Lipid organization and dynamics of the monostearoylglycerol-water system. A 2H NMR study

Self-Assembling Behavior of Glycerol Monoundecenoate in Water

The self-assembling properties of glycerol esters in water are well known. Still, few data on glycerol monoesters of undecylenic acid are available. The aim of this study was to highlight the behavior of glycerol monoundecenoate (GM-C11:1) in different diluted and concentrated states. Its self-assembling properties in water and upon solid inorganic surfaces were investigated in the diluted state using surface tension experiments, atomic force microscopy, and cryogenic transmission electron microscopy studies. In the concentrated state, the gelling properties in the presence of water were investigated using polarized light microscopy, differential scanning calorimetry (DSC), and small-angle X-ray scattering (SAXS) experiments. GM-C11:1 at 100 mg/L self-assembles at the liquid/air interfaces as aggregates of approximately 20 nm in diameter, organized into concentric forms. These aggregates are spherical globules composed of several molecules of GM-C11:1. At higher concentrations (1000 and 10⁴ mg/L), GM-C11:1 is able to uniformly coat liquid/air and liquid/solid interfaces. In bulk, GM-C11:1 forms spontaneously aggregates and vesicles. In a more concentrated state, GM-C11:1 assembles into lamellar L_β and L_α forms in water. By cross-referencing SAXS and DSC findings, we were able to distinguish between interlamellar water molecules strongly bound to GM-C11:1 and other molecules remaining unbound and considered to be "mobile" water. The percentage of water strongly bound was proportional to the percentage of GM-C11:1 in the system. In this case, GM-C11:1 appears to be an effective molecule for surface treatments for which water retention is important.

Long-timescale motions in glycerol-monopalmitate lipid bilayers investigated using molecular dynamics simulation

The occurrence of long-timescale motions in glycerol-1-monopalmitate (GMP) lipid bilayers is investigated based on previously reported 600 ns molecular dynamics simulations of a 2×8×8 GMP bilayer patch in the temperature range 302-338 K, performed at three different hydration levels, or in the presence of the cosolutes methanol or trehalose at three different concentrations. The types of long-timescale motions considered are: (i) the possible phase transitions; (ii) the precession of the relative collective tilt-angle of the two leaflets in the gel phase; (iii) the trans-gauche isomerization of the dihedral angles within the lipid aliphatic tails; and (iv) the flipping of single lipids across the two leaflets. The results provide a picture of GMP bilayers involving a rich spectrum of events occurring on a wide range of timescales, from the 100-ps range isomerization of single dihedral angles, via the 100-ns range of tilt precession motions, to the multi-μs range of phase transitions and lipid-flipping events.

Effect of cis and trans double bonds on conformational disordering of the hydrocarbon chain of lipid, unsaturated monoacylglycerols, in the lamellar phase of a binary system with water

To clarify the influence of cis and trans double bonds on conformational disordering of unsaturated hydrocarbon-chain of lipids in bilayer formed in the binary system with water, calorimetric study was conducted for systems of unsaturated monoacylglycerols; monoolein (MO), monovaccenin (MV), and monoelaidin (ME). Heat capacities of the binary systems were measured by adiabatic calorimetry. The observed entropies of transition (Δ(trs)S) from lamellar phase (L(α)) to fluid isotropic (FI) phase are very small and depend on lipids: Δ(trs)S of MO/water, MV/water, and ME/water were ca. 0.8 J K(-1) (mol of lipid) (-1), ca. 0.9 J K(-1) (mol of lipid) (-1), and ca. 0.4 J K(-1) (mol of lipid) (-1), respectively. These show that the conformational disordering of the hydrocarbon chain over gauche and trans conformations is suppresed in L(α) phase. Through the comparison of Δ(trs)S among the binary systems, the suppresed conformational disordering of hydrocarbon chain in the bilayer is discussed.

Ultrasonic velocity measurements as a method for investigating phase transitions of monoglyceride emulsifier systems in pearlescent cosmetic creams

The phase transitions of monoglyceride emulsifier systems and pearlescent effects in cosmetic creams are investigated using ultrasonic velocity measurements. The transitions between the different phases of monoglyceride emulsifier systems--the coagel, liquid-crystalline lamellar phase, and gel phase--are detected in creams by changes in the ultrasonic velocity with variation of the temperature. The phase transition temperatures correspond to DSC results. Furthermore, the slope of the ultrasound velocity curve correlates with the amount of bound water in the different phases. These insights into the ultrasonic velocity properties of the monoglyceride emulsifier system of creams make it possible to more closely study the pearlescent effect of the coagel. The temperature domain of the short time reversibility of the pearlescence as well as the back-formation time of the coagel can be determined with this method, which enable the optimization of the formulation of pearlescent creams.

