Temperature and ionic strength dependent light scattering of DMPG dispersions

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.

Lipid headgroup and side chain architecture determine manganese-induced dose dependent membrane rigidification and liposome size increase

Metal ion-membrane interactions have gained appreciable attention over the years resulting in increasing investigations into the mode of action of toxic and essential metals. More work has focused on essential ions like Ca or Mg and toxic metals like Cd and Pb, whereas this study investigates the effects of the abundant essential trace metal manganese with model lipid systems by screening zwitterionic and anionic glycerophospholipids. Despite its essentiality, deleterious impact towards cell survival is known under Mn stress. The fluorescent dyes Laurdan and diphenylhexatriene were used to assess changes in membrane fluidity both in the head group and hydrophobic core region of the membrane, respectively. Mn-rigidified membranes composed of the anionic phospholipids, phosphatidic acid, phosphatidylglycerol, cardiolipin, and phosphatidylserine. Strong binding resulted in large shifts of the phase transition temperature. The increase was in the order phosphatidylserine > phosphatidylglycerol > cardiolipin, and in all cases, saturated analogues > mono-unsaturated forms. Dynamic light scattering measurements revealed that Mn caused extensive aggregation of liposomes composed of saturated analogues of phosphatidic acid and phosphatidylserine, whilst the mono-unsaturated analogue had significant membrane swelling. Increased membrane rigidity may interfere with permeability of ions and small molecules, possibly disrupting cellular homeostasis. Moreover, liposome size changes could indicate fusion, which could also be detrimental to cellular transport. Overall, this study provided further understanding into the effects of Mn with biomembranes, whereby the altered membrane properties are consequential to the proper structural and signalling functions of membrane lipids.

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.

Solvent-assisted lipid bilayer formation on silicon dioxide and gold

Planar lipid bilayers on solid supports mimic the fundamental structure of biological membranes and can be investigated using a wide range of surface-sensitive techniques. Despite these advantages, planar bilayer fabrication is challenging, and there are no simple universal methods to form such bilayers on diverse material substrates. One of the novel methods recently proposed and proven to form a planar bilayer on silicon dioxide involves lipid deposition in organic solvent and solvent exchange to influence the phase of adsorbed lipids. To scrutinize the specifics of this solvent-assisted lipid bilayer (SALB) formation method and clarify the limits of its applicability, we have developed a simplified, continuous solvent-exchange version to form planar bilayers on silicon dioxide, gold, and alkanethiol-coated gold (in the latter case, a lipid monolayer is formed to yield a hybrid bilayer) and varied the type of organic solvent and rate of solvent exchange. By tracking the SALB formation process with simultaneous quartz crystal microbalance-dissipation (QCM-D) and ellipsometry, it was determined that the acoustic, optical, and hydration masses along with the acoustic and optical thicknesses, measured at the end of the process, are comparable to those observed by employing conventional fabrication methods (e.g., vesicle fusion). As shown by QCM-D measurements, the obtained planar bilayers are highly resistant to protein adsorption, and several, but not all, water-miscible organic solvents could be successfully used in the SALB procedure, with isopropanol yielding particularly high-quality bilayers. In addition, fluorescence recovery after photobleaching (FRAP) measurements demonstrated that the coefficient of lateral lipid diffusion in the fabricated bilayers corresponds to that measured earlier in the planar bilayers formed by vesicle fusion. With increasing rate of solvent exchange, it was also observed that the bilayer became incomplete and a phenomenological model was developed in order to explain this feature. The results obtained allowed us to clarify and discriminate likely steps of the SALB formation process as well as determine the corresponding influence of organic solvent type and flow conditions on these steps. Taken together, the findings demonstrate that the SALB formation method can be adapted to a continuous solvent-exchange procedure that is technically minimal, quick, and efficient to form planar bilayers on solid supports.

Interactions of a bacterial trehalose lipid with phosphatidylglycerol membranes at low ionic strength

Trehalose lipids are bacterial biosurfactants which present interesting physicochemical and biological properties. These glycolipids have a number of different commercial applications and there is an increasing interest in their use as therapeutic agents. The amphiphilic nature of trehalose lipids points to the membrane as their hypothetical site of action and therefore the study of the interaction between these biosurfactants and biological membranes is critical. In this study, we examine the interactions between a trehalose lipid (TL) from Rhodococcus sp. and dimyristoylphosphatidylglycerol (DMPG) membranes at low ionic strength, by means of differential scanning calorimetry, light scattering, fluorescence polarization and infrared spectroscopy. We describe that there are extensive interactions between TL and DMPG involving the perturbation of the thermotropic intermediate phase of the phospholipid, the destabilization and shifting of the DMPG gel to liquid crystalline phase transition to lower temperatures, the perturbation of the sample transparency, and the modification of the order of the phospholipid palisade in the gel phase. We also report an increase of fluidity of the phosphatidylglycerol acyl chains and dehydration of the interfacial region of the bilayer. These changes would increase the monolayer negative spontaneous curvature of the phospholipid explaining the destabilizing effect on the intermediate state exerted by this biosurfactant. The observations contribute to get insight into the biological mechanism of action of the biosurfactant and help to understand the properties of the intermediate phase display by DMPG at low ionic strength.

