Ionization and structural changes of the DMPG vesicle along its anomalous gel-fluid phase transition: a study with different lipid concentrations

Critical point for membrane bilayer formation

Unilamellar liposomes often are employed in investigations of lipid-protein interactions and the delivery of drugs in therapies for disease. Also, related lipid-containing nanoparticles have been developed as elements of a new class of mRNA vaccines. We show that only unilamellar films form in equilibrium lipid dispersions, at temperature values {T*} that depend on the identities of the lipids (e.g., T* ≈ 29 °C for DMPC). Thermodynamic analysis confirms that films at air-water surfaces can be used to monitor the properties of the lipid vesicles that form in the dispersion. When T > T*, critical exponents describing film properties as T approaches T* are μ ≈ 1.4 and ν ≈ 0.7, which are close to values for the interfacial tension and the correlation length of density fluctuations at fluid interfaces. These results, and observations that within the bilayer the lateral diffusion of fluorescent lipid probes demonstrates increases at T*, suggest that unilamellar vesicles at T* are a transition state between two different multilamellar structures. We generalize the thermodynamic arguments to explain the linkage between lipid structures in the surface and bulk dispersion within more complex samples, showing that dispersions containing total lipid extracts of cell membranes have properties similar to those in dispersions containing single lipids. Information from various independent studies indicates that T* noted for bilayer membranes of a population of cells is identical to the temperature at which the growth or gestation of the cells occurs in vivo. Examples include whole-cell lipid extracts obtained from bacteria, and poikilothermic and homeothermic animals.

Lateral diffusion of lipids in the DMPG membrane across the anomalous melting regime: effects of NaCl

The anionic dimyristoyl phosphatidylglycerol (DMPG) membrane in solvents with a low ionic strength is known to exhibit an unusually wide melting regime between the gel and fluid phase characterized by various anomalous macroscopic characteristics, such as low turbidity and high electrical conductivity and viscosity. A recent neutron spin echo study [Kelley, E. G. et al., Struct. Dyn., 7 (2020) 054704] revealed that during the extended melting phase transition the DMPG membrane becomes softer and exhibits faster collective bending fluctuation compared to the higher temperature fluid phase. In contrast, in the present work, using incoherent quasielastic neutron scattering through the anomalous phase transition regime we find that single-particle lateral and internal lipid motions in the DMPG membrane show regular temperature dependence, with no enhanced dynamics evident in the anomalous melting regime. Further, we find that incorporation of NaCl in DMPG suppresses the anomalous extended melting regime, concurrently enhancing the single-particle lipid dynamics, both the lateral diffusivity and (to a lesser extent) the internal lipid motion. This seems rather counterintuitive and in variance with the dynamic suppression effect exerted by a salt on a zwitterionic membrane. However, since incorporation of a salt in anionic DMPG leads to enhanced cooperativity, the disrupted cooperativity in the salt-free DMPG is associated with the baseline lipid dynamics that is suppressed to begin with, whereas addition of salt partially restores the cooperativity, thus enhancing lipid dynamics compared to the salt-free baseline DMPG membrane state. These results provide new insights into the ion-membrane interaction and divulge a correlation between microscopic dynamics and the structure of the lipid bilayer.

Folding of Circularly Permuted and Split Outer Membrane Protein F via Electrostatic Interactions with Terminal Residues

Membrane proteins are essential targets in drug design, biosensing, and catalysis. In this study, we explored the folding of engineered outer membrane protein F (OmpF), an abundant and functional β-barrel protein expressed in Gram-negative bacteria. We carried out circular permutation, splitting and self-complementation, and point mutation. The folding efficiency and kinetic analyses demonstrated that the N- and C-terminal residues of OmpF played critical roles in folding via electrostatic interactions with lipid headgroups. Our results indicate that native porins without charged terminal residues may be tightly downregulated to retain the integrity of the outer membrane, and this modification may facilitate the insertion and folding of modified membrane proteins under in vitro and in vivo conditions for various applications.

