Unprecedented observation of days-long remnant orientation of phospholipid bicelles: a small-angle X-ray scattering and theoretical study

A dilute nematic gel produced by intramicellar segregation of two polyoxyethylene alkyl ether carboxylic acids

MOTIVATION

Surfactants like C8E8CH2COOH have such bulky headgroups that they cannot show the common sphere-to-cylinder transition, while surfactants like C18:1E2CH2COOH are mimicking lipids and form only bilayers. Mixing these two types of surfactants allows one to investigate the competition between intramicellar segregation leading to disc-like bicelles and the temperature dependent curvature constraints imposed by the mismatch between heads and tails.

EXPERIMENTS

We establish phase diagrams as a function of temperature, surfactant mole ratio, and active matter content. We locate the isotropic liquid-isotropic liquid phase separation common to all nonionic surfactant systems, as well as nematic and lamellar phases. The stability and rheology of the nematic phase is investigated. Texture determination by polarizing microscopy allows us to distinguish between the different phases. Finally, SANS and SAXS give intermicellar distances as well as micellar sizes and shapes present for different compositions in the phase diagrams.

FINDINGS

In a defined mole ratio between the two components, intramicellar segregation wins and a viscoelastic discotic nematic phase is present at low temperature. Partial intramicellar mixing upon heating leads to disc growth and eventually to a pseudo-lamellar phase. Further heating leads to complete random mixing and an isotropic phase, showing the common liquid-liquid miscibility gap. This uncommon phase sequence, bicelles, lamellar phase, micelles, and water-poor packed micelles, is due to temperature induced mixing combined with dehydration of the headgroups. This general molecular mechanism explains also why a metastable water-poor lamellar phase quenched by cooling can be easily and reproducibly transformed into a nematic phase by gentle hand shaking at room temperature, as well as the entrapment of air bubbles of any size without encapsulation by bilayers or polymers.

High-Temperature Behavior of Early Life Membrane Models

Origin of life scenarios generally assume an onset of cell formation in terrestrial hot springs or in the deep oceans close to hot vents, where energy was available for non-enzymatic reactions. Membranes of the protocells had therefore to withstand extreme conditions different from what is found on the Earth surface today. We present here an exhaustive study of temperature stability up to 80 °C of vesicles formed by a mixture of short-chain fatty acids and alcohols, which are plausible candidates for membranes permitting the compartmentalization of protocells. We confirm that the presence of alcohol has a strong structuring and stabilizing impact on the lamellar structures. Moreover and most importantly, at a high temperature (> 60 °C), we observe a conformational transition in the vesicles, which results from vesicular fusion. Because all the most likely environments for the origin of life involve high temperatures, our results imply the need to take into account such a transition and its effect when studying the behavior of a protomembrane model.

Bicelles and nanodiscs for biophysical chemistry

Membrane nanoobjects are very important tools to study biomembrane properties. Two types are described herein: Bicelles and Nanodiscs. Bicelles are obtained by thorough water mixing of long chain and short chain lipids and may take the form of membranous discs of 10-50 nm. Temperature-composition-hydration diagrams have been established for Phosphatidylcholines and show limited domains of existence. Bicelles can be doped with charged lipids, surfactants or with cholesterol and offer a wide variety of membranous platforms for structural biology. Internal dynamics as measured by solid-state NMR is very similar to that of liposomes in their fluid phase. Because of the magnetic susceptibility anisotropy of the lipid chains, discs may be aligned along or perpendicular to the magnetic field. They may serve as weak orienting media to provide distance information in determining the 3D structure of soluble proteins. In different conditions they show strong orienting properties which may be used to study the 3D structure, topology and dynamics of membrane proteins. Lipid Bicelles with biphenyl chains or doped with lanthanides show long lasting remnant orientation after removing the magnetic field due to smectic-like properties. An alternative to pure lipid Bicelles is provided by nanodiscs where the half torus composed by short chain lipids is replaced by proteins. This renders the nano-objects less fragile as they can be used to stabilize membrane protein assemblies to be studied by electron microscopy. Internal dynamics is again similar to liposomes except that the phase transition is abolished, possibly due to lateral constrain imposed by the toroidal proteins limiting the disc size. Advantages and drawbacks of both nanoplatforms are discussed.

