Perceiving the functions of vitamin E through neutron and X-ray scattering

Take your vitamins, or don't? Vitamin E is one of the few lipophilic vitamins in the human diet and is considered an essential nutrient. Over the years it has proven to be a powerful antioxidant and is commercially used as such, but this association is far from linear in physiology. It is increasingly more likely that vitamin E has multiple legitimate biological roles. Here, we review past and current work using neutron and X-ray scattering to elucidate the influence of vitamin E on key features of model membranes that can translate to the biological function(s) of vitamin E. Although progress is being made, the hundred year-old mystery remains unsolved.

Chitosan-coated liposomal systems for delivery of antibacterial peptide LL17-32 to Porphyromonas gingivalis

Periodontal disease is triggered by surface bacterial biofilms where bacteria are less susceptible to antibiotic treatment. The development of liposome-based delivery mechanisms for the therapeutic use of antimicrobial peptides is an attractive alternative in this regard. The cationic antimicrobial peptide LL-37 (human cathelicidin) is well-known to exert antibacterial activity against P orphyromonas gingivalis, a keystone oral pathogen. However, the antibacterial activity of the 16-amino acid fragment (LL17-32) of LL-37, is unknown. In addition, there are still gaps in studies using liposomal formulations as delivery vehicles of antibacterial peptides against this pathogen. This study was designed to examine the influence of the different types of liposomal formulations to associate and deliver LL17-32 to act against P. gingivalis. Chitosans of varying Mw and degree of acetylation (DA) were adsorbed at the surface of soya lecithin (SL) liposomes. Their bulk (average hydrodynamic size, ζ-potential and membrane fluidity) and ultrastructural (d-spacing, half-bilayer thickness and the water layer thickness) biophysical properties were investigated by a panel of techniques (DLS, SAXS, M3-PALS, fluorescence spectroscopy and TEM imaging). Their association efficiency, in vitro release, stability, and efficacy in killing the periodontal pathogen P. gingivalis were also investigated. All liposomal systems possessed spherical morphologies and good shelf-life stabilities. Under physiological conditions, chitosan formulations with a high DA demonstrated enhanced stability in comparison to low DA-chitosan formulations. Chitosans and LL17-32 both decreased SL-liposomal membrane fluidity. LL17-32 exhibited a high degree of association with SL-liposomes without in vitro release. In biological studies, free LL17-32 or chitosans alone, demonstrated microbicidal activity against P. gingivalis, however this was attenuated when LL17-32 was loaded onto the SL-liposome delivery system, presumably due to the restrained release of the peptide. A property that could be harnessed in future studies (e.g., oral mucoadhesive slow-release formulations).

Effects of a Semisynthetic Catechin on Phosphatidylglycerol Membranes: A Mixed Experimental and Simulation Study

Catechins have been shown to display a great variety of biological activities, prominent among them are their chemo preventive and chemotherapeutic properties against several types of cancer. The amphiphilic nature of catechins points to the membrane as a potential target for their actions. 3,4,5-Trimethoxybenzoate of catechin (TMBC) is a modified structural analog of catechin that shows significant antiproliferative activity against melanoma and breast cancer cells. Phosphatidylglycerol is an anionic membrane phospholipid with important physical and biochemical characteristics that make it biologically relevant. In addition, phosphatidylglycerol is a preeminent component of bacterial membranes. Using biomimetic membranes, we examined the effects of TMBC on the structural and dynamic properties of phosphatidylglycerol bilayers by means of biophysical techniques such as differential scanning calorimetry, X-ray diffraction and infrared spectroscopy, together with an analysis through molecular dynamics simulation. We found that TMBC perturbs the thermotropic gel to liquid-crystalline phase transition and promotes immiscibility in both phospholipid phases. The modified catechin decreases the thickness of the bilayer and is able to form hydrogen bonds with the carbonyl groups of the phospholipid. Experimental data support the simulated data that locate TMBC as mostly forming clusters in the middle region of each monolayer approaching the carbonyl moiety of the phospholipid. The presence of TMBC modifies the structural and dynamic properties of the phosphatidylglycerol bilayer. The decrease in membrane thickness and the change of the hydrogen bonding pattern in the interfacial region of the bilayer elicited by the catechin might contribute to the alteration of the events taking place in the membrane and might help to understand the mechanism of action of the diverse effects displayed by catechins.

Partial Volumes of Phosphatidylcholines and Vitamin E: α-Tocopherol Prefers Disordered Membranes

Despite its discovery over 95 years ago, the biological and nutritional roles of vitamin E remain subjects of much controversy. Though it is known to possess antioxidant properties, recent assertions have implied that vitamin E may not be limited to this function in living systems. Through densitometry measurements and small-angle X-ray scattering we observe favorable interactions between α-tocopherol and unsaturated phospholipids, with more favorable interactions correlating to an increase in lipid chain unsaturation. Our data provide evidence that vitamin E may preferentially associate with oxygen sensitive lipids─an association that is considered innate for a viable membrane antioxidant.

Interaction of Docetaxel with Phosphatidylcholine Membranes: A Combined Experimental and Computational Study

The antineoplastic drug Docetaxel is a second generation taxane which is used against a great variety of cancers. The drug is highly lipophilic and produces a great array of severe toxic effects that limit its therapeutic effectiveness. The study of the interaction between Docetaxel and membranes is very scarce, however, it is required in order to get clues in relation with its function, mechanism of toxicity and possibilities of new formulations. Using phosphatidylcholine biomimetic membranes, we examine the interaction of Docetaxel with the phospholipid bilayer combining an experimental study, employing a series of biophysical techniques like Differential Scanning Calorimetry, X-Ray Diffraction and Infrared Spectroscopy, and a Molecular Dynamics simulation. Our experimental results indicated that Docetaxel incorporated into DPPC bilayer perturbing the gel to liquid crystalline phase transition and giving rise to immiscibility when the amount of the drug is increased. The drug promotes the gel ripple phase, increasing the bilayer thickness in the fluid phase, and is also able to alter the hydrogen-bonding interactions in the interfacial region of the bilayer producing a dehydration effect. The results from computational simulation agree with the experimental ones and located the Docetaxel molecule forming small clusters in the region of the carbon 8 of the acyl chain palisade overlapping with the carbonyl region of the phospholipid. Our results support the idea that the anticancer drug is embedded into the phospholipid bilayer to a limited amount and produces structural perturbations which might affect the function of the membrane.