Sedimentation velocity analytical ultracentrifugation in hydrogenated and deuterated solvents for the characterization of membrane proteins

This chapter is a step-by-step protocol for setting up, realizing, and analyzing sedimentation velocity experiments in hydrogenated and deuterated solvents, in the context of the characterization of membrane protein, in terms of homogeneity, association state, and amount of bound detergent, based on a real case study of the membrane protein BmrA solubilized in n-Dodecyl-β-D-Maltopyranoside) detergent.

Structure and orientation of bovine lactoferrampin in the mimetic bacterial membrane as revealed by solid-state NMR and molecular dynamics simulation

Bovine lactoferrampin (LFampinB) is a newly discovered antimicrobial peptide found in the N1-domain of bovine lactoferrin (268-284), and consists of 17 amino-acid residues. It is important to determine the orientation and structure of LFampinB in bacterial membranes to reveal the antimicrobial mechanism. We therefore performed (13)C and (31)P NMR, (13)C-(31)P rotational echo double resonance (REDOR), potassium ion-selective electrode, and quartz-crystal microbalance measurements for LFampinB with mimetic bacterial membrane and molecular-dynamics simulation in acidic membrane. (31)P NMR results indicated that LFampinB caused a defect in mimetic bacterial membranes. Ion-selective electrode measurements showed that ion leakage occurred for the mimetic bacterial membrane containing cardiolipin. Quartz-crystal microbalance measurements revealed that LFampinB had greater affinity to acidic phospholipids than that to neutral phospholipids. (13)C DD-MAS and static NMR spectra showed that LFampinB formed an α-helix in the N-terminus region and tilted 45° to the bilayer normal. REDOR dephasing patterns between carbonyl carbon nucleus in LFampinB and phosphorus nuclei in lipid phosphate groups were measured by (13)C-(31)P REDOR and the results revealed that LFampinB is located in the interfacial region of the membrane. Molecular-dynamics simulation showed the tilt angle to be 42° and the rotation angle to be 92.5° for Leu(3), which are in excellent agreement with the experimental values.

Active detergent-solubilized H+,K+-ATPase is a monomer

The H(+),K(+)-ATPase pumps protons or hydronium ions and is responsible for the acidification of the gastric fluid. It is made up of an α-catalytic and a β-glycosylated subunit. The relation between cation translocation and the organization of the protein in the membrane are not well understood. We describe here how pure and functionally active pig gastric H(+),K(+)-ATPase with an apparent Stokes radius of 6.3 nm can be obtained after solubilization with the non-ionic detergent C(12)E(8), followed by exchange of C(12)E(8) with Tween 20 on a Superose 6 column. Mass spectroscopy indicates that the β-subunit bears an excess mass of 9 kDa attributable to glycosylation. From chemical analysis, there are 0.25 g of phospholipids and around 0.024 g of cholesterol bound per g of protein. Analytical ultracentrifugation shows one main complex, sedimenting at s(20,)(w) = 7.2 ± 0.1 S, together with minor amounts of irreversibly aggregated material. From these data, a buoyant molecular mass is calculated, corresponding to an H(+),K(+)-ATPase α,β-protomer of 147.3 kDa. Complementary sedimentation velocity with deuterated water gives a picture of an α,β-protomer with 0.9-1.4 g/g of bound detergent and lipids and a reasonable frictional ratio of 1.5, corresponding to a Stokes radius of 7.1 nm. An α(2),β(2) dimer is rejected by the data. Light scattering coupled to gel filtration confirms the monomeric state of solubilized H(+),K(+)-ATPase. Thus, α,β H(+),K(+)-ATPase is active at least in detergent and may plausibly function as a monomer, as has been established for other P-type ATPases, Ca(2+)-ATPase and Na(+),K(+)-ATPase.

Response of an environment-sensitive intramolecular charge transfer probe towards solubilization of liposome membranes by a non-ionic detergent: association and dissociation kinetics

The present report describes an endeavor to follow the solubilization of DMPC and DMPG liposome membranes by a non-ionic detergent Triton X-100 on the lexicon of environment-sensitive intramolecular charge transfer (ICT) photophysics of an extrinsic molecular probe 5-(4-dimethylamino-phenyl)-penta-2, 4-dienoic acid methyl ester (DPDAME). The prospective applicability of the probe to function as a reporter for detergent-sequestered solubilization of liposome membranes is argued on the basis of comparison of the spectral properties of the probe in various environments. Fluorescence anisotropy study delineates the degree of motional restriction imposed on the probe in different microheterogeneous assemblies. The kinetics of association of the probe with the liposome membranes and the dissociation kinetics of TX-100-sequestered solubilization process of the liposomes have been monitored by the stopped-flow fluorescence technique and the results are rationalized in relevance to fluorescence anisotropy study.

Molecular structures of fluid phase phosphatidylglycerol bilayers as determined by small angle neutron and X-ray scattering

We have determined the molecular structures of commonly used phosphatidylglycerols (PGs) in the commonly accepted biologically relevant fluid phase. This was done by simultaneously analyzing small angle neutron and X-ray scattering data, with the constraint of measured lipid volumes. We report the temperature dependence of bilayer parameters obtained using the one-dimensional scattering density profile model - which was derived from molecular dynamics simulations - including the area per lipid, the overall bilayer thickness, as well as other intrabilayer parameters (e.g., hydrocarbon thickness). Lipid areas are found to be larger than their phosphatidylcholine (PC) counterparts, a result likely due to repulsive electrostatic interactions taking place between the charged PG headgroups even in the presence of sodium counterions. In general, PG and PC bilayers show a similar response to changes in temperature and chain length, but differ in their response to chain unsaturation. For example, compared to PC bilayers, the inclusion of a first double bond in PG lipids results in a smaller incremental change to the area per lipid and bilayer thickness. However, the extrapolated lipid area of saturated PG lipids to infinite chain length is found to be similar to that of PCs, an indication of the glycerol-carbonyl backbone's pivotal role in influencing the lipid-water interface.

