Multicomponent cationic lipid-DNA complex formation: role of lipid mixing

Multicomponent cationic lipid-DNA complexes (lipoplexes) were prepared by adding linear DNA to mixed lipid dispersions containing two populations of binary cationic liposomes and characterized by means of small angle X-ray scattering (SAXS). Four kinds of cationic liposomes were used. The first binary lipid mixture was made of the cationic lipid (3'[N-(N',N'-dimethylaminoethane)-carbamoyl]cholesterol (DC-Chol) and the neutral helper lipid dioleoylphosphocholine (DOPC) (DC-Chol/DOPC liposomes), the second one of the cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and the neutral dioleoylphosphatidylethanolamine (DOPE) (DOTAP/DOPE liposomes), the third one of DC-Chol and DOPE (DC-Chol/DOPE liposomes), and the fourth one of DOTAP and DOPC (DOTAP/DOPC liposomes). Upon DNA-induced fusion of liposomes, large lipid mixing at the molecular level occurs. As a result, highly organized mixed lipoplexes spontaneously form with membrane properties intermediate between those of starting liposomes. By varying the composition of lipid dispersions, different DNA packing density regimes can also be achieved. Furthermore, occurring lipid mixing was found to induce hexagonal to lamellar phase transition in DOTAP/DOPE membranes. Molecular mechanisms underlying experimental findings are discussed.     

Enhanced transfection efficiency of multicomponent lipoplexes in the regime of optimal membrane charge density

Recently, membrane charge density of lipid membranes, sigma M, has been recognized as a universal parameter that controls the transfection efficiency of complexes made of binary cationic liposomes and DNA (binary lipoplexes). Three distinct regimes, most likely related to interactions between complexes and cells, have also been identified. The purpose of this work was to investigate the transfection efficiency behavior of multicomponent lipoplexes in the regime of optimal membrane charge density (1< sigma M < 2 x 10 (-2) e/A (2)) and compare their performance with that of binary lipoplexes usually employed for gene delivery purposes. We found remarkable differences in transfection efficiency due to lipid composition, with maximum in efficiency being obtained when multicomponent lipoplexes were used to transfect NIH 3T3 cells, while binary lipoplexes were definitely less efficient. These findings suggested that multicomponent systems are especially promising lipoplex candidates. With the aim of providing new insights into the mechanism of transfection, we investigated the structural evolution of lipoplexes when interacting with anionic (cellular) lipids by means of synchrotron small-angle X-ray diffraction (SAXD), while the extent of DNA release upon interaction with anionic lipids was measured by electrophoresis on agarose gels. Interestingly, a clear trend was found that the transfection activity increased with the number of lipid components. These results highlight the compositional properties of carrier lipid/cellular lipid mixtures as decisive factors for transfection and suggest a strategy for the rational design of superior cationic lipid carriers.     

Structural stability against disintegration by anionic lipids rationalizes the efficiency of cationic liposome/DNA complexes

Reported here is the correlation between the transfection efficiency of cationic liposome/DNA complexes (lipoplexes) and the structural evolution that they undergo when interacting with anionic membrane lipids. Multicomponent lipoplexes, incorporating from three to six lipid species simultaneously, presented a much higher transfection efficiency than binary lipoplexes, which are more commonly used for gene-delivery purposes. The discovery that a high transfection efficiency can be achieved by employing multicomponent complexes at a lower-than-ever-before membrane charge density of lipoplexes was of primary significance. Synchrotron small-angle X-ray diffraction (SAXD) experiments showed that anionic liposomes made of dioleoylphosphatidylglycerol (DOPG) disintegrated the lamellar phase of lipoplexes. DNA unbinding was measured by electrophoresis on agarose gels. Most importantly, structural changes induced by anionic lipids strictly depended on the lipid composition of lipoplexes. We found evidence of the existence of three different regimes of stability related to the interaction between complexes and anionic membranes. Both unstable (with low membrane charge density, sigmaM) and highly stable lipoplexes (with high sigmaM) exhibited low transfection efficiency whereas highly efficient multicomponent lipoplexes exhibited an "optimal stability". This intermediate regime reflects a compromise between two opposing constraints: protection of DNA in the cytosol and endosomal escape. Here we advance the concept that structural stability, upon interaction with cellular anionic lipids, is a key factor governing the transfection efficiency of lipoplexes. Possible molecular mechanisms underlying experimental observations are also discussed.     

A physicochemical characterization of the interaction between DC-Chol/DOPE cationic liposomes and DNA