3,4,5-Trimethoxybenzoate of Catechin, an Anticarcinogenic Semisynthetic Catechin, Modulates the Physical Properties of Anionic Phospholipid Membranes

3,4,5-Trimethoxybenzoate of catechin (TMBC) is a semisynthetic catechin which shows strong antiproliferative activity against malignant melanoma cells. The amphiphilic nature of the molecule suggests that the membrane could be a potential site of action, hence the study of its interaction with lipid bilayers is mandatory in order to gain information on the effect of the catechin on the membrane properties and dynamics. Anionic phospholipids, though being minor components of the membrane, possess singular physical and biochemical properties that make them physiologically essential. Utilizing phosphatidylserine biomimetic membranes, we study the interaction between the catechin and anionic bilayers, bringing together a variety of experimental techniques and molecular dynamics simulation. The experimental data suggest that the molecule is embedded into the phosphatidylserine bilayers, where it perturbs the thermotropic gel to liquid crystalline phase transition. In the gel phase, the catechin promotes the formation of interdigitation, and in the liquid crystalline phase, it decreases the bilayer thickness and increases the hydrogen bonding pattern of the interfacial region of the bilayer. The simulation data agree with the experimental ones and indicate that the molecule is located in the interior of the anionic bilayer as monomer and small clusters reaching the carbonyl region of the phospholipid, where it also disturbs the intermolecular hydrogen bonding between neighboring lipids. Our observations suggest that the catechin incorporates well into phosphatidylserine bilayers, where it produces structural changes that could affect the functioning of the membrane.

Diethylstilbestrol Modifies the Structure of Model Membranes and Is Localized Close to the First Carbons of the Fatty Acyl Chains

The synthetic estrogen diethylstilbestrol (DES) is used to treat metastatic carcinomas and prostate cancer. We studied its interaction with membranes and its localization to understand its mechanism of action and side-effects. We used differential scanning calorimetry (DSC) showing that DES fluidized the membrane and has poor solubility in DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) in the fluid state. Using small-angle X-ray diffraction (SAXD), it was observed that DES increased the thickness of the water layer between phospholipid membranes, indicating effects on the membrane surface. DSC, X-ray diffraction, and ³¹P-NMR spectroscopy were used to study the effect of DES on the L_α-to-H_II phase transition, and it was observed that negative curvature of the membrane is promoted by DES, and this effect may be significant to understand its action on membrane enzymes. Using the ¹H-NOESY-NMR-MAS technique, cross-relaxation rates for different protons of DES with POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) protons were calculated, suggesting that the most likely location of DES in the membrane is with the main axis parallel to the surface and close to the first carbons of the fatty acyl chains of POPC. Molecular dynamics simulations were in close agreements with the experimental results regarding the location of DES in phospholipids bilayers.

Interaction of Vitamin K1 and Vitamin K2 with Dimyristoylphosphatidylcholine and Their Location in the Membrane

Vitamin K₁ and vitamin K₂ play very important biological roles as members of chains of electron transport as antioxidants in membranes and as cofactors for the posttranslational modification of proteins that participate in a number of physiological functions such as coagulation. The interaction of these vitamins with dimyristoylphosphatidylcholine (DMPC) model membranes has been studied by using a biophysical approach. It was observed by using differential scanning calorimetry that both vitamins have a very limited miscibility with DMPC and they form domains rich in the vitamins at high concentrations. Experiments using X-ray diffraction also showed the formation of different phases as a consequence of the inclusion of either vitamin K at temperatures below the phase transition. However, in the fluid state, a homogeneous phase was detected, and a decrease in the thickness of the membrane was accompanied by an increase in the water layer thickness. ²H NMR spectroscopy showed that both vitamins K induced a decrease in the onset of the phase transition, which was bigger for vitamin K₁, and both vitamins decreased the order of the membrane as seen through the first moment (M₁). ¹H NOESY MAS-NMR showed that protons located at the rings or at the beginning of the lateral chain of both vitamins K interacted with a clear preference with protons located in the polar part of DMPC. On the other hand, protons located on the lateral chain have a nearer proximity with the methyl end of the myristoyl chains of DMPC. In agreement with the ²H NMR, ATR-FTIR (attenuated total reflectance Fourier transform infrared spectroscopy) indicated that both vitamins decreased the order parameters of DMPC. It was additionally deduced that the lateral chains of both vitamins were oriented almost in parallel to the myristoyl chains of the phospholipid.

Towards long-acting adrenaline for cardiopulmonary resuscitation: Production and characterization of a liposomal formulation

The use of adrenaline in cardiopulmonary resuscitation is a long-standing medical procedure, recommended by several international guidelines. However, its unspecific action on adrenergic receptors and the need for repeated administrations pose serious concerns about its safety, the balance between benefits and risks being still under debate. To address this issue, a sustained release nano-formulation of adrenaline was developed. Adrenaline was encapsulated into PEGylated, anionic liposomes by a pH-driven loading technique. Particular attention was devoted to the prevention of oxidation of adrenaline by optimizing the preparative process and including an optimal amount of antioxidants in the formulation. The vesicles obtained were then characterized for size, zeta-potential, and lamellarity, while their morphology was described by cryo-TEM. The controlled release properties were confirmed by two different in vitro release-testing methods, and the biocompatibility was assayed on human endothelial cells in vitro.