Superstructure based on β-CD self-assembly induced by a small guest molecule

The size, shape and surface chemistry of nanoparticles play an important role in cellular interaction. Thus, the main objective of the present study was the determination of the β-cyclodextrin (β-CD) self-assembly thermodynamic parameters and its structure, aiming to use these assemblies as a possible controlled drug release system. Light scattering measurements led us to obtain the β-CD's critical aggregation concentration (cac) values, and consequently the thermodynamic parameters of the β-CD spontaneous self-assembly in aqueous solution: Δ(agg)G(o) = -16.31 kJ mol(-1), Δ(agg)H(o) = -26.48 kJ mol(-1) and TΔ(agg)S(o) = -10.53 kJ mol(-1) at 298.15 K. Size distribution of the self-assembled nanoparticles below and above cac was 1.5 nm and 60-120 nm, respectively. The number of β-CD molecules per cluster and the second virial coefficient were identified through Debye's plot and molecular dynamic simulations proposed the three-fold assembly for this system below cac. Ampicillin (AMP) was used as a drug model in order to investigate the key role of the guest molecule in the self-assembly process and the β-CD:AMP supramolecular system was studied in solution, aiming to determine the structure of the supramolecular aggregate. Results obtained in solution indicated that the β-CD's cac was not affected by adding AMP. Moreover, different complex stoichiometries were identified by nuclear magnetic resonance and isothermal titration calorimetry experiments.

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.

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

Dispersions of saturated anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG) have been extensively studied regarding their peculiar thermostructural behavior. At low ionic strength, the gel-fluid transition is spread along nearly 17 degrees C, displaying several thermal events in the calorimetric profile that is quite different from the single sharp peak around 23 degrees C found for higher ionic strength DMPG dispersions. To investigate the role of charge in the bilayer transition, we carefully examine the temperature dependence of the electrical conductivity of DMPG dispersions at different concentrations, correlating the data with the corresponding differential scanning calorimetry (DSC) traces. Electrical conductivity together with electrophoretic mobility measurements allowed the calculation of the dependence of the degree of ionization of DMPG vesicles on lipid concentration and temperature. It was shown that there is a decrease in vesicle charge as the lipid concentration increases, which is probably correlated with the increase in the concentration of bulk Na(+). Apart from the known increase in the electrical conductivity along the DMPG temperature transition region, a sharp rise was observed at the bilayer pretransition for all lipid concentrations studied, possibly indicating that the beginning of the chain melting process is associated with an increase in bilayer ionization. It is confirmed here that the gel-fluid transition of DMPG at low ionic strength is accompanied by a huge increase in the dispersion viscosity. However, it is shown that this measured macroviscosity is distinct from the local viscosity felt by either charged ions or DMPG charged aggregates in measurements of electrical conductivity or electrophoretic mobility. Data presented here give support to the idea that DMPG vesicles, at low ionic strength, get more ionized along the temperature transition region and could be perforated and/or deformed vesicle structures.

Laurdan spectrum decomposition as a tool for the analysis of surface bilayer structure and polarity: a study with DMPG, peptides and cholesterol

The highly hydrophobic fluorophore Laurdan (6-dodecanoyl-2-(dimethylaminonaphthalene)) has been widely used as a fluorescent probe to monitor lipid membranes. Actually, it monitors the structure and polarity of the bilayer surface, where its fluorescent moiety is supposed to reside. The present paper discusses the high sensitivity of Laurdan fluorescence through the decomposition of its emission spectrum into two Gaussian bands, which correspond to emissions from two different excited states, one more solvent relaxed than the other. It will be shown that the analysis of the area fraction of each band is more sensitive to bilayer structural changes than the largely used parameter called Generalized Polarization, possibly because the latter does not completely separate the fluorescence emission from the two different excited states of Laurdan. Moreover, it will be shown that this decomposition should be done with the spectrum as a function of energy, and not wavelength. Due to the presence of the two emission bands in Laurdan spectrum, fluorescence anisotropy should be measured around 480 nm, to be able to monitor the fluorescence emission from one excited state only, the solvent relaxed state. Laurdan will be used to monitor the complex structure of the anionic phospholipid DMPG (dimyristoyl phosphatidylglycerol) at different ionic strengths, and the alterations caused on gel and fluid membranes due to the interaction of cationic peptides and cholesterol. Analyzing both the emission spectrum decomposition and anisotropy it was possible to distinguish between effects on the packing and on the hydration of the lipid membrane surface. It could be clearly detected that a more potent analog of the melanotropic hormone alpha-MSH (Ac-Ser(1)-Tyr(2)-Ser(3)-Met(4)-Glu(5)-His(6)-Phe(7)-Arg(8)-Trp(9)-Gly(10)-Lys(11)-Pro(12)-Val(13)-NH(2)) was more effective in rigidifying the bilayer surface of fluid membranes than the hormone, though the hormone significantly decreases the bilayer surface hydration.

