Theoretical Model for the Phospholipid Bilayer System: Single Component Phase Transition and Binary Mixture Phase Diagram

A synthetic route was developed to prepare ether phospholipid (PL) ligands suitable for immobilization. PL ligand design included an ω-carboxyl functional group to assure proper naolecular orientation during inamobilization; i.e., the polar lipid head group protrudes from the surface. However, during immobilization, PL ligands required protecting groups to eliminate the possibility of the PL binding upside down. Four synthetic PL ligands were prepared that contain both ω-carboxyl groups for immobilization and protecting groups in the polar head group; these carboxyl-PL ligands are analogs of phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylglycerol (PG), and phosphatidic acid (PA)

The conformation of the polar head group of phosphatidylcholine in a bilayer in the liquid-crystalline state was deduced by analyzing the deuterium quadrupole splittings of the choline group and the phosphorus chemical shift anisotropy of the phosphate group in combination with the restriction of the choline conformation determined in laser Raman studies. The latter efficiently reduced the number of candidates for the actual conformation. A family of conformations was obtained for both the dynamic-structure and rigid-structure models, respectively. The polar head group is oriented roughly parallel to the membrane surface in both models. Furthermore, they are close to conformation A of the crystal structure of 1,2-dimyristoyl-sn-glycero-3-phosphocholine. The dynamic-structure model was concluded to be more reasonable in view of the fact that the polar head-group structures in most crystals comprise two conformations, which are nearly mirror images of each other. Conformational analysis was also carried out for the polar head group in the presence of multivalent cations. A possible conformational change of the polar head group induced by cations is discussed in the light of the present results.

Nijenrode Castle in Breukelen, the Netherlands, situated between Utrecht and Amsterdam, was home for seventeen days during August of last year to more than sixty physicists, chemists, and advanced students from sixteen countries. These people gathered to attend an intensive course on Fundamental Problems in Statistical Mechanics, held under the auspices of NUFFIC (the Netherlands Universities Foundation for International Cooperation) with support from NATO. Statistical mechanics seemed most appropriately chosen as the subject of the first summer school in theoretical physics in the Netherlands in view of the long tradition, begun by Van der Waals and continued by Ehrenfest. of Dutch research in this field. The lecturers at Nijenrode representing that tradition as it is active in Holland today included N. G. van Kampen, B. R. A. Nijboer, L. van Hove, N. M. Hugenholtz. E. G. D. Cohen, and P. Mazur. Visiting lecturers from abroad were K. Huang of the Massachusetts Institute of Technology. H. Wergeland of the Norwegian Technical High School in Trondheim, and G. E. Uhlenbeck of the Rockefeller Institute in New York.

Surface active agents (usually referred to as surfactants) are amphipathic molecules consisting of a nonpolar hydrophobic portion, usually a straight or branched hydrocarbon or fluorocarbon chain containing 8–18 carbon atoms, which is attached to a polar or ionic portion (hydrophilic). The hydrophilic portion can, therefore, be nonionic, ionic or zwitterionic, accompanied by counter ions in the last two cases. The hydrocarbon chain interacts weakly with the water molecules in an aqueous environment, whereas the polar or ionic head group interacts strongly with water molecules via dipole or ion‐dipole interactions. It is this strong interaction with the water molecules that renders the surfactant soluble in water. However, the cooperative action of dispersion and hydrogen bonding between the water molecules tends to squeeze the hydrocarbon chain out of the water and hence these chains are referred to as hydrophobic. The balance between hydrophobic and hydrophilic part of the molecule gives these systems their special properties, eg, accumulation at various interfaces and association in solution (to form micelles). The driving force for surfactant adsorption is the lowering of the free energy of the phase boundary. The interfacial free energy per unit area is the amount of work required to expand the interface. This interfacial free energy, referred to as surface or interfacial tension, γ, is given in mJm −2 or mNm −1 . Adsorption of surfactant molecules at the interface lowers γ and the higher the surfactant adsorption (ie, the more dense the layer is) the larger the reduction in γ. The degree of surfactant adsorption at the interface depends on surfactant structure and the nature of the two phases that meet the interface. Surface active agents also aggregate in solution forming micelles. The driving force micelle formation (or micellization) is the reduction of contact between the hydrocarbon chain and water, thereby reducing the free energy of the system. In the micelle, the surfactant hydrophobic groups are directed towards the interior of the aggregate and the polar head groups are directed towards the solvent. These micelles are in dynamic equilibrium and the rate of exchange between a surfactant molecule and the micelle may vary by orders of magnitude, depending on the structure of the surfactant molecule. Surfactants find application in almost every chemical industry of which the following may be worth mentioning: detergents, paints, dyestuffs, cosmetics, pharmaceuticals, agrochemicals, fibers, plastics, etc. Moreover, surfactants play a major role in the oil industry, for example, in enhanced and tertiary oil recovery. They are also occasionally used for environmental protection, eg, in oil slick dispersants. Therefore, a fundamental understanding of the physical chemistry of surface active agents, their unusual properties and their phase behavior is essential for most industrial chemists. In addition, an understanding of the basic phenomena involved in the application of surfactants, such as in the preparation of emulsions and suspensions and their subsequent stabilization, in microemulsions, in wetting spreading and adhesion, etc, is of vital importance in arriving at the right composition and control of the system involved. This is particularly the case with many formulations in the chemical industry mentioned above. It should be stated that commercially produced surfactants are not pure chemicals, and within each chemical type there can be tremendous variation. This can be understood, since surfactants are prepared from various feedstocks, namely petrochemicals, natural vegetable oils and natural animal fats. It is important to realize that in every case the hydrophobic group exists as a mixture of chains of different lengths. Products that may be given the same generic name could vary a great deal in their properties and the formulation chemist should bear this in mind when choosing a surfactant from a particular manufacturer.

