John M. Seddon

Pressure-jump X-ray studies of liquid crystal transitions in lipids

In this paper, we give an overview of our studies by static and time-resolved X-ray diffraction of inverse cubic phases and phase transitions in lipids. In §1, we briefly discuss the lyotropic phase behaviour of lipids, focusing attention on non-lamellar structures, and their geometric/topological relationship to fusion processes in lipid membranes. Possible pathways for transitions between different cubic phases are also outlined. In §2, we discuss the effects of hydrostatic pressure on lipid membranes and lipid phase transitions, and describe how the parameters required to predict the pressure dependence of lipid phase transition temperatures can be conveniently measured. We review some earlier results of inverse bicontinuous cubic phases from our laboratory, showing effects such as pressure-induced formation and swelling. In §3, we describe the technique of pressure-jump synchrotron X-ray diffraction. We present results that have been obtained from the lipid system 1u200a:u200a2 dilauroylphosphatidylcholine/lauric acid for cubic–inverse hexagonal, cubic–cubic and lamellar–cubic transitions. The rate of transition was found to increase with the amplitude of the pressure-jump and with increasing temperature. Evidence for intermediate structures occurring transiently during the transitions was also obtained. In §4, we describe an IDL-based ‘AXcess’ software package being developed in our laboratory to permit batch processing and analysis of the large X-ray datasets produced by pressure-jump synchrotron experiments. In §5, we present some recent results on the fluid lamellar–Pn3m cubic phase transition of the single-chain lipid 1-monoelaidin, which we have studied both by pressure-jump and temperature-jump X-ray diffraction. Finally, in §6, we give a few indicators of future directions of this research. We anticipate that the most useful technical advance will be the development of pressure-jump apparatus on the microsecond time-scale, which will involve the use of a stack of piezoelectric pressure actuators. The pressure-jump technique is not restricted to lipid phase transitions, but can be used to study a wide range of soft matter transitions, ranging from protein unfolding and DNA unwinding and transitions, to phase transitions in thermotropic liquid crystals, surfactants and block copolymers.

Inverse lyotropic phases of lipids and membrane curvature.

In recent years it has become evident that many biological functions and processes are associated with the adoption by cellular membranes of complex geometries, at least locally. In this paper, we initially discuss the range of self-assembled structures that lipids, the building blocks of biological membranes, may form, focusing specifically on the inverse lyotropic phases of negative interfacial mean curvature. We describe the roles of curvature elasticity and packing frustration in controlling the stability of these inverse phases, and the experimental determination of the spontaneous curvature and the curvature elastic parameters. We discuss how the lyotropic phase behaviour can be tuned by the addition of compounds such as long-chain alkanes, which can relieve packing frustration. The latter section of the paper elaborates further on the structure, geometric properties, and stability of the inverse bicontinuous cubic phases.

Methylene- and ether-linked liquid crystal dimers II. Effects of mesogenic linking unit and terminal chain length

Four series of liquid crystal dimers have been prepared containing either ether‐linked or methylene‐linked spacers. Changing the spacer from being ether‐linked, i.e. O(CH2) n O, to methylene‐linked, i.e. (CH2) n +2, results in decreased nematic–isotropic transition temperatures, and this reduction is more pronounced for odd‐membered spacers. By contrast, the entropy change associated with the nematic–isotropic transition is higher for an even‐membered methylene‐linked dimer than for the corresponding ether‐linked material. This trend is reversed for odd members. These observations are completely in accord with the predictions of a theoretical model developed by Luckhurst and co‐workers in which the only difference between the dimers is their shape. For the highly non‐linear pentamethylene‐linked dimers, only those with a short terminal chain exhibited fluid smectic behaviour, specifically, a monotropic alternating SmC structure which allowed for the efficient packing of the bent molecules. Once the terminal chain reached a value of m = 9, a modulated ordered smectic phase was observed. For even‐membered dimers, which exhibit only nematic phases upon melting for short terminal chain lengths, smectic phase behaviour was promoted with increasing terminal chain length, as is conventionally observed. Even‐membered ether‐linked dimers exhibited a SmC phase whereas even‐membered methylene‐linked dimers exhibited an ordered smectic G/J phase. Thus, it would appear that the differences in the transitional properties of ether‐ and methylene‐linked dimers can be accounted for largely in terms of geometrical factors.

Membrane-protein crystallization in cubo: temperature-dependent phase behaviour of monoolein–detergent mixtures

The lipidic cubic phase of monoolein has proved to be a matrix well suited to the production of three-dimensional crystals of membrane proteins. It consists of a single continuous bilayer, which is contorted in three-dimensional space and separates two distinct water channels. It has previously been proposed that on the addition of precipitants, membrane proteins embedded in the cubic phase migrate through the matrix to nucleation sites and that this process is dependent upon the stability of the lipidic cubic phase. Here, the effect of detergent type (C(8)-C(12) glucosides, C(8)-C(12) maltosides and C(7) thioglucoside) and concentration (1-3x the critical micelle concentration; CMC) on cubic phase stability are reported in the form of the temperature-dependent phase behaviour (268-313 K) in 40% aqueous solution. The results are tabulated to show the best monoolein (MO)-detergent mixtures, mixing temperatures and crystallization temperatures identified. Monoolein-detergent mixtures suited for low-temperature in cubo crystallization of temperature-sensitive proteins are also reported for the first time. These mixtures can be prepared at low temperatures (mixed at <or=288 K) and remain stable at 277 K for a period of at least one Month. They include MO-heptyl thioglucoside (1x and 3x CMC), MO-nonyl glucoside (3x CMC), MO-octyl maltoside (3x CMC), MO-nonyl maltoside (1x CMC) and MO-decyl maltoside (1x CMC).