Melting regime of the anionic phospholipid DMPG: new lamellar phase and porous bilayer model

Aqueous dispersions of the anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG) at pH above the apparent pK of DMPG and concentrations in the interval 70-300 mM have been investigated by small (SAXS) and wide-angle X-ray scattering, differential scanning calorimetry, and polarized optical microscopy. The order-disorder transition of the hydrocarbon chains occurs along an interval of about 10 degrees C (between T(m)(on) approximately 20 degrees C and T(m)(off) approximately 30 degrees C). Such melting regime was previously characterized at lower concentrations, up to 70 mM DMPG, when sample transparency was correlated with the presence of pores across the bilayer. At higher concentrations considered here, the melting regime persists but is not transparent. Defined SAXS peaks appear and a new lamellar phase L(p) with pores is proposed to exist above 70 mM DMPG, starting at approximately 23 degrees C (approximately 3 degrees C above T(m)(on)) and losing correlation after T(m)(off). A new model for describing the X-ray scattering of bilayers with pores, presented here, is able to explain the broad band attributed to in-plane correlation between pores. The majority of cell membranes have a net negative charge, and the opening of pores across the membrane tuned by ionic strength, temperature, and lipid composition is likely to have biological relevance.

The intermediate state of DMPG is stabilized by enhanced positive spontaneous curvature

1,2-Dimyristoyl-sn-glycero-3-phospho-rac-glycerol (DMPG) at low salt concentrations has a complex endotherm with at least four components and extending over the span of 20 degrees. During this ongoing melting, the solution becomes viscous and scatters light poorly. This multipeak endotherm was suggested to result from the effects of curvature on the relative free energies of gel and fluid DMPG bilayers, further relating to the formation of an intermediate sponge phase between the lamellar gel and fluid phases. Although later studies appear to exclude a connected bilayer network, the relation of the endotherm peaks to curvature remains an appealing hypothesis. This was tested by including in the system both water-soluble small molecules (dimethyl sulfoxide, ethanol, and urea) as well as amphiphiles (myristoyl-lyso-PG, cholesterol, cholesterol-3-sulfate, and dimyristoylglycerol) known to alter the spontaneous curvature of bilayers. All compounds increasing the monolayer positive spontaneous curvature (ethanol, urea, myristoyl-lyso-PG, cholesterol-3-sulfate) increased the temperature span of the intermediate state and elevated the temperature of its dissolution, while all compounds increasing the negative spontaneous curvature (dimethyl sulfoxide, cholesterol, dimyristoylglycerol) had the opposite effect, implying that the intermediate state contains a structure with positive curvature. The results support the view that the intermediate state consists of vesicles with a large number of holes. The viscosity increase could be related to vesicle expansion needed to accommodate the numerous holes.

Extensive bilayer perforation coupled with the phase transition region of an anionic phospholipid

At low ionic strength dimyristoylphosphatidylglycerol (DMPG) exhibits a broad phase transition region characterized by several superimposed calorimetric peaks. Peculiar properties, such as sample transparency, are observed only in the transition region. In this work we use differential scanning calorimetry (DSC), turbidity, and optical microscopy to study the narrowing of the transition region with the increase of ionic strength (0-500 mM NaCl). Upon addition of salt, the temperature extension of the transition region is reduced, and the number of calorimetric peaks decreases until a single cooperative event at T(m) = 23 degrees C is observed in the presence of 500 mM NaCl. The transition region is always coupled with a decrease in turbidity, but a transparent region is detected within the melting process only in the presence of up to 20 mM NaCl. The vanishing of the transparent region is associated with one of the calorimetric peaks. Optical microscopy of giant vesicles shows that bilayers first rupture when the transition region is reached and subsequently lose optical contrast. Fluorescence microscopy reveals a blurry and undefined image in the transparent region, suggesting a different lipid self-assembly. Overall sample turbidity can be directly related to the bilayer optical contrast. Our observations are discussed in terms of the bilayer being perforated along the transition region. In the narrower temperature interval of the transparent region, dependent on the ionic strength, the perforation is extensive and the bilayer completely loses the optical contrast.

On the interaction of the anthraquinone barbaloin with negatively charged DMPG bilayers

Barbaloin is a bioactive glycosilated 1,8-dihydroxyanthraquinone present in several exudates from plants, such as Aloe vera, which are used for cosmetic or food purposes. It has been shown that barbaloin interacts with DMPG (dimyristoylphosphatidylglycerol) model membranes, altering the bilayer structure (Alves, D. S.; Pérez-Fons, L.; Estepa, A.; Micol, V. Biochem. Pharm. 2004, 68, 549). Considering that ESR (electron spin resonance) of spin labels is one of the best techniques to monitor structural properties at the molecular level, the alterations caused by the anthraquinone barbaloin on phospholipid bilayers will be discussed here via the ESR signal of phospholipid spin probes intercalated into the membranes. In DMPG at high ionic strength (10 mM Hepes pH 7.4 + 100 mM NaCl), a system that presents a gel-fluid transition around 23 degrees C, 20 mol % barbaloin turns the gel phase more rigid, does not alter much the fluid phase packing, but makes the lipid thermal transition less sharp. However, in a low-salt DMPG dispersion (10 mM Hepes pH 7.4 + 2 mM NaCl), which presents a rather complex gel-fluid thermal transition (Lamy-Freund, M. T.; Riske, K. A. Chem. Phys. Lipids 2003, 122, 19), barbaloin strongly affects bilayer structural properties, both in the gel and fluid phases, extending the transition region to much higher temperature values. The position of barbaloin in DMPG bilayers will be discussed on the basis of ESR results, in parallel with data from sample viscosity, DSC (differential scanning calorimetry), and SAXS (small-angle X-ray scattering).