Superflexibility of graphene oxide

Graphene oxide (GO), the main precursor of graphene-based materials made by solution processing, is known to be very stiff. Indeed, it has a Young's modulus comparable to steel, on the order of 300 GPa. Despite its very high stiffness, we show here that GO is superflexible. We quantitatively measure the GO bending rigidity by characterizing the flattening of thermal undulations in response to shear forces in solution. Characterizations are performed by the combination of synchrotron X-ray diffraction at small angles and in situ rheology (rheo-SAXS) experiments using the high X-ray flux of a synchrotron source. The bending modulus is found to be 1 kT, which is about two orders of magnitude lower than the bending rigidity of neat graphene. This superflexibility compares with the fluidity of self-assembled liquid bilayers. This behavior is discussed by considering the mechanisms at play in bending and stretching deformations of atomic monolayers. The superflexibility of GO is a unique feature to develop bendable electronics after reduction, films, coatings, and fibers. This unique combination of properties of GO allows for flexibility in processing and fabrication coupled with a robustness in the fabricated structure.

pH-Cleavable Nucleoside Lipids: A New Paradigm for Controlling the Stability of Lipid-Based Delivery Systems

Lipid-based delivery systems are an established technology with considerable clinical acceptance and several applications in human. Herein, we report the design, synthesis and evaluation of novel orthoester nucleoside lipids (ONLs) for the modulation of liposome stability. The ONLs contain head groups with 3'-orthoester nucleoside derivatives featuring positive or negative charges. The insertion of the orthoester function in the NL structures allows the formation of pH-sensitive liposomes. ONL-based liposomes can be hydrolyzed to provide nontoxic products, including nucleoside derivatives and hexadecanol. To allow the release to be tunable at different hydrolysis rates, the charge of the polar head structure is modulated, and the head group can be released at a biologically relevant pH. Crucially, when ONLs are mixed with natural phosphocholine lipids (PC), the resultant liposome evolves toward the formation of a hexadecanol/PC lamellar system. Biological evaluation shows that stable nucleic acid lipid particles (SNALPs) formulated with ONLs and siRNAs can effectively enter into tumor cells and release their nucleic acid payload in response to an intracellular acidic environment. This results in a much higher antitumor activity than conventional SNALPs. The ability to use pH-cleavable nucleolipids to control the stability of lipid-based delivery systems represents a promising approach for the intracellular delivery of drug cargos.

Chemically Locked Bicelles with High Thermal and Kinetic Stability

In situ polymerization of a bicellar mixture composed of a phospholipid and polymerizable surfactants afforded unprecedented stable bicelles. The polymerized composite showed an aligned phase over a wide thermal range (25 to >90 °C) with excellent (2)H quadrupole splitting of the solvent signal, thus implying versatility as an alignment medium for NMR studies. Crosslinking of the surfactants also brought favorable effects on the kinetic stability and alignment morphology of the bicelles. This system could thus offer a new class of scaffolds for biomembrane models.

Temperature-resistant bicelles for structural studies by solid-state NMR spectroscopy

Three-dimensional structure determination of membrane proteins is important to fully understand their biological functions. However, obtaining a high-resolution structure has been a major challenge mainly due to the difficulties in retaining the native folding and function of membrane proteins outside of the cellular membrane environment. These challenges are acute if the protein contains a large soluble domain, as it needs bulk water unlike the transmembrane domains of an integral membrane protein. For structural studies on such proteins either by nuclear magnetic resonance (NMR) spectroscopy or X-ray crystallography, bicelles have been demonstrated to be superior to conventional micelles, yet their temperature restrictions attributed to their thermal instabilities are a major disadvantage. Here, we report an approach to overcome this drawback through searching for an optimum combination of bicellar compositions. We demonstrate that bicelles composed of 1,2-didecanoyl-sn-glycero-3-phosphocholine (DDPC) and 1,2-diheptanoyl-sn-glycero-3-phosphocholin (DHepPC), without utilizing additional stabilizing chemicals, are quite stable and are resistant to temperature variations. These temperature-resistant bicelles have a robust bicellar phase and magnetic alignment over a broad range of temperatures, between -15 and 80 °C, retain the native structure of a membrane protein, and increase the sensitivity of solid-state NMR experiments performed at low temperatures. Advantages of two-dimensional separated-local field (SLF) solid-state NMR experiments at a low temperature are demonstrated on magnetically aligned bicelles containing an electron carrier membrane protein, cytochrome b5. Morphological information on different DDPC-based bicellar compositions, varying q ratio/size, and hydration levels obtained from (31)P NMR experiments in this study is also beneficial for a variety of biophysical and spectroscopic techniques, including solution NMR and magic-angle-spinning (MAS) NMR for a wide range of temperatures.