Synthetic analogues of the cell-wall lipid monomycoloyl glycerol (MMG) are promising as next-generation vaccine adjuvants. In the present study, the thermotropic phase behavior of an array of synthetic MMG analogues was examined by using simultaneous small- and wide-angle X-ray scattering under excess water conditions. The MMG analogues differed in the alkyl chain lengths and in the stereochemistry of the polar glycerol headgroup or of the lipid tails (native-like versus alternative compounds). All MMG analogues formed poorly hydrated lamellar phases at low temperatures and inverse hexagonal (H2) phases at higher temperatures prior to melting. MMG analogues with a native-like lipid acid configuration self-assembled into noninterdigitated bilayers whereas the analogues displaying an alternative lipid acid configuration formed interdigitated bilayers in a subgel (Lc') state. This is in contrast to previously described interdigitated phases for other lipids, which are usually in a gel (Lβ) state. All investigated MMG analogues displayed an abrupt direct temperature-induced phase transition from Lc' to H2. This transition is ultimately driven by the lipid chain melting and the accompanying change in molecular shape. No intermediate structures were found, but the entire array of MMG analogues displayed phase coexistence during the lamellar to H2 transition. The structural data also showed that the headgroups of the MMG analogues adopting the alternative lipid acid configuration were ordered and formed a two-dimensional molecular superlattice, which was conserved regardless of the lipid tail length. To our knowledge, the MMG analogues with an alternative lipid acid configuration represent the first example of a lipid system showing both interdigitation and superlattice formation, and as such could serve as an interesting model system for future studies. The MMG analogues are also relevant from a subunit vaccine perspective because they are well-tolerated and display promising immunopotentiating activity. The structural characterization described here will serve as a prerequisite for the rational design of nanoparticulate adjuvants with specific and tailored structural features.

Bimolecular membrane of lipids are in a liquid crystalline state capable of reversible structural modifications and permeation properties of biomembranes depend upon such reversible change. Therefore, attempts have been made to confirm understanding of characteristic behaviors of the solid state, thermotropic and lyotropic mesomorphic states of biomembranes composed of phospholipids. The principal chemical structure of biological phospholipids is classified into the two parts of a polar head group and two longer hydrocarbon chains. This amphiphilic characteristic and the hydrophilic-hydrophobic balance must be taken into consideration to synthesize totally artificial amphiphiles which have membrane structure and permeation property similar to those of biological membranes. Ralston et al. studied the amphiphilic behavior of quaternary ammonium salts containing two long alkyl chains(1). They suggested that the simple positively-charged hydrophilic moiety was sufficient to form bimolecular lamellae instead of complicated polar head groups of biological phospholipids. Kunitake et al. have succeeded in synthesis of totally artificial amphiphiles being capable of formation of monolayer of bilayer membranes(2–4). Their artificial amphiphiles are composed of combinations of the one- or two- headed polar groups and the monoalkyl or dialkyl long chains.