Interactions of Cationic Lipids with DNA: A Structural Approach

Colloidal nucleic acid carrier systems based on cationic lipids are a promising pharmaceutical tool in the implementation of gene therapeutic strategies. This study demonstrates the complex behavior of DNA at the lipid-solvent interface facilitating structural changes of the lyotropic liquid-crystalline phases. For this study, the structural properties of six malonic acid based cationic lipids were determined using small- and wide-angle X-ray scattering (SAXS and WAXS) as well as differential scanning calorimetry (DSC). Selected lipids (lipid 3 and lipid 6) with high nucleic acid transfer activity have been investigated in detail because of the strong influence of the zwitterionic helper lipid 1,2-di(9 Z-octadecenoyl)- sn-glycero-3-phosphoethanolamine (DOPE) on the structural properties as well as of the complex formation of lipid-DNA complexes (lipoplexes). In the case of lipid 3, DNA stabilizes a metastable cubic mesophase with Im3 m symmetry and an Im3 m Q_α^c lipoplex is formed, which is rarely described for DNA lipoplexes in literature. In the case of lipid 6, a cubic mesophase with Im3 m symmetry turns into a fluid lamellar phase while mixing with DOPE and complexing DNA.

Structural Modifications of DPPC Bilayers upon Inclusion of an Antibacterial Cationic Bolaamphiphile

The emergence of antibiotic-resistant bacterial strains has fostered fundamental research to develop alternative antimicrobial strategies. Among the several systems proposed so far, the association complexes (nanoplexes) formed by transcription factor decoys (TFDs), i.e., short oligonucleotides targeting a crucial bacterial transcription factor, and a bolaform cationic amphiphile, 10,10'-(dodecane-1,12-diyl)-bis-(9-amino-1,2,3,4-tetrahydroacridinium) chloride (12-bis-THA), have demonstrated their potential in vitro and in vivo. The application of these nanoplexes is hampered by a scarce colloidal stability, which can be addressed by including the bolaamphiphile in a liposomal carrier, which is then associated to the TFD. The present study reports an investigation on the effects of 12-bis-THA on the structure of synthetic lipid bilayers to assess the morphology of the mixed assemblies, gain insight into the location of the host within the bilayer, and determine the loading capacity of the carrier. Our results demonstrate that 12-bis-THA promptly inserts within 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) bilayers, bending its C-12 spacer chain to adopt a conelike shape and shifting the gel-liquid crystalline transition of the chains to lower temperatures. The host liposomal structure is retained for a bolaamphiphile concentration of up to 3.2% mol to DPPC, whereas higher concentrations lead to the destabilization by means of a detergency-like mechanism, with the simultaneous existence of different lamellar-based structures, such as liposomes, bicelles, and rafts, in which DPPC and 12-bis-THA could be present in different molar ratios. Overall, these results shed light on the interaction of the bolaamphiphile with a lipid bilayer and provide valuable insight to better formulate the antimicrobial amphiphile in liposomal carriers to circumvent the colloidal instability of nanoplexes.

Phenolic Group of α-Tocopherol Anchors at the Lipid-Water Interface of Fully Saturated Membranes

α-Tocopherol is considered to carry on a very important role as an antioxidant for membranes and lipoproteins and other biological roles as membrane stabilizers and bioactive lipids. Given its essential role, it is very important to fully understand its location in the membrane. In this work, the vertical location of vitamin E in saturated membranes has been studied using biophysical techniques. Small- and wide-angle X-ray diffraction experiments show that α-tocopherol alters the water layer between bilayers in both 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC), indicating its proximity to this surface. The quenching of the intrinsic fluorescence of α-tocopherol indicates a low quenching efficiency by acrylamide and a higher quenching by 5-doxyl-PC than by 9- and 16-doxyl-PC. These results suggest that in both DMPC and DPPC membranes, the chromanol ring is not far away from the surface of the membrane but within the bilayer. ¹H nuclear Overhauser enhancement spectroscopy magic-angle spinning-nuclear magnetic resonance studies showed that α-tocopherol is localized in a similar manner in DMPC and DPPC membranes, with the chromanol ring embedded in the upper part of the hydrophobic bilayer. Using attenuated total reflection-Fourier transform infrared spectroscopy, it was observed that the tail chain of α-tocopherol lies nearly parallel to the acyl chains of DMPC and DPPC. Taking these results together, it was concluded that in both DMPC and DPPC, the hydroxyl group of the chromanol ring will establish hydrogen bonding with water on the membrane surface, and the main axis of the α-tocopherol molecule will be perpendicular to the bilayer plane.

The vertical location of α-tocopherol in phosphatidylcholine membranes is not altered as a function of the degree of unsaturation of the fatty acyl chains

α-Tocopherol is a natural preservative that prevents free radical chain oxidations in biomembranes. We have studied the location of α-tocopherol in model membranes formed by different unsaturated phosphatidylcholines, namely 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (PLPC), 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (PAPC) and 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (PDPC). Small angle X-ray diffraction revealed that α-tocopherol was well mixed with all the phospholipids. In all the cases only one lamellar phase was detected. Very modest changes occasioned by α-tocopherol were observed in the electron density profiles. The results obtained from quenching of α-tocopherol intrinsic fluorescence by acrylamide showed that this vitamin was inefficiently quenched in the four types of membranes, indicating that the fluorescent chromanol ring was poorly accessible for this hydrophilic quencher. Compatible with that, quenching by doxyl derivatives of phosphatidylcholines indicated that the chromanol ring was close in the four membranes to the nitroxide probe located at position 5. Quenching by doxyl-phosphatidylcholines also indicated that the efficiency of quenching was higher in POPC than in the other unsaturated phospholipids. ¹H-MAS-NMR showed that α-tocopherol induced chemical shifts of protons from the phospholipids, especially of those bonded to carbons 2 and 3 of the acyl chains of the four phospholipids studied. The ¹H-MAS-NMR NOESY results suggested that the lower part of the chromanol ring was located between the C3 of the fatty acyl chains and the centre of the hydrophobic monolayer for the four phospholipid membranes studied. Taken together, these results suggest that α-tocopherol is located, in all the membranes studied, with the chromanol ring within the hydrophobic palisade but not far away from the lipid-water interface.

