Intramolecular disorder and its relation to mesophase structure in lipid/water mixtures

NMR spin-half pair dipolar echo measurements are reported for the lamellar (dispersions and multibilayer stacks) and hexagonal phases of potassium palmitate/²H₂O mixtures. In the lamellar L_β and L_γ (gel) phases the alkyl chains are rigid and perfectly ordered, while in the lamellar L_α and hexagonal phases they are flexible and disordered. In particular, the measurements show that in the fluid lamellar L_α phase the chain is “bent” at the C₉–C₁₀ segment; but is “straight” in the hexagonal phase.     

Lipid membrane structure and interactions in dimethyl sulfoxide/water mixtures.

In this paper we have investigated via x-ray diffraction the influence of dimethyl sulfoxide (DMSO), known for its biological and therapeutic properties, on the structure of lipid membranes of dipalmitoylphosphatidylcholine (DPPC) in excess of the solvent (DMSO/water) at mole DMSO fractions XDMSO in (0.1) and under equilibrium conditions. At small XDMSO </= 0.133 the repeat distance d is reduced remarkably, whereas wide-angle x-ray diffraction pattern remains almost unchanged with the increase in XDMSO. It agrees well with previous study (Yu and Quinn, 1995). At 0.133 < XDMSO < 0.3 the repeat period d reduces slowly; however, an orthorombic in-plane lattice of hydrocarbon chains transfers to a disordered quasihexagonal lattice. The increase in XDMSO from 0.3 up to approximately 0.9 leaves d almost unchanged, whereas it leads to less disordered packing of hydrocarbon chains. At XDMSO approximately 0.9, Lbeta' phase transfers into interdigitated phase. The chain-melting phase transition temperature of DPPC membranes increases by several degrees with the increase of DMSO concentration. It points to a strong concentration-dependent solvation of membrane surface by DMSO. Thus DMSO strongly interacts with the membrane surface, probably displacing water and modifying the structure of the lipid bilayer. It appears to determine some of the properties of DMSO as a biologically and therapeutically active substance.     

The structure of haem in pyridine/water mixtures and its implication in studies of haem catabolism.

1. A study of haem spectra in pyridine/water mixtures at low pyridine concentrations revealed changes in haemochrome structure consistent with an aggregation process. No corresponding change in the structure of the haemichrome species was observed. 2. This aggregation has been correlated with a previously observed sharp decrease in the rate of coupled oxidation (degradation) of haem as pyridine concentration is decreased. The decrease appears to be due primarily to haem aggregation and not to changes in the hydrophobic nature of the solvent. The effect of ethanol and butanone addition was examined and supports this conclusion. 3. Evidence is presented that coupled oxidation occurs via the iron (II) species (haemochrome).     

The temperature-composition phase diagram and mesophase structure characterization of monopentadecenoin in water.

The temperature-composition phase diagram of monopentadecenoin, a monoacylglycerol with a cis monounsaturated fatty acid 15 carbon atoms long (C15:1c10) in water was constructed using x-ray diffraction. Low- and wide-angle diffraction patterns were collected from samples of fixed hydration as a function of temperature in the heating direction on x-ray-sensitive film. The temperature and hydration ranges investigated were 0-104 degrees C and 0-60% (w/w) water, respectively. The phases identified in the system include the lamellar crystalline phase, the lamellar liquid crystalline phase, the fluid isotropic phase, and two inverted cubic phases belonging to space groups la3d (Q230) and Pn3m (Q244). Particular attention has been devoted to the issues of phase equilibrium, phase boundary verification, and structure characterization. The phase diagrams of monopentadecenoin, monomyristolein (C14:1c9), and monoolein (C18:1c9) are compared, and the impact of molecular structure on mesophase stability and structure is discussed.     

Phase diagrams for impure lipid systems. Application to lipid/anaesthetic mixtures

A physical model is presented to describe theoretically the temperature-dependent interactions of lipid bilayers with small molecules such as anaesthetics. Based on an earlier model, a triangular lattice in which each site is occupied by a single lipid chain is constructed and the small (anaesthetic) molecules are assumed to occupy interstitial sites in the centre of each lattice triangle. The phase characteristics of such lipid/anaesthetic mixtures are described in terms of the interaction parameters between lipid-lipid, lipid-anaesthetic and anaesthetic-anaesthetic molecules. Depending on the chemical nature of the interacting species the following three models are formulated: Model I. An interstitial model in which the only perturbation is in the head-group region of the bilayer and direct interactions between neighbouring anaesthetic molecules are taken into account. Model II. Here, only hydrophobic interactions between anaesthetics and lipids are considered. Model III. Both van der Waals' and coulombic interactions are taken into account. Phase diagrams for the three models are obtained by numerical calculation over a wide range of interaction parameters. It is shown that in all three models, lateral phase separation takes place due to the presence of anaesthetics. The heat of transition, however, is found to be virtually independent of the anaesthetic concentration.     

Fluorescence excitation profiles of beta-carotene in solution and in lipid/water mixtures

Intrinsic fluorescence from all-trans β-carotene molecules in solution and embedded in lipid/water mixtures has been observed under laser excitation and its excitation profiles measured. The profiles closely correspond to the absorption spectra. The observations can be explained in terms of a low-lying ¹A_g excited state.     

