The influence of α-, γ- and δ-tocopherols on the structure and phase behavior of dipalmitoyl phosphatidylcholine (DPPC) bilayers has been determined from X-ray diffraction studies on oriented multilayers. In all the three cases the main-transition temperature (T(m)) of DPPC was found to decrease with increasing tocopherol concentration up to around 25 mol%. Beyond this the main transition is suppressed in the case of γ-tocopherol, whereas T(m) becomes insensitive to composition in the other two cases. The pre-transition is found to be suppressed over a narrow tocopherol concentration range between 7.5 and 10 mol% in DPPC-γ-tocopherol and DPPC-δ-tocopherol bilayers, and the ripple phase occurs down to the lowest temperature studied. In all the three cases a modulated phase is observed above a tocopherol concentration of about 10 mol%, which is similar to the P(β) phase reported in DPPC-cholesterol bilayers. This phase is found to occur even in excess water conditions at lower tocopherol concentrations, and consists of bilayers with periodic height modulation. These results indicate the ability of tocopherols to induce local curvature in membranes, which could be important for some of their biological functions.

We have studied the influence of some major phytosterols on the structure and phase behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) membranes. Phase diagrams of the binary systems have been determined from X-ray diffraction data. All DPPC-phytosterol membranes studied exhibit a sterolinduced modulated phase (P b ) below the main transition, first reported in DPPC-cholesterol membranes. Structural features of this phase have been deduced from electron density maps calculated from the diffraction data. In addition, a re-entrant ripple phase is observed in DPPC-stigmasterol membranes as a function of temperature. The increase in the bilayer thickness in the fluid phase, due to the ordering of the lipid chains by the sterol molecules, has also been determined from the electron density profiles of the bilayers. All the phytosterols studied are found to increase the bilayer thickness, but to a lower extent compared to cholesterol. Some of the phytosterols phase separate from POPE even at very low concentrations, whereas the others have a similar effect on the phase behavior of POPE, with the temperature range over which the fluid lamellar phase occurs first decreasing and then increasing with increasing phytosterol concentration.Phytosterols are important structural components of plant membranes. 1-3 Unlike mammalian and fungal cells, which generally contain one major sterol, cholesterol 4 and ergosterol, 5 respectively, plants have a characteristically complex mixture of sterols. Over 250 different sterols and related compounds have been reported in various plant and marine species. The most abundant of all plant sterols is b-sitosterol, followed by stigmasterol and campesterol. In any arbitrary phytosterol mixture, one is most likely to find about 70% of b-sitosterol, 20% of stigmasterol, 5% of campesterol, the remaining being a mixture of less abundant sterols. 1

We present a phenomenological theory of phase transitions in achiral lipid membranes in terms of two coupled order parameters -a scalar order parameter describing lipid chain melting, and a vector order parameter describing the tilt of the hydrocarbon chains below the chain-melting transition. Existing theoretical models fail to account for all the observed features of the phase diagram, in particular the detailed microstructure of the asymmetric ripple phase lying between the fluid and the tilted gel phase. In contrast, our two-component theory reproduces all the salient structural features of the ripple phase, providing a unified description of the phase diagram and microstructure. PACS numbers: 87.16.D-,61.30.DkPhospholipids self-assemble in water to form a rich variety of spatially modulated phases [1]. The simplest of these is the 1-dimensionally modulated fluid lamellar phase (L α ) consisting of periodic stacks of lipid bilayer membranes separated by water, where the hydrocarbon chains are floppy with liquid-like in-plane order. Changing the temperature or water content induces a sequence of symmetry breaking transitions characterized by unique microstructures.On reducing the temperature below the chain melting (main) transition (T m ), the L α phase of phosphatidylcholines (PCs) transforms to a gel phase (L β ′ ), characterized by fully-stretched all − trans chains which are tilted with respect to the bilayer normal [2][3][4]. In addition, an asymmetric ripple phase (P β ′ ) is found to occur in between the L α and L β ′ phases in many PCs at high water content [1,2,5].Extensive studies using a variety of experimental techniques [1,[6][7][8][9][10][11][12][13][14][15][16][17][18], reveal that the P β ′ phase is characterized by a periodic saw-tooth height modulation of the bilayers having an amplitude of ∼ 1 nm and a wavelength of ∼ 15 nm, and a bilayer thickness that is different in the two arms of the ripple ( fig. 1) [9,10]. As a result, the rippled bilayers lack a mirror plane normal to the rippling direction. While in principle, this discrete symmetry breaking can arise from an asymmetry in either shape (unequal lengths of the two arms) or bilayer thickness (unequal bilayer thickness in the two arms), in practice these asymmetries seem to appear simultaneously.At first it was believed that the origins of the asymmetric ripple lay in the chirality of lipid molecules [20]. However, subsequent experiments using racemic mixtures showed this was not the case [8,15]. More recently, all-atom molecular dynamics simulations of lipid bilayers have observed that the degree of chain ordering is different in the two arms of the ripple [21]. The occurrence of the ripple phase only in those lipids that exhibit a L β ′ phase at lower temperatures [22], and in isolated bilayers [23], suggests an intimate connection between chain tilt and the ability of the bilayers to form ripples.Several theoretical models have been proposed to describe the sequence of phase transitions in such lipid bilayers and the microstructure ...