Malonic acid based cationic lipids - The way to highly efficient DNA-carriers

Cationic lipids play an important role as non-viral nucleic acid carriers in gene therapy since 3 decades. This review will introduce malonic acid derived cationic lipids as nucleic acid carriers which appeared in the literature dealing with lipofection 10years ago. The family of amino-functionalized branched fatty acid amides will be presented as well as different generations of malonic acid diamides. Both groups of cationic lipids yield lipid mixtures with highly efficient nucleic acid transfer activities in in-vitro cell culture models. The DNA transfer screening of lipid libraries with directed structural variations in the lipophilic as well as in the hydrophilic part of the amphiphiles yields structure/activity relationships. Furthermore, the detailed characterizations of selected lipid composites at the air/water interface and in bulk systems are summarized with regard to transfection determining physical-chemical properties. The findings are also discussed in comparison to results obtained with other families of cationic lipids.

Interaction of the Lipopeptide Biosurfactant Lichenysin with Phosphatidylcholine Model Membranes

Lichenysins produced by Bacillus licheniformis are anionic lipopeptide biosurfactants with cytotoxic, antimicrobial, and hemolytic activities that possess enormous potential for chemical and biological applications. Through the use of physical techniques such as differential scanning calorimetry, small- and wide-angle X-ray diffraction, and Fourier-transform infrared spectroscopy as well as molecular dynamics simulations, we report on the interaction of Lichenysin with synthetic phosphatidylcholines differing in hydrocarbon chain length. Lichenysin alters the thermotropic phase behavior of phosphatidylcholines, displaying fluid-phase immiscibility and showing a preferential partitioning into fluid domains. The interlamellar repeat distance of dipalmitoylphosphatidylcholine (DPPC) is modified, affecting both the phospholipid palisade and the lipid/water interface, which also experiences a strong dehydration. Molecular dynamics confirms that Lichenysin is capable of interacting both with the hydrophobic portion of DPPC and with the polar headgroup region, which is of particular relevance to explain much of its properties. The results presented here help to establish a molecular basis for the Lichenysin-induced perturbation of model and biological membranes previously described in the literature.

Inclusion of oligonucleotide antimicrobials in biocompatible cationic liposomes: A structural study

HYPOTHESIS

Transcription factor decoys (TFD) are short oligonucleotides designed to block essential genetic pathways in bacteria and defeat resistant infections. TFD protection in biological fluids and their delivery to the site of infection require formulation in appropriate delivery systems. In this work, we build on a classical phosphatidylcholine/phosphatidylethanolamine (POPC/DOPE) scaffold to design TFD-loaded cationic liposomes by combining the DNA-complexing abilities of a bolaamphiphile, (1,1'-(dodecane-1,12-diyl)-bis-(9-amino-1,2,3,4-tetrahydroacridinium) chloride (12-bis-THA), with the biocompatible cationic lipid ethyl-phosphatidylcholine (DPePC). The goal is to perform a structural study to determine the impact of the bolaamphiphile and TFD incorporation on the liposome structure, the capacity for TFD encapsulation, and the colloidal stability in saline media and cell culture environments.

EXPERIMENTS

The systems are characterized by means of dynamic light scattering, small-angle X-ray scattering, and ζ-potential measurements, to provide a clear picture of the liposome structure. Circular dichroism (CD) spectroscopy is used to assess the compaction of the oligonucleotide in a psi form, while steady-state fluorescence and fluorescence correlation spectroscopies give insight into the entrapment rate and distribution of the TFD in the liposomes.

FINDINGS

We found that the combination of the two cationic species, 12-bis-THA and DPePC, allows encapsulation of 90% of the TFD. Results of CD experiments revealed that the TFD is condensed, therefore likely protected from the lytic action of serum nucleases. Finally, the systems showed colloidal stability in aqueous dispersion with ionic strength comparable to biologically relevant media.

Cationic liposomal vectors incorporating a bolaamphiphile for oligonucleotide antimicrobials

Antibacterial resistance has become a serious crisis for world health over the last few decades, so that new therapeutic approaches are strongly needed to face the threat of resistant infections. Transcription factor decoys (TFD) are a promising new class of antimicrobial oligonucleotides with proven in vivo activity when combined with a bolaamphiphilic cationic molecule, 12-bis-THA. These two molecular species form stable nanoplexes which, however, present very scarce colloidal stability in physiological media, which poses the challenge of drug formulation and delivery. In this work, we reformulated the 12-bis-THA/TFD nanoplexes in a liposomal carrier, which retains the ability to protect the oligonucleotide therapeutic from degradation and deliver it across the bacterial cell wall. We performed a physical-chemical study to investigate how the incorporation of 12-bis-THA and TFD affects the structure of POPC- and POPC/DOPE liposomes. Analysis was performed using dynamic light scattering (DLS), ζ-potential measurements, small-angle x-ray scattering (SAXS), and steady-state fluorescence spectroscopy to better understand the structure of the liposomal formulations containing the 12-bis-THA/TFD complexes. Oligonucleotide delivery to model Escherichia coli bacteria was assessed by means of confocal scanning laser microscopy (CLSM), evidencing the requirement of a fusogenic helper lipid for transfection. Preliminary biological assessments suggested the necessity of further development by modulation of 12-bis-THA concentration in order to optimize its therapeutic index, i.e. the ratio of antibacterial activity to the observed cytotoxicity. In summary, POPC/DOPE/12-bis-THA liposomes appear as promising formulations for TFD delivery.

