuterated Lyso-PPC were 40–50 kHz which is strongly indicative of an Lo phase. This evidence was supported by wide angle X-ray scattering data which showed a characteristic diffuse peak centred at 4.2 Å. The Lo phase coexists with an isotropic Lyso-PPC phase at Chol concentrations up to 70 mol% Chol, and with Chol crystals at Chol concentrations above this value. Below 70 mol% Chol, an increase in the concentration of Chol in the system caused a corresponding increase in the proportion of the Lo phase present compared with the amount of isotropic Lyso-PPC. The chemical-shift anisotropy (CSA) of the static 31P-NMR spectra of the Lo phase showed the symmetry of a lamellar phase, but the linewidth, Δσ, was much narrower than CSA powder patterns obtained for diacylphospholipids in similar conditions, being ∼20 ppm as opposed to ∼40 ppm, respectively.

Mesostructured silica can be prepared by surfactant templating either under conditions of low surfactant template concentration (so-called liquid crystal templating) or from a pre-formed mesophase at higher surfactant concentrations (true liquid crystal templating). In this study, the structural properties of such silica products prepared using a surfactant bipyridine complex of Ru(II) are compared with similar products templated with a conventional cationic surfactant, CTAB (cetyltrimethylammonium bromide). Templating with this ruthenium metallosurfactant leads to well-ordered mesoporous silica with catalytically active RuO2nanoparticles distributed uniformly within the silica pores.

A homologous series of three octakis-alkyl-substituted porphyrin (OAP) derivatives with alkyl chain lengths of C8, C10 and C12 has been synthesised. For each compound, both the free-base (i.e. two H-atoms) and the fourfold coordinated zinc complex were investigated, where the latter for each chain length was found to exhibit liquid-crystalline mesophase behaviour. At low temperatures all three metal coordinated compounds formed a solid S phase with a structure based on a square packing of columns with a stacking periodicity of approx. 4.5 Å. At increased temperatures the square packed solid S phases undergo phase transitions to liquid-crystalline columnar phases based on centred rectangular lattices of C2/m symmetry, with the ratio of the lattice parameters, a/b, ranging between 1 and 1.732. The a/b ratio exhibited two discontinuous jumps with increasing temperature prior to melting to the isotropic phase for Zn-OAP-C8, implying the existence of three columnar phases denoted Colr1, Colr2 and Colr3. For the longer chain length compounds, the number of mesophases formed was reduced, first to two for Zn-OAP-C10, then to one for Zn-OAP-C12.

Over a range of hydration, unsaturated diacylglycerol/phosphatidylcholine mixtures adopt an inverse micellar cubic phase, of crystallographic space group Fd3m. In this study hydrated DOPC:DOG mixtures with a molar ratio close to 1∶2 were examined as a function of hydrostatic pressure, using synchrotron X-ray diffraction. The small-angle diffraction pattern at atmospheric pressure was used to calculate 2-D sections through the electron density map. Pressure initially has very little effect on the structure of the Fd3m cubic phase, in contrast to its effect on hydrated inverse bicontinuous cubic phases. At close to 2 kbar, a sharp transition occurs from the Fd3m phase to a pair of coexisting phases, an inverse hexagonal HII phase plus an (ordered) lamellar phase. Upon increasing the pressure to 3 kbar, a further sharp transition occurs from the HII phase to a (fluid) lamellar phase, in coexistence with the ordered lamellar phase. These transitions are fully reversible, but show hysteresis. Remarkably, the lattice parameter of the Fd3m phase is practically independent of pressure. These results show that these two lipids are miscible at low pressure, adopting a single lyotropic phase (Fd3m); they then become immiscible above a critical pressure, phase separating into DOPC-rich and DOG-rich phases.