Phase structure of liposome in lipid mixtures

Gas microbubbles present in ultrasound imaging contrast agents are stabilized by lipid aggregates that typically contain a mixture of lipids. In this study, the phase structure of the lipid mixtures that contained two or three lipids was investigated using three different methods: dynamic light scattering, ¹H NMR, and microfluidity measurements with fluorescence probes. Three lipids that are commonly present in imaging agents (DPPC, DPPE-PEG, and DPPA) were used. Two types of systems, two-lipid model systems and simulated imaging systems were investigated. The results show that liposomes were the dominant aggregates in all the samples studied. The polar PEG side chains from the PEGylated lipid lead to the formation of micelles and micellar aggregates in small sizes. In the ternary lipid systems, almost all the lipids were present in bilayers with micelles absent and free lipids at very low concentration. These results suggest that liposomes, not micelles, contribute to the stabilization of microbubbles in an ultrasound imaging contrast agent.     

Partial specific volumes of lipid and water in mixtures of egg lecithin and water.

We describe in this paper measurements of the partial specific volumes of lipid (upsilon L) and water (upsilon W) in mechanical mixtures of egg yolk lecithin and water over a range of hydrations (5-55 wt% water) that includes the region at which excess water appears. upsilon L and upsilon W are found to be very nearly 1 cm3/g over the entire range. The water activities of the mixtures were also determined and found to be the same as for lipid deposited as oriented multilayers on solid substrates.     

Modification of membrane lipid: physical properties in relation to fatty acid structure.

Differential scanning calorimetry (DSC) and electron spin resonance (ESR) measurements were made to characterize how modifications in the fatty acid composition of Escherichia coli affected the thermotropic phase transition(s) of the membrane lipd. When the fatty acid composition contained between 20 and 60% saturated fatty acids, the DSC curves for isolated phospholipids and cytoplasmic membranes showed a broad (15-25 degree C) gel to liquid-crystalline phase transition, the position of which depended on the particular fatty acid composition. Utilizing multiple lipid mutants, enrichment of the membrane phospholipids with a single long-chain cis-monoenoic fatty acid in excess of that possible in a fatty acid levels less than 20% and gradually replaced the broad peak as the cis-monoenoic fatty acid content increased. These results were obtained with phospholipids, cytoplasmic membranes, and whole cells. With these same phopholipids, plots of 2,2,6,6-tetramethylpiperidinyl-1-oxy partitioning and ESR order parameters vs. 1/T revealed discontinuities at temperatures 40-60 degrees C above the calorimetrica-ly measured gel to liquid-crystalline phase transitions. Moreover, when the membrane phospholipids were enriched with certain combinations of cis-monenoic fatty acids (e.g., cis-delta 9-16:1 plus cis-delta 11-18:1) the DSC curve showed a broad gel to liquid crystalline phase change below 0 degrees C but the ESR studies revealed no discontinuities at temperatures above those of the gel to liquid-crystalline transition. These results demonstrated that enrichment of the membrane lipids with molecules in which both fatty acyl chains are identical cis-monoenoic residues led to a distinct type of liquid-crystalline phase. Furthermore, a general conclusion from this study is that Escherichia coli normally maintains a heterogeneous mixture of lipid molecules and, by so doing, prevents strong lipid-lipid associations that lead to the formation of lipid domains in the membrane.     

Long-call structure and its relation to taxonomy in lion tamarins

Recent evidence on craniodental morphology suggests the acceptance of three species of lion tamarins (Leontopithecus). Confirmatory evidence is presented here using the morphology of long-call vocalizations recorded from several individuals of each type of lion tamarin. Recordings were made of Leontopicthecus rosalia, Leontopithecus chrysopygus, and Leontopithecus chrysomelas at the Centro de Primatologia do Rio de Janeiro (FEEMA) and of L. rosalia at Monkey Jungle in Florida. Thirty separate parameters were measured, and 17 of them differed significantly between populations. In general, L. chrysomelas had higher pitched calls with shorter note duration, while L. chrysopygus, the larger of the animals, had lower pitched calls with longer note duration. L. rosalia was either intermediate to the other two populations or resembled L. chrysopygus. Thus, the results from the analysis of vocal structures closely paralleled the results obtained with more traditional taxonomic methods and suggests that the quantitative analysis of vocal strucures can be a useful adjunct in taxonomy.     

The N-terminal beta-barrel structure of lipid body lipoxygenase mediates its binding to liposomes and lipid bodies.

Phospholipase A2 and a particular isoform of lipoxygenase are synthesized and transferred to lipid bodies during the stage of triacylglycerol mobilization in germinating cucumber seedlings. Lipid body lipoxygenase (LBLOX) is post-translationally transported to lipid bodies without proteolytic modification. Fractionation of homogenates from cucumber cotyledons or transgenic tobacco leaves expressing LBLOX showed that a small but significant amount was detectable in the microsomal fraction. A beta-barrel-forming N-terminal domain in the structure of LBLOX, as deduced from sequence data, was shown to be crucial for selective intracellular transport from the cytosol to lipid bodies. Although a specific signal sequence for targeting protein domains to the lipid bodies could not be established, it was evident that the beta-barrel represents a membrane-binding domain that is functionally comparable with the C2 domains of mammalian phospholipases. The intact beta-barrel of LBLOX was demonstrated to be sufficient to target in vitro a fusion protein of LBLOX beta-barrel with glutathione S-transferase (GST) to lipid bodies. In addition, binding experiments on liposomes using lipoxygenase isoforms, LBLOX deletions and the GST-fusion protein confirmed the role of the beta-barrel as the membrane-targeting domain. In this respect, the cucumber LBLOX differs from cytosolic isoforms in cucumber and from the soybean LOX-1. When the beta-barrel of LBLOX was destroyed by insertion of an additional peptide sequence, its ability to target proteins to membranes was abolished.