Experimental Modeling of Flavonoid-Biomembrane Interactions

Nonspecific interactions of flavonoids with lipids can alter the membrane's features (e.g., thickness and fluctuations) as well as influence their therapeutic potentials. However, relatively little is known about the details of how flavonoids interact with lipid components. Structure-dependent interactions of a variety of flavonoids with phospholipid monolayers on a mercury (Hg) film electrode were established by rapid cyclic voltammetry (RCV). The data revealed that flavonoids adopting a planar configuration altered the membrane properties more significantly than nonplanar flavonoids. Quercetin, rutin, and tiliroside were selected for follow-up experiments with Langmuir monolayers, Brewster angle microscopy (BAM), and small-angle X-ray scattering (SAXS). Relaxation phenomena in DOPC monolayers and visualization of the surface with BAM revealed a pronounced monolayer stabilization effect with both quercetin and tiliroside, whereas rutin disrupted the monolayer structure rendering the surface entirely smooth. SAXS showed a monotonous membrane thinning for all compounds studied associated with an increase in the mean fluctuations of the membrane. Rutin, quercetin, and tiliroside decreased the bilayer thickness of DOPC by ∼0.45, 0.8, and 1.1 Å at 6 mol %, respectively. In addition to the novelty of using lipid monolayers to systematically characterize the structure-activity relationship (SAR) of a variety of flavonoids, this is the first report investigating the effect of tiliroside with biomimetic membrane models. All the flavonoids studied are believed to be localized in the lipid/water interface region. Both this localization and the membrane perturbations have implications for their therapeutic activity.

X-ray diffraction and NMR data for the study of the location of idebenone and idebenol in model membranes

Here we present some of our data about the interaction of idebenone and idebenol with dipalmitoyl-phosphatidylcholine (DPPC). In particular, we include data of small angle X-ray diffraction (SAXD) and wide angle X-ray diffraction experiments, obtention of electronic profiles of the membranes, (2)H-NMR and (31)P-NMR, as part of the research article: "Both idebenone and idebenol are localized near the lipid-water interface of the membrane and increase its fluidity" (Gomez-Murcia et al., 2016) [1]. These data were obtained from model membranes that included different proportions of idebenone and idebenol, at temperatures both above and below of the gel to fluid phase. The X-ray experiments were carried out by using a modified Kratky compact camera (MBraun-Graz-Optical Systems, Graz Austria), incorporating two coupled linear position sensitive detectors. The NMR data were collected from a a Bruker Avance 600 instrument.

Small Angle X-ray and Neutron Scattering: Powerful Tools for Studying the Structure of Drug-Loaded Liposomes

Nanovectors, such as liposomes, micelles and lipid nanoparticles, are recognized as efficient platforms for delivering therapeutic agents, especially those with low solubility in water. Besides being safe and non-toxic, drug carriers with improved performance should meet the requirements of (i) appropriate size and shape and (ii) cargo upload/release with unmodified properties. Structural issues are of primary importance to control the mechanism of action of loaded vectors. Overall properties, such as mean diameter and surface charge, can be obtained using bench instruments (Dynamic Light Scattering and Zeta potential). However, techniques with higher space and time resolution are needed for in-depth structural characterization. Small-angle X-ray (SAXS) and neutron (SANS) scattering techniques provide information at the nanoscale and have therefore been largely used to investigate nanovectors loaded with drugs or other biologically relevant molecules. Here we revise recent applications of these complementary scattering techniques in the field of drug delivery in pharmaceutics and medicine with a focus to liposomal carriers. In particular, we highlight those aspects that can be more commonly accessed by the interested users.

Bending Rigidities and Interdomain Forces in Membranes with Coexisting Lipid Domains

To precisely quantify the fundamental interactions between heterogeneous lipid membranes with coexisting liquid-ordered (Lo) and liquid-disordered (Ld) domains, we performed detailed osmotic stress small-angle x-ray scattering experiments by exploiting the domain alignment in raft-mimicking lipid multibilayers. Performing a Monte Carlo-based analysis allowed us to determine with high reliability the magnitude and functional dependence of interdomain forces concurrently with the bending elasticity moduli. In contrast to previous methodologies, this approach enabled us to consider the entropic undulation repulsions on a fundamental level, without having to take recourse to crudely justified mean-field-like additivity assumptions. Our detailed Hamaker-coefficient calculations indicated only small differences in the van der Waals attractions of coexisting Lo and Ld phases. In contrast, the repulsive hydration and undulation interactions differed significantly, with the latter dominating the overall repulsions in the Ld phase. Thus, alignment of like domains in multibilayers appears to originate from both, hydration and undulation repulsions.

Capsaicin Fluidifies the Membrane and Localizes Itself near the Lipid-Water Interface

Capsaicin is the chemical responsible for making some peppers spicy hot, but additionally it is used as a pharmaceutical to alleviate different pain conditions. Capsaicin binds to the vanilloid receptor TRPV1, which plays a role in coordinating chemical and physical painful stimuli. A number of reports have also shown that capsaicin inserts in membranes and its capacity to modify them may be part of its molecular mode of action, affecting the activity of other membrane proteins. We have used differential scanning calorimetry, X-ray diffraction, (31)P NMR, and (2)H NMR spectroscopy to show that capsaicin increases the fluidity and disorder of 1,2-palmitoyl-sn-glycero-3-phosphocholine membrane models. By using (1)H NOESY MAS NMR based on proton-proton cross-peaks between capsaicin and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine resonances, we determined the location profile of this molecule in a fluid membrane concluding that it occupies the upper part of the phospholipid monolayer, between the lipid-water interface and the double bond of the acyl chain in position sn-2. This location explains the disorganization of the membrane of both the lipid-water interface and the hydrophobic palisade.