Influence of salt on the structure of DMPG studied by SAXS and optical microscopy

Aqueous dispersions of 50 mM dimyristoylphosphatidylglycerol (DMPG) in the presence of increasing salt concentrations (2-500 mM NaCl) were studied by small angle X-ray scattering (SAXS) and optical microscopy between 15 and 35 degrees C. SAXS data show the presence of a broad peak around q approximately 0.12 A(-1) at all temperatures and conditions, arising from the electron density contrasts within the bilayer. Up to 100 mM NaCl, this broad peak is the main feature observed in the gel and fluid phases. At higher ionic strength (250-500 mM NaCl), an incipient lamellar repeat distance around d=90-100 A is detected superimposed to the bilayer form factor. The data with high salt were fit and showed that the emergent Bragg peak is due to loose multilamellar structures, with the local order vanishing after approximately 4d. Optical microscopy revealed that up to 20 mM NaCl, DMPG is arranged in submicroscopic vesicles. Giant (loose) multilamellar vesicles (MLVs) start to appear with 50 mM NaCl, although most lipids are arranged in small vesicles. As the ionic strength increases, more and denser MLVs are seen, up to 500 mM NaCl, when MLVs are the prevailing structure. The DLVO theory could account for the experimentally found interbilayer distances.

Role of membranes on the antibacterial and anti-inflammatory activities of the bioactive compounds from Hypoxis rooperi corm extract

Hypoxis rooperi corm extract ('African potato') is known for its traditional and ethnomedical uses in the treatment of a large variety of diseases. Its main bioactive compound hypoxoside (HYP) and its aglycone derivative rooperol (RO) were isolated and the interaction of these compounds with several types of model membranes was studied in order to contribute to the understanding of their molecular mechanism. The results show that RO abolishes the main transition phase and perturb the van der Waals interactions between phospholipid acyl chains in a stronger way than HYP in dimiristoylphosphatidylcholine (DMPC), dielaidoylphosphatidylethanolamine (DEPE) and dimiristoylphosphatidylglycerol membranes (DMPG), probably indicating that this molecule inserts into the bilayer. This effect decreases as the acyl chain length of the phospholipid increases. RO also promoted the formation of hexagonal H(II) phases at lower temperatures compared to pure DEPE. On the contrary, HYP showed a shallow interaction with phospholipids. This compound promoted the formation of gel-fluid like intermediate structures with isotropic motion in phosphatidylglycerol membranes at physiological pH, and affected the phospholipid/water interface probably through the variation of the surface charge of the phospholipid phosphate groups. Moreover, RO inhibited Staphylococcus aureus in a stronger manner than Escherichia coli and promoted a higher leakage level in E. coli, PG and PE-containing synthetic membranes. Furthermore, RO showed a significant degree of inhibition of cyclooxygenase-2 (COX-2) and cyclooxygenase-1 (COX-1) evidencing an approximate COX-2/COX-1 IC50 ratio of 1.9, therefore this compound may be responsible for the anti-inflammatory activity of H. rooperi corm extract. These results may contribute to understand the molecular mechanism of the antibacterial and/or anti-inflammatory properties of the bioactive compounds deriving from the African potato corm extract.

On the propensity of phosphatidylglycerols to form interdigitated phases

We have determined the phase behavior of disaturated phosphatidylglycerols (PGs) of chain lengths n(CH2) = 14-18 at high pH and ionic strength using calorimetry, dilatometry, as well as x-ray diffraction. PGs with n(CH2) = 14 and 16 show thermotropic behavior similar to that of phosphatidylcholines (PCs). The area/lipid obtained in the gel phase is smaller than that reported for PCs despite the expected larger effective headgroup size. This can be explained by the tilting of the PG headgroup out of the bilayer plane, and we provide experimental evidence for a headgroup tilt transition. For distearoyl PG, we further find that the "usual" gel phase coexists with an interdigitated phase, which exhibits a transition from an orthorhombic into a hexagonal chain packing. The total amount of the interdigitated phase depends significantly on the temperature but is found to be largely independent of temperature equilibration time and different sample preparation protocols. Thus, the development of the interdigitated phase appears to be kinetically trapped. The formation of interdigitated phases in PGs at much smaller chain lengths than in PCs is of high relevance to interaction studies with antimicrobial peptides, as it provides a mechanism for the discrimination of membranes composed of different lipid species.

Thermal phase behavior of DMPG: the exclusion of continuous network and dense aggregates

1,2-Dimyristoyl-sn-glycero-3-phospho-rac-glycerol has been suggested to form at intermediate temperatures and at high concentrations in low-salt solutions as a continuous sponge phase (Heimburg, T.; Biltonen, R. L. Biochemistry 1994, 33, 9477-9488). In the present study, the changes in signals seen for a range of fluorescent probes during phase transformations of this phospholipid indicate continuous melting and a change in lipid packing, in accordance with previous reports. However, in accordance with Lamy-Freund and Riske (Lamy-Freund, M. T.; Riske, K. A. Chem. Phys. Lipids 2003, 122, 19-32), no enhancement of lipid mixing within the putative sponge phase region was seen, suggesting a lack of a connected lipid surface. Accordingly, a typical sponge phase cannot account for the properties of the intermediate phase. The low scattering intensities of the latter have also been taken as evidence for disaggregation. While dynamic light scattering and data for membranes containing poly(ethylene glycol)-ylated lipids could lend credence to disaggregation, the most likely explanation for the scattering data would appear to be a shape transition without significant changes in neither vesicle aggregation nor bilayer connectivity. An abrupt change in light scattering and signals from some of the fluorescent probes used reveals a new transition at Tt approximately 43 degrees C, with the formation of a more ordered interface.

