The distribution of alpha-tocopherol in mixed aqueous dispersions of phosphatidylcholine and phosphatidylethanolamine

Vitamin E - phosphatidylethanolamine interactions in mixed membranes with sphingomyelin: Studies by 2H NMR

Among the structurally diverse collection of lipids that comprise the membrane lipidome, polyunsaturated phospholipids are particularly vulnerable to oxidation. The role of α-tocopherol (vitamin E) is to protect this influential class of membrane phospholipid from oxidative damage. Whether lipid-lipid interactions play a role in supporting this function is an unanswered question. Here, we compare the molecular organization of polyunsaturated 1-[²H₃₁]palmitoyl-2-docosahexaenoylphosphatidylethanolamine (PDPE-d₃₁) and, as a control, monounsaturated 1-[²H₃₁]palmitoyl-2-oleoylphosphatidylethanolamine (POPE-d₃₁) mixed with sphingomyelin (SM) and α-tocopherol (α-toc) (2:2:1 mol) by solid-state ²H NMR spectroscopy. In both cases the effect of α-toc appears similar. Spectral moments reveal that the main chain melting transition of POPE-d₃₁ and PDPE-d₃₁ is broadened beyond detection. A spectral component attributed to the formation of inverted hexagonal H_II phase in coexistence with lamellar L_α phase by POPE-d₃₁ (20 %) and PDPE-d₃₁ (18 %) is resolved following the addition of α-toc. Order parameters in the remaining L_α phase are increased slightly more for POPE-d₃₁ (7%) than PDPE-d₃₁ (4%). Preferential interaction with polyunsaturated phospholipid is not apparent in these results. The propensity for α-toc to form phase structure with negative curvature that is more tightly packed at the membrane surface, nevertheless, may restrict the contact of free radicals with lipid chains on phosphatidylethanolamine molecules that accumulate polyunsaturated fatty acids.

Disruptive effect of tocopherol oxalate on DPPC liposome structure: DSC, SAXS, and fluorescence anisotropy studies

α-Tocopherol oxalate (TO), a tocopherol ester derivative, was investigated for its effect on the structural changes of fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes, as a function of concentration and temperature, by applying differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS), and DPH fluorescence anisotropy methods. The DSC and DPH anisotropy data indicated that TO embedded into DPPC membrane lowered the enthalpy (ΔH_m) and temperature (T_m) of the main phase transition as well its cooperativity. Fluidization of the membrane at a lowered temperature was accompanied by formation of mixed structures of tocopherol-enriched domains. SAXS studies showed the formation of various ordered structures in DPPC gel-phase during incorporation of TO into the bilayer, as evidenced by the existence of lamellar phases with repeat distances (d) of 6.13 and 6.87 nm, assigned to TO-enriched domains and a lamellar, liquid-ordered DPPC phase with d = 8.45 nm at increasing TO concentrations with lowering and broadening of the Bragg peaks, and diffuse scattering, characteristic of a fluid L_α phase, were observed. In DPPC fluid-phase, the increasing presence of TO at low concentrations resulted in the appearance of a liquid-ordered phase with repeat d = 6.9 nm coexistent with a lamellar structure with d = 9.2 nm, assigned to liquid-disordered structures. An increasing repeat distance observed with raising the TO amount in the DPPC bilayer evolved from an increasing interlamellar water layer of increasing thickness. Presence of TO facilitated penetration of water molecules into the acyl chain region which decreased van der Waals interactions in the bilayer. The DSC, SAXS, and fluorescence anisotropy data established that TO exhibited pronounced disruptive activity in DPPC membranes compared to α-tocopherol. The driving force of the observed action was attributed to electrostatic and dipole interactions of the acidic moiety with the polar head group of phospholipids in the interface region of the bilayer.