Magnetically induced anisotropic orientation of graphene oxide locked by in situ hydrogelation

A general method to prepare polymer gels containing anisotropically oriented graphene oxide (GO) or reduced graphene oxide (RGO) was developed, by using the magnetically induced orientation of GO. Under a magnetic field, an aqueous dispersion of GO was gelated by in situ cross-linking polymerization of an acryl monomer and a cross-linker. In the resultant hydrogel, the orientation of GO was retained even in the absence of the magnetic field, because the gel network trapped GO via noncovalent interactions and efficiently suppressed the structural relaxation of GO. The locked structure enabled quantitative investigation on the magnetic orientation of GO using 2D small-angle X-ray scattering, which revealed that GO nanosheets orient parallel to the magnetic field with an order parameter of up to 0.80. Systematic studies with varying gelation conditions indicate that the present method can afford a wide range of GO-hybridized anisotropic materials, in terms of GO alignment direction, sample shape, and GO concentration. Also by virtue of the locked structure, the orientation of GO in the hydrogel was well preserved throughout the in situ chemical reduction of GO, yielding an RGO-hybridized anisotropic hydrogel, as well as the conversion of the hydrogel into organo- and ionogels through the replacement of the internal water with solvents. As a preliminary demonstration of the present method for practical application, a polymer-composite film containing RGO oriented vertical to the film surface was prepared, and its anisotropically enhanced electroconductivity along the orientation direction of RGO was confirmed by the flash-photolysis time-resolved microwave conductivity measurement.

Investigation of the mechanism of action of novel amphipathic peptides: insights from solid-state NMR studies of oriented lipid bilayers

We have investigated in the present study the effect of both non-selective and selective cationic 14-mer peptides on the lipid orientation of DMPC bilayers by (31)P solid-state nuclear magnetic resonance (NMR) spectroscopy. Depending on the position of substitution, these peptides adopt mainly either an α-helical structure able to permeabilize DMPC and DMPG vesicles (non-selective peptides) or an intermolecular β-sheet structure only able to permeabilize DMPG vesicles (selective peptides). Several systems have been investigated, namely bilayers mechanically oriented between glass plates as well as bicelles oriented with their normal perpendicular or parallel to the external magnetic field. The results have been compared with spectral simulations with the goal of elucidating the difference in the interaction of these two types of peptides with zwitterionic lipid bilayers. The results indicate that the perturbation induced by selective peptides is much greater than that induced by non-selective peptides in all the lipid systems investigated, and this perturbation has been associated to the aggregation of the selective β-sheet peptides in these systems. On the other hand, the oriented lipid spectra obtained in the presence of non-selective peptides suggest the presence of toroidal pores. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Amino acid-nucleotide-lipids: effect of amino acid on the self-assembly properties

Hybrid amphiphiles composed of a lipid covalently linked to biomolecules are attracting considerable attention, owing to their unique physicochemical and biological properties. Herein, we have synthesized novel amino acid-nucleotide-lipids (ANLs), presenting phenylalanine and thymidine residues and saturated or unsaturated diacyl glycerol lipid moieties to investigate the effect of the specific aminoacid moieties on both aggregation properties and interactions of ANLs with single strand polyA RNA. Physicochemical studies (DLS, cryo-TEM, and small angle X-ray scattering) indicate that phenylanaline amino acids inserted at the 5' position of the nucleotide-lipids stabilize multilamellar systems, whereas unilamellar vesicles are formed preferentially in the case of nucleotide-lipids (NLs). Both NLs and ANLs exhibit weak interactions with complementary polyA RNA as revealed by isothermal titration calorimetry investigations. The multilamellar vesicles obtained with ANLs could be used as a versatile carrier, suitable for both hydrophobic and hydrophilic therapeutic molecules.