Temperature-dependent optical properties of individual vascular wall components measured by optical coherence tomography

Optical properties of tissues and tissue components are important parameters in biomedical optics. We report measurements of tissue refractive index n and the attenuation coefficient mu(t) using optical coherence tomography (OCT) of individual vascular wall layers and plaque components. Moreover, since the temperature dependence of optical properties is widely known, we compare measurements at room and body temperatures. A decrease of n and mu(t) is observed in all samples, with the most profound effect on samples with high lipid content. The sample temperature is of influence on the quantitative measurements within OCT images. For extrapolation of ex-vivo experimental results, especially for structures with high lipid content, this effect should be taken into account.

Determination of phase transition temperatures of lipids by light scattering

Various techniques have been proposed to specify the phase transition temperatures of surfactant molecules. The work reported herein deals with a new general method of T(c) determination based on the optical properties' modifications of aqueous surfactant solutions when the phase transitions occur in the phospholipid membrane. The shape alteration of supramolecular systems induced by the phase transition was correlated with the refraction and absorption coefficients of their aqueous dispersion. The mean count rate (average number of photons detected per second) measured with a Zetasizer Nano-S model ZEN1600 Dynamic Light Scattering Instrument, is representative of an emerging macroscopic phenomenon, but not directly size dependent and has been adapted to our expectations. Changes in the measured scattering intensity reflect changes in the optical properties of the material during temperature variations. Thus, this method allowed to specify the phase transition temperature of many natural or synthetic surfactants independently of their polar head or hydrophobic part.

Aqueous suspensions of charged spherical colloids: dependence of the surface charge on ionic strength, acidity, and colloid concentration

We theoretically investigate the dependence of the surface charge developed on charged spherical colloids upon several environmental parameters: the ionic strength of the monovalent added electrolyte, acidity (stabilized by a pH buffer solution), and colloid concentration. In the framework of the mean-field Poisson-Boltzmann spherical cell model, we include the charged colloid-microion correlations into the buffer equation, and we allow for the specific binding of ions to the ionizable groups on the colloid surface. Theoretical predictions are compared to the results obtained under the planar-symmetry Gouy-Chapman approximation and analyzed for the experimental conditions of an aqueous dispersion of the phospholipid dimyristoyl phosphatidylglycerol (DMPG). Experimental measurements of the partition ratio of an aqueous soluble cationic spin label on buffered dispersions of polyanionic unilamellar vesicles of DMPG in the presence of added monovalent salt are theoretically interpreted in terms of ion partition due to electrostatic interactions. We show that the specific binding of the probe must be admitted to explain the experimental results.

Differential effects of oleuropein, a biophenol from Olea europaea, on anionic and zwiterionic phospholipid model membranes

Oleuropein (Ole) is the major phenolic constituent of the olive leaf (Olea europaea) and it is also present in olive oil and fruit. In the last years several compounds from olive tree, oleuropein among them, have shown a variety of biological activities such as antimicrobial or antioxidant. A phospholipid model membrane system was used to study whether the Ole biological effects could be membrane related. Ole showed a significant partition level in phospholipid membranes, i.e. 80%, at lipid-saturating conditions. Moreover, fluorescence quenching experiments indicated a shallow location for Ole in membranes. Ole promoted weak effects on zwiterionic phospholipids such as phosphatidylcholine or phosphatidylethanolamine. In contrast, differential scanning microcalorimetry, light scattering and fluorescence anisotropy pH titration studies revealed strong effects on anionic phospholipids such as phosphatidylglycerol at physiological pH and salt conditions. These effects consisted on perturbations at the phospholipid membrane surface, which might involve specific molecular interactions between Ole and the negatively charged phosphate group and therefore modify the phospholipid/water interface properties. It is proposed that Ole induces lipid structures similar to the gel-fluid intermediate phase (IP) described for PG membranes, in a similar way than low ionic strength does. These effects on phosphatidylglycerol may account for the antimicrobial activity of Ole.

Mesoscopic structure in the chain-melting regime of anionic phospholipid vesicles: DMPG

In a range of low ionic strength, aqueous dispersions of the anionic phospholipid DMPG (dimyristoylphosphatidylglycerol) have a transparent intermediate phase (IP, between T(m)(on) congruent with 20 degrees C and T(m)(off) congruent with 30 degrees C) between the turbid gel and fluid membrane phases, evidenced in turbidity data. Small angle x-ray scattering results on DMPG dispersions show that, besides the bilayer peak present in all phases, a peak corresponding to a mesoscopic structure at approximately 400 A is detected only in IP. The dependence of this peak position on DMPG concentration suggests a correlation in the bilayer plane, consistent with the stability of vesicles in IP. Moreover, observation of giant DMPG vesicles with phase contrast light microscopy show that vesicles "disappear" upon cooling below T(m)(off) and "reappear" after reheating. This further proves that although vesicles cannot be visualized in IP, their overall structure is maintained. We propose that the IP in the melting regime corresponds to unilamellar vesicles with perforations, a model which is consistent with all described experimental observations. Furthermore, the opening of pores across the membrane tuned by ionic strength, temperature, and lipid composition is likely to have biological relevance and could be used in applications for controlled release from nanocompartments.