Adsorption Behavior of Oxyethylated Surfactants at the Air/Water Interface

Lipids are important natural chemical compounds, as they comprise the major component in biological membranes. Biological membranes are composed of lipids in a bilayer structure and proteins incorporated into the bilayers or bound to the bilayer surface. Due to the amphiphilic nature of lipids, they self-assemble in aqueous solution into a variety of lyotropic phases, the most important one being the lamellar phase made up of stacks of lipid bilayers separated by water layers. The bilayer structure can be easily produced by dispersing lipids in water. Lipids are amphiphilic structures built up of a polar headgroup and one or two hydrophobic alkyl chains. They are, therefore, surface active and form the so-called insoluble monolayers or Langmuir films at the air–water interface. These monolayers resemble half of a lipid bilayer and are, therefore, widely used as model systems for bilayer membranes. In this review, the properties and the phase behavior of phospholipid monolayers, as well as the techniques used for studying these monolayers will be described. In addition, examples for the information gained using different techniques will be shown.

Additive chemical structure and its effect on the wetting behaviour of oil at 100 °C

Surfactants are a class of chemical compounds with unique properties, structure of which is consisted of hydrophobic and hydrophilic parts. This double structure gives rise to their unique behaviors in various conditions. Bolaforms are organic molecules compounds of two charged groups which are connected via a linear poly methylen chain.Bolaforms\' surfactants don\'t work as well as Conventional surfactant (having a head group and one alkyl chain) in reducing surface tension. Bolaforms are absorbed in water – air interface, however, their effectiveness in terms of reducing water surface tension is not considerable compared to common surfactants. In this study, synthesis of two kinds of bolaforms which the same spacer length and different heads is investigated. The structural formula for synthesized kinds is as follows: bolaform A with structural formula Br-(CH3)3N+(CH2)3 N+(CH3)­3 Br – ­with methyl side branches in head group and bolaform B with structural formula of Br-(CH3 CH2)3N+(CH2)3 N+(CH3 CH2)3Br –. IR and NMR identification techniques approves obtained product. Using surface tension measurement device it can be observed that synthesized bolaforms have high CMC; in other words, bolaform\'s surfactants with short spacer play electrolytic role.

Transmembrane movement of phospholipids is a fundamental step in the process of biological membrane assembly and intracellular lipid sorting. To facilitate study of transmembrane movement, we have synthesized analogues of phosphatidylglycerol and diacylglycerol in which a sulfhydryl group replaces a hydroxyl group in the polar head group. A rapid, continuous assay for the movement of phospholipids across single-walled lipid vesicles was developed that exploits the reactivity of these analogues toward 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), a nonpenetrating, colorimetric, sulfhydryl reagent. In the reaction of DTNB with vesicles containing phosphatidylthioglycerol, a phosphatidylglycerol analogue, two kinetic phases were seen, which represent the reaction of DTNB with phosphatidylthioglycerol in the outer and inner leaflets of the bilayer. Analysis of the slow second phase indicated that the half-time for phosphatidylthioglycerol transbilayer movement was in excess of 8 days. In a similar experiment using dioleoylthioglycerol, a diacylglycerol analogue, the reaction was complete within 15 s. The large difference in translocation rates between these two lipids indicates that the primary barrier to transmembrane movement is the polar head group and implies that phospholipid translocation events in biological membranes may not be unlike those for molecules similar to the polar head groups alone.

A new class of extended surfactants was prepared in which the spacer arm between the polar portion and the hydrophobic alkyl chain was a polymer of propylene glycol with an average length of six propylene oxide units. The polar head was a single or double xylitol moiety or a xylitol molecule with carboxylic acid functionality. Surfactants containing double xylitol polar head groups showed a much higher critical micelle concentration value than surfactants with a single polar head.

Comprehensive glycerol ether lipid fingerprints through a novel reversed phase liquid chromatography–mass spectrometry protocol

It has been demonstrated that histamine is released into rabbit plasma during the acute allergic reaction following IgE-dependent activa-tion of basophils (1–3) and this was ascribed to a factor actively released from leukocytes by a calcium — and temperature — dependent process (4,5). From 1972 onwards, Benveniste et al. (6–7), described the method used to obtain this substance which they named platelet-activating factor (PAF), and they started its characterization, demonstrating that it was released from rabbit basophils by an IgE-dependent process. It was subsequently evidenced by structural elucidation that PAF is a phospholipase A2 sensitive phospholipid (8–10), identified as 1-alkyl-2(R)- acetyl-glycero-3-phosphorylcholine (10,11). The two main molecular species of PAF produced by human neutrophils or rabbit basophils have a hexadecyl or octadecyl alkyl moiety, but more advanced separation techniques have shown that PAF comprises multiple (at least 16) molecular species containing saturated or unsaturated alkyl or acyl chains, or even a polar head group other than choline (12,13).

On the entropy minimization problem in Statistical Mechanics