Enhanced dynamics in the anomalous melting regime of DMPG lipid membranes

Like many soft materials, lipids undergo a melting transition associated with a significant increase in their dynamics. At temperatures below the main melting transition (T_m ), all molecular and collective dynamics are suppressed, while above T_m the alkyl tail motions, lipid diffusivity, and collective membrane undulations are at least an order of magnitude faster. Here we study the collective dynamics of dimyristoylphosphatidylglycerol (DMPG, di 14:0 PG) using neutron spin echo spectroscopy throughout its anomalous phase transition that occurs over a 12 °C-20° C wide temperature window. Our results reveal that the membranes are softer and more dynamic during the phase transition than at higher temperatures corresponding to the fluid phase and provide direct experimental evidence for the predicted increase in membrane fluctuations during lipid melting. These results provide new insights into the nanoscale lipid membrane dynamics during the melting transition and demonstrate how these dynamics are coupled to changes in the membrane structure.

Pore Model in the Melting Regime of a Lyotropic Biomembrane with an Anionic Phospholipid

Aqueous dispersions of the anionic phospholipid dimyristoyl phosphatidyl glycerol (DMPG) exhibit an unusual "melting regime", at the phase transition between the ordered (gel) and the disordered (fluid liquid crystal) state of hydrocarbon chains, depending on the ionic strength and DMPG concentration, previously attributed to the pore formation. Dispersions with 150 mM DMPG present a lamellar phase above 23 °C, within the melting regime. In this study, we present a detailed pore model for the analysis of small-angle X-ray scattering (SAXS) results and their variation with temperature, focused on the surface fractions of pores in the bilayers. Large and small toroidal pores are necessary to explain the SAXS results. Pores have DMPG in the fluid conformation, whereas the flat region of the bilayer has DMPG molecules in fluid and in gel conformations. A particular strategy was developed to estimate the charges due to the localization of mobile ions in the system, which is based on the calculation of electron densities by duly considering all molecular and ionic species that characterize the system, and the temperature dependency of their volumes. The best fit to the model of SAXS curves defines that the gel phase transforms initially, at 19.4 °C, in uncoupled bilayers with large pores (radius 93.2 ± 0.5 Å, with water channel diameter 137 ± 1 Å), which transform into small pores along the lamellar phase. The minimum intensity of the SAXS bilayer peak at 30 °C corresponds to a maximum number of small pores, and above 35 °C, the system enters into the normal lamellar fluid phase, without pores. The charge is estimated and shows that the regions with pores contains less Na⁺ ions per polar head; hence, when they are forming, there is a release of Na⁺ ions toward the bulk.

A convenient and versatile synthesis of Laurdan-like fluorescent membrane probes: characterization of their fluorescence properties

A simple and versatile synthetic scheme leading to Laurdan-derived fluorescent probes for biological membranes. Libraries of Laurdan derivatives will allow addressing the effect of the polar group on probes capacity to monitor lipids physical state.

Lattice solution model for order-disorder transitions in membranes and Langmuir monolayers

Lipid monolayers and bilayers have been used as experimental models for the investigation of membrane thermal transitions. The main transition takes place near ambient temperatures for several lipids and reflects the order-disorder transition of lipid hydrocarbonic chains, which is accompanied by a surface density gap. Equivalence between the transitions in the two systems has been argued by several authors. The two-state statistical model adopted by numerous authors for different properties of the membrane, such as permeability, diffusion, and mixture or insertion of cholesterol or protein, is inadequate for the description of charged membranes, since it lacks a proper description of surface density. We propose a lattice solution model which adds interactions with water molecules to lipid-lipid interactions and obtain its thermal properties under a mean-field approach. Density variations, although concomitant with chain order variations, are independent of the latter. The model presents both chain order and gas-liquid transitions, and extends the range of applicability of previous models, yielding Langmuir isotherms in the full range of pressures and areas.

Temperature-dependence of cationic lipid bilayer intermixing: possible role of interdigitation

In this work, we investigated the properties of a fusogenic cationic lipid, diC14-amidine, and show that this lipid possesses per se the capacity to adopt either an interdigitated structure (below and around its transition temperature) or a lamellar structure (above the transition temperature). To provide experimental evidence of this lipid bilayer organization, phospholipids spin-labeled at different positions of the hydrocarbon chain were incorporated into the membrane and their electron spin resonance (ESR) spectra were recorded at different temperatures. For comparison, similar experiments were performed with dimyristoyl phosphatidylcholine, a zwitterionic lipid (DMPC) which adopts a bilayer organization over a broad temperature range. Lipid mixing between diC14-amidine and asolectin liposomes was more efficient below (10-15 °C) than above the transition temperature (above 25 °C). This temperature-dependent "fusogenic" activity of diC14-amidine liposomes is opposite to what has been observed so far for peptides or virus-induced fusion. Altogether, our data suggest that interdigitation is a highly fusogenic state and that interdigitation-mediated fusion occurs via an unusual temperature-dependent mechanism that remains to be deciphered.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: theory versus experiment