Stearic acid solubility and cubic phase volume

Stearic acid (SA) is highly soluble in structurally diverse solvents. SA/solvent packing within a (24.8 A)3 cubic volume explains the stoichiometry of SA solubility at multiple temperatures in multiple solvents. In the absence of solvent, the cubic volume contains 25 molecules at van der Waals distances from each other. At 55 degrees C, SA occupied half the cubic volume in saturated solution of four structurally diverse solvents. Below 4% SA/volume (e.g. in acetonitrile), the head and foot of each SA molecules on average is more than one solvent molecule away from the head and foot of a neighboring SA molecule. At 50% SA/cubic volume, -CH2- groups on SA molecules are separated from neighboring -CH2- groups on SA molecules by a monolayer of solvent molecules. Lowering the temperature from 55 to 25 degrees C, the volume fraction of SA decreased by a factor of 2 (or more) for every 6 degrees C. Lowering temperature increased the relative number of column of solvent molecules in the cubic phase, and correspondingly, the distance between SA molecules within the cubic volume increased. In three of five solvents, molecular mechanics calculations demonstrated the van der Waals stabilization that occurs from SA/SA affinity in the absence of solvent is similar in magnitude to the van der Waals stabilization from SA/solvent affinity. Methyl-t-butyl ether was less stabilized than hexane, acetone or methanol because the more bulky molecules packed less efficiently within the cubic volume. The most efficient/most stable packing however was still as columns of solvent between columns of SA. The efficiency and stability of SA and solvent packing optimal within the (24.8 A)3 cubic volume. Between 100 and 8% SA, multiple SA molecules present within the cubic volume function as SA aggregates. Both inter- and intra-cubic (phase) volume properties of SA aggregates coexist. Although acetonitrile and SA at the molecular level are both rod shaped, acetonitrile disrupted the packing of SA molecules within the cubic phase. The disrupted packing explains the much lower solubility of SA in acetonitrile than in the other solvents. The same molecular structures (e.g. methanol) can either stabilize or disrupt the packing of aggregated SA molecules, depending upon temperature. The mechanisms of aggregation within cubic volumes could also occur with structurally more complicated lipids. Aggregation and dispersion from such cubic phases could also be present in more complex chemical and/or macromolecular environments.

Phase transitions and microstructure of emulsion systems prepared with acylglycerols/zinc stearate emulsifier

The emulsification processes, during which acylglycerols/zinc stearate emulsifier, water, and oil phase formed ternary systems, such as water-in-oil (W/O) emulsions, oil-in-water (O/W) dispersions, and unstable oil-water mixtures, were investigated in order to characterize the progressive transformations of the dispersed systems. The type, structure, and phase transitions of the systems were found to be determined by temperature and water phase content. Crystallization of the emulsifier caused the destabilization and subsequent phase inversion of the emulsions studied, at a temperature of 60-61 degrees C. The observed destabilization was temporary and led, at lower temperature, to W/O emulsions, "O/W + O" systems, or O/W dispersions, depending on the water content. Simultaneous emulsification and cooling of 20-50 wt % water systems resulted in the formation of stable W/O emulsions that contained a number of large water droplets with dispersed oil globules inside them ("W/O + O/W/O"). In water-rich systems (60-80 wt % of water), crystallization of the emulsifier was found to influence the formation of crystalline vesicle structures that coexisted, in the external water phase, with globules of crystallized oil phase. Results of calorimetric, rheological, and light scattering experiments, for the O/W dispersions obtained, indicate the possible transition of a monostearoylglycerol-based alpha-crystalline gel phase to a coagel state, in these multicomponent systems.

Phenolic compounds from the fruit of Garcinia dulcis

Dulcinoside (1), dulcisisoflavone (2), dulcisxanthone A (3) and sphaerobioside acetate (6) together with 22 known compounds were isolated from the green fruit of G. dulcis. Dulcisflavan (4), dulcisxanthone B (5) and isonormangostin (7) together with 22 known compounds were isolated from the ripe fruit. Compounds 6 and 7 were synthetic known compounds. Their structures were determined by spectroscopic methods. The radical scavenging and antibacterial activities of some of the compounds were investigated.