Non-symmetric liquid crystal trimers. The first example of a triply-intercalated alternating smectic C phase

Non-symmetric liquid crystal trimers containing three different mesogenic units and two flexible spacers have been synthesised, and exhibit a new smectic modification, the triply-intercalated alternating smectic C phase.

Field-induced alignment of a smectic-A phase: a time-resolved x-ray diffraction investigation.

The field-induced alignment of a smectic-A phase is, in principle, a complicated process involving the director rotation via the interaction with the field and the layer rotation via the molecular interactions. Time-resolved nuclear magnetic resonance spectroscopy has revealed this complexity in the case of the director alignment, but provides no direct information on the motion of the layers. Here we describe a time-resolved x-ray diffraction experiment using synchrotron radiation to solve the challenging problem of capturing the diffraction pattern on a time scale which is fast in comparison with that for the alignment of the smectic layers. We have investigated the alignment of the smectic-A phase of 4-octyl-4()-cyanobiphenyl by a magnetic field. The experiment consists of creating a monodomain sample of the smectic-A phase by slow cooling from the nematic phase in a magnetic field with a flux density of 7 T. The sample is then turned quickly through an angle phi(0) about an axis parallel to the x-ray beam direction but orthogonal to the field. A sequence of two-dimensional small angle x-ray diffraction patterns are then collected at short time intervals. Experiments were carried out for different values of phi(0), and at different temperatures. The results show that the alignment behavior changes fundamentally when phi(0) exceeds 45 degrees, and that there is a sharp change in the alignment process when the temperature is less than 3 degrees C below the smectic-A-nematic transition. The results of the x-ray experiments are in broad agreement with the NMR results, but reveal major phenomena concerning the maintenance of the integrity of the smectic-A layer structure during the alignment process.

Highly non-linear liquid crystal tetramers

Three new homologous series of liquid crystal tetramers in which four mesogenic groups are linked via three alkyl spacers have been synthesised and characterised. In each series the length of the outer two spacers, n, is varied from 3 to 12 methylene units while the central spacer is held at 5 methylene units. The three series differ only in the substitution pattern around the inner two mesogenic units. The series in which the inner spacer is attached to the 4-position on both mesogenic groups is referred to as n-p5p-n while n-p5m-n refers to attachment at the 4-position on one group and the 3-position on the other. n-m5m-n refers to attachment at the 3-position on both groups. All the members of the n-p5p-n and n-p5m-n series exhibit nematic behaviour. The nematic–isotropic ntransition temperatures exhibit a pronounced alternation as the length and parity of the outer spacers are varied but which attenuates as n is increased. By contrast, the odd–even effects shown by the associated entropy changes are not attenuated as n is increased. 4-p5m-4 and 5-p5m-5 also exhibit smectic behaviour. 4-m5m-4 shows a nematic phase and an alternating smectic phase analogous in structure to that observed for certain semi-flexible main chain liquid crystal polymers. The n-m5m-n compounds with nxa0=xa06, 8, 9 and 10 also exhibit a nematic phase and a disordered crystalline phase. The trends in the transitional properties of the series are interpreted in terms of average molecular shapes and a model developed to understand the behaviour of liquid crystal dimers.

Differing applications of Raman scattering to liquid crystals

An overview of the differing possibilities offered by Raman scattering in the study of liquid crystals is given, together with a few experimental examples of the least studied phenomena. Among them, changes in the intrinsic Raman polarizabilities and phonon self-energies as a function of temperature which provide, in principle, an experimental method for the study of coexisting and/or inhomogeneous phases through Raman imaging. We show that the C-H in-plane deformation mode of the phenyl rings is particularly suited to monitor both intrinsic scattering efficiencies and frequency changes. This mode is present in a large family of liquid crystal compounds and we present evidence indicating that the origin of these changes is its electron-phonon coupling to the lowest allowed electronic transition in the ultraviolet.

STRUCTURE AND PHASE BEHAVIOUR OF SYNTHETIC GLYCOLIPIDS

We have used X-ray diffraction and differential scanning calorimetry (DSC) to study the structure and lyotropic phase behaviour of two synthetic dialkyl glycolipids having β-D-glucose and β-D-galactose headgroups and two saturated tetradecyl (C14) chains. The diastereomeric compounds, 1,2-di-O-tetradecyl-3-O-(β-D-glucopyranosyl)-sn-glycerol (di-14:0–β-D-GlcDAG) and 1,2-di-O-tetradecyl-3-O-(β-D-galactopyranosyl)-sn-glycerol (di-14:0-β-D-GalDAG) show striking differences in their phase behaviour, particularly in the ordered lamellar phase region. Both compounds adopt the fluid lamellar Lα phase upon cooling from the HII phase, but below the chain-melting transition,di-14:0-β-D-GlcDAG forms a metastable Lβ gel phase, whereas di-14:0–β-D-GalDAG forms only crystalline lamellar phases, on the timescale of our measurements. We have determined the limiting hydrations of the various phases, and compare the findings from these glycolipids with those from our previous studies of the phospholipid didodecyl phosphatidylethanolamine (di-12:0-PE).

A Spectroscopic Study of Group IV Transition-Metal-Incorporated Direct Templated Mesoporous Catalysts. Part 2. A Comparison of Ti-, Zr- and Hf-Containing Materials

Ti, Zr and Hf ions have been incorporated into mesoporous silica using a novel one-pot procedure based on the true liquid crystal templating (TLCT) synthesis method. The nature of the metallic sites was assessed using diffuse reflectance UV-visible, photoluminescence and FT-IR spectroscopy. In situ adsorption of oxygen and ammonia probe molecules provided information on the accessibility of optically active species.