Membrane-related effects underlying the biological activity of the anthraquinones emodin and barbaloin

Commercial plant extracts containing anthraquinones are being increasingly used for cosmetics, food and pharmaceuticals due to their wide therapeutic and pharmacological properties. In this work, the interaction with model membranes of two representative 1,8-dihydroxyanthraquinones, barbaloin (Aloe) and emodin (Rheum, Polygonum), has been studied in order to explain their effects in biological membranes. Emodin showed a higher affinity for phospholipid membranes than barbaloin did, and was more effective in weakening hydrophobic interactions between hydrocarbon chains in phospholipid bilayers. Whereas emodin induced the formation of hexagonal-H(II) phase, barbaloin stabilized lamellar structures. Barbaloin promoted the formation of gel-fluid intermediate structures in phosphatidylglycerol membranes at physiological pH and ionic strength values. It is proposed that emodin's chromophore group is located at the upper half of the membrane, whereas barbaloin's one is in a deeper position but having its glucopyranosyl moiety near the phospholipid/water interface. Moreover, membrane disruption by emodin or barbaloin showed specificity for the two major phospholipids present in bacterial membranes, phosphatidylethanolamine and phosphatidylglycerol. In order to relate their strong effects on membranes to their biological activity, the capacity of these compounds to inhibit the infectivity of the viral haemorrhagic septicaemia rhabdovirus (VHSV), a negative RNA enveloped virus, or the growth of Escherichia coli was tested. Anthraquinone-loaded liposomes showed a strong antimicrobial activity whereas these compounds in their free form did not. Both anthraquinones showed antiviral activity but only emodin was a virucidal agent. In conclusion, a molecular mechanism based on the effect of these compounds on the structure of biological membranes is proposed to account for their multiple biological activities. Anthraquinone-loaded liposomes may suppose an alternative for antimicrobial, pharmaceutical or cosmetic applications.

DMPG gel-fluid thermal transition monitored by a phospholipid spin labeled at the acyl chain end

Low ionic strength aqueous dispersion of dimyristoyl phosphatidylglycerol (DMPG) presents a rather peculiar gel-fluid thermal transition behavior. The lipid main phase transition occurs over a large temperature interval (ca. 17 degrees C), along which several calorimetric peaks are observed. Using lipids spin labeled at the acyl chain end, a two-peak electron spin resonance (ESR) spectrum is observed along that temperature transition region (named intermediate phase), at three different microwave frequencies: L-, X- and Q-bands. The intermediate phase ESR spectra are analyzed, and shown to be most likely due to spin labels probing two distinct types of lipid organization in the DMPG bilayer. Based on the ESR spectra parameters, a model for the DMPG intermediate phase is proposed, where rather fluid and hydrated domains, possibly high curvature regions, coexist with patches that are more rigid and hydrophobic.

The peculiar thermo-structural behavior of the anionic lipid DMPG

Aqueous dispersions of the anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG), around 100 mM ionic strength, are known to exhibit a thermal behavior similar to that of the largely studied lipid dimyristoyl phosphatidylcholine (DMPC), which undergoes a gel to liquid crystalline phase transition at 23 degrees C, well characterized by differential scanning calorimetry (DSC), and other methods. However, at low ionic strength, DMPG has been shown to present a large gel-fluid transition region, ranging from 18 to 35 degrees C. This intermediate phase is optically transparent and characterized by a continuous change in membrane packing. Structural properties of the DMPG gel-fluid transition region will be discussed, based on results obtained by several techniques: electron spin resonance (ESR) of spin labels at the membrane surface and intercalated at different depths in the bilayer; light scattering; DSC; small angle X-ray scattering (SAXS); and fluorescence spectroscopy of probes in the bilayer.

Structure of polydispersed colloids characterised by light scattering and electron microscopy

The dynamics of growth and aggregation of colloidal silver iodide particles was followed by the static light scattering method. The particles were treated as spheres and they were stable in size in the defined time interval. This approach enabled the use of the Zimm plots in order to determine the radii of gyration and the radii of spherical particles. Stable AgI colloids, either positively or negatively charged, showed the usual Zimm diagrams, while the diagrams were untypical when the stability of the colloids decreased. The untypical Zimm diagrams showed 'curves' with envelopes and 'curves' with minima in the unstable domain and in the domain where the most rapid nucleation occurs, respectively. Satisfactory agreement of particle sizes within the limits of accuracy, determined using static--and dynamic light scattering data and of the values obtained from the electron microscopic images was shown. Fitting the theoretical and experimental data, P(theta) functions showed that the particle shapes approach the theoretical model for spheres and thin discs. The colloid stability of polydispersed aggregates was also explained using the second virial coefficient, its negative sign implying interaction of particles in the solution, its positive value indicating formation of new particles from the supernatant solution. In addition, the colloid stability can be characterised by the mass fractal dimension. For positive stable colloids, Dm = 2.70 +/- 0.26, it can be related to the reaction controlled processes, whereas for negative stable colloids, Dm = 1.97 +/- 0.19, it was attributed to the diffusion controlled processes.