Microstructural investigation of monoglyceride–water coagel systems by NMR and CryoSEM

Monoglyceride coagels consist of a network of plate-like crystals and are formed from a swollen gel state (alpha-gel). In order to resolve the transition mechanism, coagels were prepared with monoglycerides that differ in fatty acid composition (monomyristate and palmitate/stearate, respectively). Rheology provided information on kinetics of coagel formation and the strength of the resulting crystal network. From NMR measurements, the surface-to-volume ratio, tortuosity, and dimensionality of the network were obtained. These findings were in line with qualitative and quantitative structural information obtained from CryoSEM. As a model for the behaviour of non-monoglyceride species, the dynamics of (perdeuterated) palmitic acid was monitored in both alpha-gels and coagels. The experimental data support a two-stage mechanism. In the first stage, two-dimensional separation of D- and L-isomers in the monoglyceride bilayers of the alpha-gel occurs. This process depends primarily on lateral diffusion rate of the monoglycerides. Palmitic acid can be accommodated in the alpha-gel bilayer, but in the coagels it is separated into relative mobile and mechanically weak junction zones between the crystal plates. In the second stage of coagel formation, the crystal plates also grow in the third dimension. Both monoglyceride type and concentration determine the kinetics of this process.

Glycerol Derivatives of Cutin and Suberin Monomers: Synthesis and Self-Assembly

Glycerol derivatives of cutin and suberin monomers were synthesized by acid catalysis. Their dispersion in an aqueous solution was examined by phase contrast microscopy, neutron scattering, and solid state NMR. It is shown that the phase behavior strongly depends on the nature of the derivatives forming either lumps of aggregated membranes or well dispersed membranes.

Phase behavior of the palmitic acid/palmitin system. A 2H NMR study

The phase behavior of mixtures of palmitic acid (PA) and 1-monohexadecanoyl-rac-glycerol, palmitin, was studied by phase contrast microscopy and deuterium solid-state NMR. At pH 5, mixtures remained precipitated as lumps in solution. The NMR spectrum of the perdeuterated PA (PAd31) at 300 K exhibited a shape and quadrupolar splittings, deltav, characteristic of lipids embedded in a gel phase. The alkyl chains remained in a trans conformation with their long molecular axis oriented at about 15 degrees with respect to the bilayer normal. However, gauche defects were shown to occur at the end of the alkyl chain. At 330 K, the system underwent a phase transition to a hexagonal phase followed by an isotropic phase at 340 K. Upon cooling to 330 K, the spectrum in the hexagonal phase was oriented at 0 degrees showing that the cylinders were oriented with their long axis parallel to the field. Up to 11 positions (from 15) of PAd31 could be assigned. At pH 7 and 9 at room temperature, the mixtures were fully dispersed in a viscous solution of vesicles. The system underwent a phase transition at 320 K from a gel phase to a fluid phase with the bilayer normal oriented at 90 degrees with respect to the field. Analogous experiments performed with PA selectively labeled on carbon C2 allowed for the assignment of deltav for that position and suggested different conformations of the headgroup in the gel and fluid or hexagonal phases. The implications of these findings for the bio-availability of these fatty acids, in the understanding of the contribution of hydroxyl and carboxyl groups in the membrane formation, and for the production of simple self-oriented systems are discussed.

Effect of phospholipids and a transmembrane peptide on the stability of the cubic phase of monoolein: implication for protein crystallization from a cubic phase

The cubic phase of monoolein has successfully been used for crystallization of a number of membrane proteins. However, the mechanism of protein crystallization in the cubic phase is still unknown. It was hypothesized, that crystallization occurs at locally formed patches of bilayers. To get insight into the stability of the cubic phase, we investigated the effect of different phospholipids and a model transmembrane peptide on the lipid organization in mixed monoolein systems. Deuterium-labeled 1-oleoyl-rac-[(2)H(5)]-glycerol was used as a selective probe for (2)H NMR. The phase behavior of the phospholipids was followed by (31)P NMR. Upon incorporation of phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, or phosphatidic acid, the cubic phase of monoolein transformed into the L(alpha) or H(II) phase depending on the phase preference of the phospholipid and its concentration. The ability of phospholipids to destabilize the cubic phase was found to be dependent on the phospholipid packing properties. Electrostatic repulsion facilitated the cubic-to-L(alpha) transition. Incorporation of the transmembrane peptide KALP31 induced formation of the L(alpha) phase with tightly packed lipid molecules. In all cases when phase separation occurs, monoolein and phospholipid participate in both phases. The implications of these findings for protein crystallization are discussed.