Aqueous dispersions of phosphatidylglycerol (PG) lipids may present an anomalous chain-melting transition at low ionic strengths, as seen by different experimental techniques such as calorimetry or light scattering. The anomaly disappears at high ionic strengths or for longer acyl-chain lengths. In this article, we use a statistical model for the bilayer that distinguishes both lipid chain and headgroup states in order to compare model and experimental thermotropic and electrical properties. The effective van der Waals interactions among hydrophobic chains compete with the electrostatic repulsions between polar headgroups, which may be ionized (counterion dissociated) or electrically neutral (associated with counterions). Electric degrees of freedom introduce new thermotropic charge-ordered phases in which headgroup charges may be spatially ordered, depending on the electrolyte ionic strength, introducing a new rationale for experimental data on PGs. The thermal phases presented by the model for different chain lengths, at fixed ionic strength, compare well with an experimental phase diagram constructed on the basis of differential scanning calorimetry profiles. In the case of dispersions of DMPG (dimyristoyl phosphatidylglycerol) with added monovalent salt, the model properties reproduce the main features displayed by data from differential scanning calorimetry as well as the characteristic profile for the degree of ionization of the bilayer surface across the anomalous transition region, obtained from the theoretical interpretation of electrokinetic (conductivity and electrophoretic mobility) measurements.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: a statistical model

We propose a statistical model to account for the gel-fluid anomalous phase transitions in charged bilayer- or lamellae-forming ionic lipids. The model Hamiltonian comprises effective attractive interactions to describe neutral-lipid membranes as well as the effect of electrostatic repulsions of the discrete ionic charges on the lipid headgroups. The latter can be counterion dissociated (charged) or counterion associated (neutral), while the lipid acyl chains may be in gel (low-temperature or high-lateral-pressure) or fluid (high-temperature or low-lateral-pressure) states. The system is modeled as a lattice gas with two distinct particle types--each one associated, respectively, with the polar-headgroup and the acyl-chain states--which can be mapped onto an Ashkin-Teller model with the inclusion of cubic terms. The model displays a rich thermodynamic behavior in terms of the chemical potential of counterions (related to added salt concentration) and lateral pressure. In particular, we show the existence of semidissociated thermodynamic phases related to the onset of charge order in the system. This type of order stems from spatially ordered counterion association to the lipid headgroups, in which charged and neutral lipids alternate in a checkerboard-like order. Within the mean-field approximation, we predict that the acyl-chain order-disorder transition is discontinuous, with the first-order line ending at a critical point, as in the neutral case. Moreover, the charge order gives rise to continuous transitions, with the associated second-order lines joining the aforementioned first-order line at critical end points. We explore the thermodynamic behavior of some physical quantities, like the specific heat at constant lateral pressure and the degree of ionization, associated with the fraction of charged lipid headgroups.

Thermal phase behavior of DMPG bilayers in aqueous dispersions as revealed by 2H- and 31P-NMR

The synthetic lipid 1,2-dimyristoyl-sn-3-phosphoglycerol (DMPG), when dispersed in water/NaCl exhibits a complex phase behavior caused by its almost unlimited swelling in excess water. Using deuterium ((2)H)- and phosphorus ((31)P)-NMR we have studied the molecular properties of DMPG/water/NaCl dispersions as a function of lipid and NaCl concentration. We have measured the order profile of the hydrophobic part of the lipid bilayer with deuterated DMPG while the orientation of the phosphoglycerol headgroup was deduced from the (31)P NMR chemical shielding anisotropy. At temperatures >30 °C we observe well-resolved (2)H- and (31)P NMR spectra not much different from other liquid crystalline bilayers. From the order profiles it is possible to deduce the average length of the flexible fatty acyl chain. Unusual spectra are obtained in the temperature interval of 20-25 °C, indicating one or several phase transitions. The most dramatic changes are seen at low lipid concentration and low ionic strength. Under these conditions and at 25 °C, the phosphoglycerol headgroup rotates into the hydrocarbon layer and the hydrocarbon chains show larger flexing motions than at higher temperatures. The orientation of the phosphoglycerol headgroup depends on the bilayer surface charge and correlates with the degree of dissociation of DMPG-Na(+). The larger the negative surface charge, the more the headgroup rotates toward the nonpolar region.