Phospholipid-driven differences determine the action of the synthetic antimicrobial peptide OP-145 on Gram-positive bacterial and mammalian membrane model systems

OP-145, a synthetic antimicrobial peptide developed from a screen of the human cathelicidin LL-37, displays strong antibacterial activities and is--at considerably higher concentrations--lytic to human cells. To obtain more insight into its actions, we investigated the interactions between OP-145 and liposomes composed of phosphatidylglycerol (PG) and phosphatidylcholine (PC), resembling bacterial and mammalian membranes, respectively. Circular dichroism analyses of OP-145 demonstrated a predominant α-helical conformation in the presence of both membrane mimics, indicating that the different membrane-perturbation mechanisms are not due to different secondary structures. Membrane thinning and formation of quasi-interdigitated lipid-peptide structures was observed in PG bilayers, while OP-145 led to disintegration of PC liposomes into disk-like micelles and bilayer sheets. Although OP-145 was capable of binding lipoteichoic acid and peptidoglycan, the presence of these bacterial cell wall components did not retain OP-145 and hence did not interfere with the activity of the peptide toward PG membranes. Furthermore, physiological Ca++ concentrations did neither influence the membrane activity of OP-145 in model systems nor the killing of Staphylococcus aureus. However, addition of OP-145 at physiological Ca++-concentrations to PG membranes, but not PC membranes, resulted in the formation of elongated enrolled structures similar to cochleate-like structures. In summary, phospholipid-driven differences in incorporation of OP-145 into the lipid bilayers govern the membrane activity of the peptide on bacterial and mammalian membrane mimics.

Lamellar gel (lβ) phases of ternary lipid composition containing ceramide and cholesterol

Lipid lateral segregation into specific domains in cellular membranes is associated with cell signaling and metabolic regulation. This phenomenon partially arises as a consequence of the very distinct bilayer-associated lipid physico-chemical properties that give rise to defined phase states at a given temperature. Until now lamellar gel (Lβ) phases have been described in detail in single or two-lipid systems. Using x-ray scattering, differential scanning calorimetry, confocal fluorescence microscopy, and atomic force microscopy, we have characterized phases of ternary lipid compositions in the presence of saturated phospholipids, cholesterol, and palmitoyl ceramide mixtures. These phases stabilized by direct cholesterol-ceramide interaction can exist either with palmitoyl sphingomyelin or with dipalmitoyl phosphatidylcholine and present intermediate properties between raft-associated phospholipid-cholesterol liquid-ordered and phospholipid-ceramide Lβ phases. The present data provide novel, to our knowledge, evidence of a chemically defined, multicomponent lipid system that could cooperate in building heterogeneous segregated platforms in cell membranes.

Initial step of pH-jump-induced lamellar to bicontinuous cubic phase transition in dioleoylphosphatidylserine/monoolein

Electrostatic interactions (EI) are an important factor for phase transitions between lamellar liquid-crystalline (L(α)) and inverse bicontinuous cubic (Q(II)) phases. We investigated the low pH-induced L(α) to double-diamond cubic (Q(II)(D)) phase transition in dioleoylphosphatidylserine (DOPS)/monoolein (MO) using time-resolved small-angle X-ray scattering. Using a stopped-flow apparatus, a suspension of liposomes (multilamellar vesicles (MLVs) or large unilamellar vesicles (LUVs)) of 20%-DOPS/80%-MO membrane at neutral pH was rapidly mixed with a low pH buffer, and then the structural change of the membranes in the resultant suspension was observed as a function of time (i.e., pH-jump experiment). At the initial step, the L(α) phase was directly transformed into the hexagonal II (H(II)) phase, and subsequently, the H(II) phase slowly converted into the Q(II)(D) phase. We obtained the rate constants of the initial step (i.e., the L(α) to H(II) phase transition) and of the second step (i.e., the H(II) to Q(II)(D) phase transition) using the non-negative matrix factorization method. The rate constant of the initial step was independent of the MLV concentration, indicating that single MLVs can convert into the HII phase without any interaction with other MLVs. On the other hand, the rate constant of the initial step increased with a decrease in pH, 0.041 s(-1) at pH 2.6 and 0.013 s(-1) at pH 2.8, and also exhibited a size dependence; for smaller vesicles such as LUVs and smaller MLVs with diameters of ~1 μm, the rate constant was smaller. They were reasonably explained by the classical nucleation theory. These results provide the first experimental evidence of the total kinetics of EI-induced L(α)/Q(II) phase transitions.

Recent developments in the field of bending rigidity measurements on membranes

This review gives a brief overview of experimental approaches used to assess the bending rigidity of membranes. Emphasis is placed on techniques based on the use of giant unilamellar vesicles. We summarize the effect on the bending rigidity of membranes as a function of membrane composition, presence of various inclusions in the bilayer and molecules and ions in the bathing solutions. Examples for the impact of temperature, cholesterol, some peptides and proteins, sugars and salts are provided and the literature data are discussed critically. Future directions, open questions and possible developments in this research field are also included.

Steric-induced effects on stabilizing a lamellar structure

We investigate the behavior of multilamellar phases composed of lecithin and a commercial cosurfactant (Simusol), which is a mixture of ethoxylated fatty acids. Using X-ray scattering and a new procedure to fit the data, relevant parameters characterizing the lamellar structure were determined as a function of membrane composition, varying from 100% of lecithin to 100% of Simulsol. Scattering data illustrating the swelling of the lamellae for different amounts of cosurfactant are presented with the respective behavior of the Caillé parameter. With this experimental approach, we show that the incorporation of ethoxy brushes onto the lipid surface enhances repulsive interactions arising from membrane fluctuations and changes the interactions at the interface between bilayers.