The effect of protein stability on protein-monoglyceride interactions

We have previously shown that proteins such as beta-lactoglobulin and lysozyme insert into monoglyceride monolayers and are able to induce an L(beta) to coagel phase transition in monoglyceride bilayers. These studies gave a first indication that protein stability could be an important factor for these interactions. This study therefore aims at further investigating the potential role of protein stability on protein-monoglyceride interactions. To this end we studied the interaction of stable and destabilized alpha-lactalbumin with monostearoylglycerol. Our results show that protein stability is important for the insertion of proteins into a monostearoylglycerol monolayer, such that the lower the stability of the protein the better the protein inserts. In marked contrast to beta-lactoglobulin and lysozyme we found that destabilized alpha-lactalbumin does not induce the L(beta) to coagel phase transition in monoglyceride bilayers. We propose that this is due to an increased surface coverage by the protein which could result from the unfolding of the protein upon binding to the interface.

Rheological Characterization, Crystallization, and Gelation Behavior of Monoglyceride Gels

Saturated monoglycerides can form firm gels in water. These gels are networks of stiff plate-like beta-crystals of monoglycerides (a "cardhouse"), grown from a space-filling lamellar liquid-crystalline phase. The molecular mechanism of crystallization is discussed in the light of network formation. The concentration dependence of gel development of (shear-cooled) monoglyceride gels has been studied by rheology. A gelation mechanism has been proposed, consisting of two steps: (i) After formation of a nucleus, rapid crystallization in a lateral direction occurs (probably within one bilayer) by which the first space-filling network is formed. (ii) This is followed by reinforcement of the network by which stacks of crystalline bilayers are formed. The plate-like crystals are linked in connective domains or junction zones, probably containing all the material (cosurfactants, diglycerides, etc.) that does not fit in the crystalline array. Small deformation rheology shows that above about 2 wt% monoglyceride a percolating network is formed. The large deformation rheology is typical for a particle gel with a relatively small strain at failure (both in shear deformation and compression). The connective domains or junction zones already fail when relatively small deformations are put on the system.

Thermotropic phase behavior of monoglyceride-dicetylphosphate dispersions and interactions with proteins: a (2)H and (31)P NMR study

The phase behavior of a 1-[(2)H(35)]-stearoyl-rac-glycerol ([(2)H(35)]-MSG)/dicetylphosphate (DCP) mixture and its interaction with beta-lactoglobulin and lysozyme were studied by (2)H and (31)P nuclear magnetic resonance (NMR). The behavior of the lipids was monitored by using deuterium-labeled [(2)H(35)]-MSG as a selective probe for (2)H NMR and DCP for (31)P NMR. Both (2)H and (31)P NMR spectra exhibit characteristic features representative of different phases. In the lamellar phases, (31)P NMR spectra of DCP are different from the spectra of natural phospholipids, which is attributable to differences in the intramolecular motions and the orientation of the shielding tensor of DCP compared with phospholipids. The presence of the negatively charged amphiphile DCP has a large effect on the phase behavior of [(2)H(35)]-MSG. At low temperature, the presence of DCP inhibits crystallization of the gel phase into the coagel. Upon increasing the temperature, the gel phase of [(2)H(35)]-MSG transforms in the liquid-crystalline lamellar phase. In the presence of DCP, the gel phase directly transforms into an isotropic phase. The negatively charged beta-lactoglobulin and the positively charged lysozyme completely neutralize the destabilizing effect of DCP on the monoglyceride liquid-crystalline phase and they even stabilize this phase. Without DCP the proteins do not seem to interact with the monoglyceride. These results suggest that interaction is facilitated by electrostatic interactions between the negatively charged DCP and positively charged residues in the proteins. In addition, the nonbilayer-forming DCP creates insertion sites for proteins in the bilayer.

Fructans insert between the headgroups of phospholipids

Fructans are polysaccharides consisting of one glucose unit and two or more fructose units. It was hypothesized that fructans play a role in drought tolerance in plants by interacting directly with the membrane. In this paper we investigated this hypothesis by studying fructan-membrane interactions in hydrated mono- and bilayer systems. It was found that fructans inserted between the headgroups of different kinds of phospholipids with some preference for phosphatidylethanolamine. Insertion occurred even under conditions of very tight lipid packing. The presence of a surface associated layer of fructan was observed in both model systems. This layer was able to reduce the ability of a surface-active protein to interact with the lipids. Fructans showed a much stronger effect on the different lipid systems than other (poly)saccharides, which appears to be related to their hydrophobic properties. Fructans were able to stabilize the liquid-crystalline lamellar phase, which is consistent with a drought protecting role in plants.