α-Tocopherol-loaded niosome prepared by heating method and its release behavior

α-Tocopherol-loaded niosome was developed using modified heating method. The influence of surfactants (Span60 and Tween60) in different mole ratios, presence or absence of cholesterol (Chol) and dicetyl phosphate (DCP) as well as different concentration of α-tocopherol (α-TOC) on mean size, polydispersity index, zeta potential and entrapment efficiency (EE) was evaluated. The results showed that α-TOC loaded niosomes exhibited a small mean size (73.85±0.6-186±0.58nm), narrow size distribution and high EE (61.13±0.52-98.92±0.92). By decreasing the HLB, the EE and stability of the niosomes increased. The DCP and Chol improved the physicochemical properties of niosomes. 3:1 mole ratio of Span 60:Tween 60, 4mg/ml of α-TOC and 25:12.5:2.5 mole ratio of surfactant:Chol:DCP was the optimum formulation in the encapsulation of α-TOC applying niosome system. The niosomes had sustained release profile in the simulated gastric and intestinal fluid.

Biosynthesis, structural, and functional attributes of tocopherols in planta; past, present, and future perspectives

Tocopherols are lipophilic molecules, ubiquitously synthesized in all photosynthetic organisms. Being a group of vitamin E compounds, they play an essential role in human nutrition and health. Despite their structural and functional attributes as important antioxidants in plants, it would be misleading to ignore the potential roles of tocopherols beyond their antioxidant properties in planta. Detailed characterization of mutants and transgenic plants, including Arabidopsis (vte1, vte2, vte4, and so on), maize (sxd1) mutants, and transgenic potato and tobacco lines altered in tocopherol biosynthesis and contents, has led to surprising outcomes regarding the additional functions of these molecules. Thus, the aim of this review is to highlight the past and present research findings on tocopherols' structural, biosynthesis, and functional properties in plants. Special emphasis is given to their suggested functions in planta, such as cell signaling, hormonal interactions, and coordinated response of tocopherols to other antioxidants under abiotic stresses. Moreover, some important questions about possible new functions of tocopherols will be discussed as future prospects to stimulate further research.

Modulated phases of phospholipid bilayers induced by tocopherols

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.

Molecular associations of vitamin E

To understand how vitamin E fulfills its functions in membranes and lipoproteins, it is necessary to know how it associates with the lipid components of these structures and the effects its presence has on their structure and stability. Studies of model membrane systems containing vitamin E have proved to be an informative approach to address these questions. A review of the way vitamin E interacts with phospholipid bilayers, how it distributes within the structure, its motional diffusion characteristics, and orientation has been undertaken. The effect of vitamin E on membrane stability and permeability has been described. The tendency of vitamin E to form complexes with certain phospholipids is examined as is the way modulation of protein functions takes place. Finally, recent evidence relevant to the putative role of vitamin E in protecting membranes from free radical attack and the consequences of lipid oxidation in lipoproteins and membranes is examined.

The structure and phase behaviour of α-tocopherol-rich domains in 1-palmitoyl-2-oleoyl-phosphatidylethanolamine

The effect of alpha-tocopherol on the structure and thermotropic phase behaviour of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine dispersed in excess water was examined by synchrotron X-ray diffraction and differential scanning calorimetry. Small- and wide-angle X-ray scattering intensity profiles were recorded from mixed dispersions containing up to 20 mol% alpha-tocopherol during temperature scans over the range 10-75 degrees C. These showed that a domain enriched in alpha-tocopherol phase separated from pure phospholipid in the mixture. This domain tends to have inverted hexagonal structure which coexists with phospholipid bilayers depleted of alpha-tocopherol. The scattering intensity and dimensions of the phase are dependent on the temperature and proportion of alpha-tocopherol in the mixture. Phase separations were also manifest in calorimetric scans of the mixed dispersions evidenced from the appearance of multiple peaks at temperatures corresponding to transitions observed in the X-ray scattering experiments. The effect of alpha-tocopherol in the range 0-20 mol% on the phase behaviour and structure of the phospholipid as observed from the X-ray scattering and calorimetric results have been used to construct a partial phase diagram of the mixture in the temperature range 10-75 degrees C. This shows that alpha-tocopherol has a marked tendency to partition from bilayers of the phospholipid to form an enriched domain in which the phospholipid assumes a hexagonal-II structure.