Thermal transitions of DMPG bilayers in aqueous solution: SAXS structural studies

Dimyristoylphosphatidylglycerol (DMPG) has been extensively studied as a model for biological membranes, since phosphatidylglycerol is the most abundant anionic phospholipid in prokaryotic cells. At low ionic strengths, this lipid presents a peculiar thermal behavior, with two sharp changes in the light scattering profile, at temperatures named here T(on)(m) and T(off)(m). Structural changes involved in the DMPG thermal transitions are here investigated by small angle X-ray scattering (SAXS), and compared to the results yielded by differential scanning calorimetry (DSC) and electron spin resonance (ESR). The SAXS results show a broad peak, indicating that DMPG is organized in single bilayers, for the range of temperature studied (10-45 degrees C). SAXS intensity shows an unusual effect, starting to decrease at T(on)(m), and presenting a sharp increase at T(off)(m). The bilayer electron density profiles, obtained from modeling the SAXS curves, show a gradual decrease in electron density contrast (attributed to separation between charged head groups) and in bilayer thickness between T(on)(m) and T(off)(m). Results yielded by SAXS, DSC and ESR indicate that a chain melting process starts at T(on)(m), but a complete fluid phase exists only for temperatures above T(off)(m), with structural changes occurring at the bilayer level in the intermediate region.

Correlation between the effects of a cationic peptide on the hydration and fluidity of anionic lipid bilayers: a comparative study with sodium ions and cholesterol

The cationic tridecapeptide alpha-melanocyte stimulating hormone (alpha-MSH) is known to interact with anionic vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG), partially penetrating the lipid membrane. In the lipid liquid crystal phase, phospholipid derivatives spin labeled at the different C-atoms along the acyl chain, show that the peptide increases the bilayer packing at all depths. Parallel to that, there is an increase in the probe's isotropic hyperfine splittings, indicating that the peptide significantly decreases the membrane hydrophobic barrier. Accordingly, it is suggested that the increase in membrane packing yielded by alpha-MSH is partly due to a greater level of interchain hydration. This result is compared to the increase in packing and decrease in polarity yielded by cholesterol, and the absence of structural or polar alterations with Na+. The latter result shows that the peptide effect is not related to an increase of positive charges at the anionic vesicle surface. Alterations on the lipid bilayer polar profile measured by the nitroxide hyperfine splitting z component in frozen samples are shown to be different from those obtained at room temperature. However, it is shown here that a certain correlation can be drawn between the increase in polarity measured in frozen samples and the packing effect caused by the different molecules in the lipid gel phase.

How bradykinin alters the lipid membrane structure: a spin label comparative study with bradykinin fragments and other cations

Electron spin resonance spectroscopy of several different spin labels was used to comparatively study the interaction of the cationic peptide hormone bradykinin (BK; Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg), and some BK fragments (des-Arg(9)-BK, des-Arg(1)-BK, and Arg-Pro-Pro-Gly-Phe or BK(1-5)), with anionic vesicles of dimyristoyl phosphatidylglycerol (DMPG). For temperatures above the lipid gel-liquid crystal thermal transition (T(m) approximately 20 degrees C), membrane-incorporated spin labels indicated that all peptides (total concentration of 10 mol % relative to lipid) interact with the bilayer, turning the membrane less fluid, both at its surface and center, suggesting a partial penetration of the peptides into the membrane core. However, in the lipid gel phase (t < T(m)), BK was found to display a much stronger interaction with the membrane, decreasing the bilayer fluidity. At temperatures around 15 degrees C the BK-DMPG system was found to present a hysteresis, evinced by the different electron spin resonance spectra yielded upon cooling and heating the sample. System reversibility was found at all other temperatures (0-45 degrees C). That effect could not be assigned to the BK higher concentration at the membrane surface, due to its higher net charge (2(+)) compared to the fragments (1(+)), because ten times more des-Arg(9)-BK (100 mol %) yielded opposite result. Further, that was found to be a result rather different from those elicited by the other cations tested: the monovalent Na(+), the divalent Zn(2+), and the peptide pentalysine. The data presented here are discussed in the light of the different BK and BK fragments biological activities.

Network formation of lipid membranes: triggering structural transitions by chain melting

Phospholipids when dispersed in excess water generally form vesicular membrane structures. Cryo-transmission and freeze-fracture electron microscopy are combined here with calorimetry and viscometry to demonstrate the reversible conversion of phosphatidylglycerol aqueous vesicle suspensions to a three-dimensional structure that consists of extended bilayer networks. Thermodynamic analysis indicates that the structural transitions arise from two effects: (i) the enhanced membrane elasticity accompanying the lipid state fluctuations on chain melting and (ii) solvent-associated interactions (including electrostatics) that favor a change in membrane curvature. The material properties of the hydrogels and their reversible formation offer the possibility of future applications, for example in drug delivery, the design of structural switches, or for understanding vesicle fusion or fission processes.

Role of soft and hard aggregates in the thermodynamics of lipid dispersions

We study the thermodynamics of a two-dimensional polydisperse ideal gas model of different species of aggregates. We show that if these aggregates are distinguished not only by their sizes but also by their ability to display shape fluctuations, the system presents dominance of one or other species, depending on the temperature region. This result, which emerges solely from the statistics of the model in total absence of interaggregate interactions, describes well the observed temperature dependence of light scattering in dispersions of dimyristoyl phosphatidylglycerol, a negatively charged lipid.