The interdigitated phase of the lipid bilayer results when acyl chains from opposing monolayers fully interpenetrate such that the terminal methyl groups of the respective lipid chains are located at the interfacial region on the opposite sides of the bilayer. Usually, chain interdigitation is not encountered in a symmetric chain phosphatidylcholine (PC) membrane but can be induced under certain special conditions. In this article, we elucidate the contribution of small amphiphatic molecules in altering the physical properties of a symmetric chain PC bilayer membrane, which results in acyl chain interdigitation. Using small-angle X-ray scattering (SAXS), we have carried out a systematic investigation of the physical interactions of three naphthalene derivatives containing hydroxyl groups: β-naphthol, 2,3-dihydroxynaphthalene, and 2,7-dihydroxynaphthalene, with dipalmitoylphosphatidylcholine (DPPC) bilayers. On the basis of the diffraction patterns, we have determined the temperature-composition phase diagrams of these binary mixtures. The present study not only enables us to gain insight into the role played by small molecules in altering the packing arrangement of the acyl chains of the constituting PC lipids of the bilayer but also brings to light some important features that have not yet been reported hitherto. One such feature is the stabilization of the enigmatic asymmetric ripple phase over a wide temperature and concentration range. The results presented here strongly point toward a clear correlation between chain interdigitation and the stability of the ripple phase.

An anisotropic colloidal shape in combination with an externally tunable interaction potential results in a plethora of self-assembled structures with potential applications toward the fabrication of smart materials. Here we present our investigation on the influence of an external magnetic field on the self-assembly of hematite-silica core–shell prolate colloids for two aspect ratios ρ = 2.9 and 3.69. Our study shows a rather counterintuitive but interesting phenomenon, where prolate colloids self-assemble into oblate liquid crystalline (LC) phases. With increasing concentration, particles with smaller ρ reveal a sequence of LC phases involving para-nematic, nematic, smectic, and oriented glass phases. The occurrence of a smectic phase for colloidal ellipsoids has been neither predicted nor reported before. Quantitative shape analysis of the particles together with extensive computer simulations indicate that in addition to ρ, a subtle deviation from the ideal ellipsoidal shape dictates the formation of this unusual sequence of field-induced structures. Particles with ρ = 2.9 exhibit a hybrid shape containing features from both spherocylinders and ellipsoids, which make their self-assembly behavior richer than that observed for either of the “pure” shapes. The shape of the particles with higher ρ matches closely with the ideal ellipsoids, as a result their phase behavior follows the one expected for a “pure” ellipsoidal shape. Using anisotropic building blocks and external fields, our study demonstrates the ramifications of the subtle changes in the particle shape on the field-directed self-assembled structures with externally tunable properties.