Supramolecular Assemblies of Nucleoside Phosphocholine Amphiphiles

A family of new uridine phosphocholine amphiphiles that were prepared using a convenient four-step synthetic route is described. Physicochemical studies (differential scanning calorimetry, small-angle X-ray scattering, UV-vis and circular dichroism spectroscopies, light microscopy, transmission electronic microscopy, and scanning electron microscopy) show that these amphiphiles spontaneously assemble into supramolecular structures including vesicles, fibers, hydrogels, and organogels. In aqueous solution, the amphiphiles possessing saturated alkyl chains self-assemble into DNA-like helical fibers in the crystalline state below T(m) and compact bilayers above the melting temperature (T(m)). The transition from bilayers to fibers is thermally reversible. Above a threshold concentration (>6% w/w), a hydrogel is formed due to an entangled network of the fibers. A therapeutic agent such as DNA can be entrapped within the hydrogel structure. In addition to forming bilayer vesicles and hydrogels in aqueous solution, these nucleoside amphiphiles also form organogels in cyclohexane above T(m). Scanning electron microscopy shows a continuous multilamellar phase in the organogels.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

The interaction of alpha-tocopherol with bilayers of 1-palmitoyl-2-oleoyl-phosphatidylcholine

The effect of alpha-tocopherol on the structure and phase behaviour of 1-palmitoyl-2-oleoyl-phosphatidylcholine was examined by real-time synchrotron X-ray diffraction and freeze-fracture electron microscopic methods. X-ray scattering intensity was recorded from mixed aqueous dispersions of phospholipid with 2.5, 5, 10 and 20 mol% alpha-tocopherol during temperature scans at 3 degrees /min between -25 and 10 degrees C. A ripple structure is induced by the presence of alpha-tocopherol that coexists with the ripple phase characteristic of the pure phospholipid in mixtures containing 2.5 mol% alpha-tocopherol but completely replaces it in mixtures containing greater proportions of alpha-tocopherol. Freeze-fracture replicas of dispersions containing 5 mol% alpha-tocopherol indicate a ripple phase with a periodicity of about 9 nm. Increasing amounts of alpha-tocopherol result in a progressive reduction in temperature of the gel to liquid-crystal phase transition and broadening of the transition. Two lamellar phases coexist in the liquid-crystal state, one with a spacing of 6.4 nm assigned to an alpha-tocopherol-enriched lamellar structure and the other with a lamellar repeat of 6.1 nm corresponding to bilayers of pure phospholipid.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Phase separations of alpha-tocopherol in aqueous dispersions of distearoylphosphatidylethanolamine

The effect of alpha-tocopherol on the structure and thermotropic phase behaviour of distearoylphosphatidylethanolamine was examined by using synchrotron X-ray diffraction methods. There was evidence that alpha-tocopherol does not distribute randomly in the dispersed phospholipid but instead phospholipid phases enriched in alpha-tocopherol are formed. Heating codispersions from lamellar gel phase induced formation of hexagonal-II phase at temperatures below the main transition of the pure phospholipid and which were enriched in alpha-tocopherol. Codispersions containing 5 or 10 mol% alpha-tocopherol were induced to form a cubic phase at temperatures above the lamellar to hexagonal-II phase transition. Such phases were not observed in codispersions containing 2.5 or 20 mol% alpha-tocopherol in which only lamellar and hexagonal-II phases were formed. The space group of the cubic phases were tentatively assigned as Pn3m. Equilibration of codispersions at 4 degrees C results in the formation of lamellar crystalline phases enriched in alpha-tocopherol and phase separated domains of pure phospholipid. Two lamellar crystalline phases were characterized on the basis of their particular wide-angle X-ray scattering patterns. The lamellar crystalline phases were also distinguished from other lamellar phases of the pure phospholipid by the lamellar repeat. Partitioning of alpha-tocopherol into phosphatidylethanolamine domains in membranes may introduce instability into the structure.