Physicochemical, cytotoxic, and dermal release features of a novel cationic liposome nanocarrier

A novel cationic liposome nanocarrier, having interesting performance in topical drug delivery, is here presented and evaluated for its features. Two penetration enhancers, namely monoolein and lauroylcholine chloride, are combined to rapidly formulate (15 min) a cationic liposome nanostructure endowed of excellent stability (>6 months) and skin penetration ability, along with low short-term cytotoxicity, as evaluated via the MTT test. Cytotoxicity tests and lipid droplet analysis give a strong indication that monoolein and lauroylcholine synergistically endanger long-term cells viability. The physicochemical features, investigated through SAXS, DLS, and cryo-TEM techniques, reveal that the nanostructure is retained after loading with diclofenac in its acid (hydrophobic) form. The drug release performances are studied using intact newborn pig skin. Analysis of the different skin strata proves that the drug mainly accumulates into the viable epidermis with almost no deposition into the derma. Indeed, the flux of the drug across the skin is exceptionally low, with only 1% release after 24 h. These results validate the use of this novel formulation for topical drug release when the delivery to the systemic circulation should be avoided.

Losartan's affinity to fluid bilayers modulates lipid-cholesterol interactions

Losartan is an angiotensin II receptor antagonist mainly used for the regulation of high blood pressure. Since it was anticipated that losartan reaches the receptor site via membrane diffusion, the impact of losartan on model membranes has been investigated by small angle X-ray scattering. For this purpose 2-20 mol% losartan was incorporated into dimyristoyl-phosphatidylcholine (DMPC) and palmitoyl-oleoyl-phosphatidylcholine (POPC) bilayers and into their binary mixtures with cholesterol in the concentration range of 0 to 40 mol%. Effects of losartan on single component bilayers are alike. Partitioning of losartan into the membranes confers a negative charge to the lipid bilayers that causes the formation of unilamellar vesicles and a reduction of the bilayer thickness by 3-4%. Analysis of the structural data resulted in an estimate for the partial area of losartan, A(Los) ≈ 40 Å(2). In the presence of cholesterol, differences between the effects of losartan on POPC and DMPC are striking. Membrane condensation by cholesterol is retarded by losartan in POPC. This contrasts with DMPC, where an increase of the cholesterol content shifts the partitioning equilibrium of losartan towards the aqueous phase, such that losartan gets depleted from the bilayers from 20 mol% cholesterol onwards. This indicates (i) a chain-saturation dependent competition of losartan with lipid-cholesterol interactions, and (ii) the insolubility of losartan in the liquid ordered phase of PCs. Consequently, losartan's action is more likely to take place in fluid plasma membrane patches rather than in domains rich in cholesterol and saturated lipid species such as in membrane rafts.

Structural Versatility of Bicellar Systems and Their Possibilities as Colloidal Carriers

Bicellar systems are lipid nanostructures formed by long- and short-chained phospholipids dispersed in aqueous solution. The morphological transitions of bicellar aggregates due to temperature, composition and time variations have been revised in this work. To this end, two bicellar systems have been considered; one formed by dimyristoyl-phosphatidylcholine (DMPC) and dihexanoyl- phosphatidylcholine (DHPC) and another formed by dipalmitoyl-phosphatidylcholine (DPPC) and DHPC. The relationship between the magnetic alignment, the morphology of the aggregates and the phase transition temperature (T_m) of lipids is discussed. In general terms, the non-alignable samples present rounded objects at temperature below the T_m. Above this temperature, an increase of viscosity is followed by the formation of large elongated aggregates. Alignable samples presented discoidal objects below the T_m. The best alignment was achieved above this temperature with large areas of lamellar stacked bilayers and some multilamellar vesicles. The effect of the inclusion of ceramides with different chain lengths in the structure of bicelles is also revised in the present article. A number of physical techniques show that the bicellar structures are affected by both the concentration and the type of ceramide. Systems are able to incorporate 10% mol of ceramides that probably are organized forming domains. The addition of 20% mol of ceramides promotes destabilization of bicelles, promoting the formation of mixed systems that include large structures. Bicellar systems have demonstrated to be morphologically stable with time, able to encapsulate different actives and to induce specific effects on the skin. These facts make bicellar systems good candidates as colloidal carriers for dermal delivery. However, water dilution induces structural changes and formation of vesicular structures in the systems; stabilization strategies have been been explored in recent works and are also updated here.

Effects of a bacterial trehalose lipid on phosphatidylglycerol membranes

Bacterial trehalose lipids are biosurfactants with potential application in the biomedical/healthcare industry due to their interesting biological properties. Given the amphiphilic nature of trehalose lipids, the understanding of the molecular mechanism of their biological action requires that the interaction between biosurfactant and membranes is known. In this study we examine the interactions between a trehalose lipid from Rhodococcus sp. and dimyristoylphosphatidylglycerol membranes by means of differential scanning calorimetry, X-ray diffraction, infrared spectroscopy and fluorescence polarization. We report that there are extensive interactions between trehalose lipid and dimyristoylphosphatidylglycerol involving the perturbation of the thermotropic gel to liquid-crystalline phase transition of the phospholipid, the increase of fluidity of the phosphatidylglycerol acyl chains and dehydration of the interfacial region of the bilayer, and the modulation of the order of the phospholipid bilayer. The observations are interpreted in terms of structural perturbations affecting the function of the membrane that might underline the biological actions of the trehalose lipid.

Curcumin disorders 1,2-dipalmitoyl-sn-glycero-3-phosphocholine membranes and favors the formation of nonlamellar structures by 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine

Curcumin is a polyphenol present in turmeric, a spice widely used in Asian traditional medicine and cooking. It has many and diverse biological effects and is incorporated in cell membranes. This paper describes the mode in which curcumin modulates the physical properties of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dielaidyl-sn-glycero-3-phosphoetnanolamine (DEPE) multilamellar membranes. Curcumin disordered DPPC membranes at temperatures below T(c) as seen by DSC, FT-IR, (2)H NMR, WAXD, and SAXD. The decrease induced by curcumin in T(c) suggested that it is oriented in the bilayer with its main axis parallel to the acyl chains. Above T(c), too, curcumin introduced disorder as seen by infrared spectroscopy which showed that curcumin also alters the conformation of the polar group of DPPC, increasing the percentage of unhydrated C=O groups, but does not form hydrogen bonds with either the C=O group or the phosphate group of DPPC. Small angle X-ray diffraction showed a notable increase in the repeating spacings as a result of the presence of curcumin, suggesting the formation of a rippled phase. Increasing concentrations of curcumin progressively modified the onset and completion of the phase transition and also DeltaH up to a 6:1 DPPC/curcumin molar ratio. A further increase of curcumin concentration did not produce effects on the transition parameters, suggesting that there is a limit for the solubility of curcumin in DPPC. Additionally, when DEPE was used to test the effect of curcumin on the phospholipid polymorphism, it was found that the temperature at which the H(II) phase is formed decreased, indicating that curcumin favors negative curvature of the membrane, which may be important for explaining its effect on membrane dynamics and on membrane proteins or on proteins which may be activated through membrane insertion.

New pH-sensitive liposomes containing phosphatidylethanolamine and a bacterial dirhamnolipid

Phosphatidylethanolamine-based pH-sensitive liposomes of various compositions have been described as efficient systems for cytoplasmic delivery of molecules into cells. Incorporation of an amphiphile of appropriate structure is needed for the stabilization and performance of these vesicles. Among the wide variety of interesting activities displayed by Pseudomonas aeruginosa dirhamnolipids (diRL), is their capacity to stabilize bilayer structures in phosphatidylethanolamine systems. In this work, X-ray scattering, dynamic light scattering, fluorescence spectroscopy and fluorescence microscopy have been used to study the structure and pH-dependent behaviour of phosphatidylethanolamine/diRL liposomes. We show that diRL, in combination with dioleoylphosphatidylethanolamine (DOPE), forms stable multilamellar and unilamellar liposomes. Acidification of DOPE/diRL vesicles leads to membrane destabilization, fusion, and release of entrapped aqueous vesicle contents. Finally, DOPE/diRL pH-sensitive liposomes act as efficient vehicles for the cytoplasmic delivery of fluorescent probes into cultured cells. It is concluded that DOPE/diRL form stable pH-sensitive liposomes, and that these liposomes are incorporated into cultured cells through the endocytic pathway, delivering its contents into the cytoplasm, which means a potential use of these liposomes for the delivery of foreign substances into living cells. Our results establish a new application of diRL as a bilayer stabilizer in phospholipid vesicles, and the use of diRL-containing pH-sensitive liposomes as delivery vehicles.

Cholesterol orientation and tilt modulus in DMPC bilayers

We performed molecular dynamics (MD) simulations of hydrated bilayers containing mixtures of dimyristoylphosphatidylcholine (DMPC) and cholesterol at various ratios, to study the effect of cholesterol concentration on its orientation, and to characterize the link between cholesterol tilt and overall phospholipid membrane organization. The simulations show a substantial probability for cholesterol molecules to transiently orient perpendicular to the bilayer normal, and suggest that cholesterol tilt may be an important factor for inducing membrane ordering. In particular, we find that as cholesterol concentration increases (1-40% cholesterol) the average cholesterol orientation changes in a manner strongly (anti)correlated with the variation in membrane thickness. Furthermore, cholesterol orientation is found to be determined by the aligning force exerted by other cholesterol molecules. To quantify this aligning field, we analyzed cholesterol orientation using, to our knowledge, the first estimates of the cholesterol tilt modulus chi from MD simulations. Our calculations suggest that the aligning field that determines chi is indeed strongly linked to sterol composition. This empirical parameter (chi) should therefore become a useful quantitative measure to describe cholesterol interaction with other lipids in bilayers, particularly in various coarse-grained force fields.

The membrane insertion of helical antimicrobial peptides from the N-terminus of Helicobacter pylori ribosomal protein L1

The interaction of two helical antimicrobial peptides, HPA3 and HPA3P with planar supported lipid membranes was quantitatively analysed using two complementary optical biosensors. The peptides are analogues of Hp(2-20) derived from the N-terminus of Helicobacter pylori ribosomal protein L1 (RpL1). The binding of these two peptide analogues to zwitterionic dimyristoyl-phosphatidylcholine (DMPC) and negatively charged membranes composed of DMPC/dimyristoylphosphatidylglycerol (DMPG) (4:1) was determined using surface plasmon resonance (SPR) and dual polarisation interferometry (DPI). Using SPR analysis, it was shown that the proline substitution in HPA3P resulted in much lower binding for both zwitterionic and anionic membranes than HPA3. Structural changes in the planar DMPC and DMPC/DMPG (4:1) bilayers induced by the binding of both Hp(2-20) analogues were then resolved in real-time with DPI. The overall process of peptide-induced changes in membrane structure was analysed by the real-time changes in bound peptide mass as a function of bilayer birefringence. The insertion of both HPA3 and HPA3P into the supported lipid bilayers resulted in a decrease in birefringence with increasing amounts of bound peptide which reflects a decrease in the order of the bilayer. The binding of HPA3 to each membrane was associated with a higher level of bound peptide and greater membrane lipid disordering and a faster and higher degree of insertion into the membrane than HPA3P. Furthermore, the binding of both HPA3 and HPA3P to negatively charged DMPC/DMPG bilayers also leads to a greater disruption of the lipid ordering. These results demonstrate the geometrical changes in the membrane upon peptide insertion and the extent of membrane structural changes can be obtained quantitatively. Moreover, monitoring the effect of peptides on a structurally characterised bilayer has provided further insight into the role of membrane structure changes in the molecular basis of peptide selectivity and activity and may assist in defining the mode of antimicrobial action.