A 1:1 mixture of the cationic lipid 3beta-[ N-( N', N'-dimethylaminoethane)-carbamoyl]cholesterol hydrochloride (DC-Chol) and the zwitterionic lipid, 1,2-dioleoyl- sn-glycero-3-phosphoetanolamine (DOPE), has been used to compact calf-thymus DNA (CT-DNA) in aqueous buffered solution at 298.15 K. The formation process of this lipoplex has been analyzed by means of electrophoretic mobility, cryo-TEM, dynamic light scattering, and fluorescence spectroscopy techniques. The experimental results indicate that DC-Chol/DOPE liposomes are mostly spherical and unilamellar, with a mean diameter of around 99 +/- 10 nm and a bilayer with a thickness of 4.5 +/- 0.5 nm. In the presence of CT-DNA, DC-Chol/DOPE/CT-DNA lipoplexes are formed by means of a strong entropically driven surface electrostatic interaction, as confirmed by zeta potential and fluorescence results, as a consequence of which DNA is compacted and condensed at the surface of the cationic liposomes. The negative charges of DNA phosphate groups are neutralized by the positive charges of cationic liposomes at the isoneutrality L/ D ratio, ( L/ D) varphi around 4, obtained from electrophoretic, fluorescence, and DLS measurements. The decrease in the fluorescence emission intensity of ethidium bromide, EtBr, initially intercalated between DNA base pairs, as long as the association between the biopolymer and the cationic liposomes takes place has permitted one to confirm its electrostatic character as well as to evaluate the different microenvironments of varying polarity of DNA-double helix, liposomes, and/or lipoplexes. Electronic microscopy reveals a rich scenario of possible nanostructures and morphologies for the lipoplexes, from unilamellar DNA-coated liposomes to multilamellar lipoplexes passing through cluster-like structures and several intermediate morphologies.     

Label-free quantitative analysis for studying the interactions between nanoparticles and plasma proteins

A shotgun proteomics approach was used to compare human plasma protein binding capability with cationic liposomes, DNA–cationic lipid complexes (lipoplexes), and lipid–polycation–DNA (LPD) complexes. Nano-high-performance liquid chromatography coupled with a high-resolution LTQ Orbitrap XL mass spectrometer was used to characterize and compare their protein corona. Spectral counting and area under curve methods were used to perform label-free quantification. Substantial qualitative and quantitative differences were found among proteins bound to the three different systems investigated. Protein variety found on lipoplexes and LPD complexes was richer than that found on cationic liposomes. There were also significant differences between the amounts of protein. Such results could help in the design of gene-delivery systems, because some proteins could be more selectively bound rather than others, and their bio-distribution could be driven in vivo for more efficient and effective gene therapy.     

One-dimensional thermotropic dilatation area of lipid headgroups within lamellar lipid/DNA complexes

Using simultaneous synchrotron small- and wide-angle X-ray diffraction (SWAXD), we investigated the thermotropic behavior of a cationic lipid mixture of DOTAP-DOPC (1,2-dioleoyl-3-trimethylammonium-propane-dioleoylphosphatidylcholine) liposomes complexed with calf thymus DNA. The DOTAP-DOPC/DNA complex reacts to temperature change by a bilayer compression normal to its surface and an expansion of the DNA in the plane of the rod lattice. By applying two independent recently developed models, we show here for the first time that the thermotropic dilatation area of lipid headgroups within the complexes is not isotropic but occurs parallel to the 1D DNA lattice (i.e., along the direction perpendicular to the DNA axis). Our results shed light on the role of spatial dimensionality in the DNA packing density within lamellar lipoplexes and provide experimental evidence that the interaction between DNA molecules confined between lipid bilayers can be regarded as a 1D problem.     

Interaction of lipoplexes with anionic lipids resulting in DNA release is a two-stage process

We propose a mechanism for DNA release from lipoplexes in cells that accounts for various observations of lipoplex-anionic lipid interactions. We examined the structural evolution of lipoplexes upon interaction with cellular lipids by synchrotron small-angle X-ray diffraction (SAXD), and the extent of DNA release from lipoplexes was determined by gel electrophoresis. We find that the interaction of lipoplexes with anionic cellular lipids is a two-stage process. In the first step, anionic lipids laterally diffuse into the complex and neutralize the cationic lipids. As a result, the membrane charge density of lipoplexes decreases and interactions between cationic lipids and DNA become weaker, but DNA is extremely poorly released. Only after the cationic charge of lipoplex membranes is completely neutralized by anionic lipids does DNA starts to be released significantly.     

The analysis of serum effects on structure, size and toxicity of DDAB-DOPE and DC-Chol-DOPE lipoplexes contributes to explain their different transfection efficiency

The effect of serum on structural properties of dimethyl-dioctadecyl-ammonium bromide (DDAB)–1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) liposomes and DDAB–DOPE/DNA lipoplexes has been investigated by energy dispersive X-ray diffraction (EDXD) technique, at different cationic lipid/DNA weight ratios (ρ). The role of serum on the size of lipoplexes has also been studied by dynamic light scattering. Lipoplex transfection efficiency (TE) as a function of ρ, and lipoplex toxicity to C6 rat glioma cells have been evaluated in Dulbecco's Modified Eagle Medium (DMEM) with and without serum. A multi-parametric analysis concerning the role of size, structure and cytotoxicity on transfection efficiency contributes to explain the experimental observation that 3β-[N-(N′,N′-dimethylaminoethane)carbamoyl]-cholesterol (DC-Chol)–DOPE/DNA transfect C6 cells better than DDAB–DOPE/DNA lipoplexes.     

The analysis of serum effects on structure,size and toxicity of DDAB–DOPE and DC-Chol–DOPE lipoplexes contributes to explain their different transfection efficiency

The effect of serum on structural properties of dimethyl-dioctadecyl-ammonium bromide (DDAB)–1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) liposomes and DDAB–DOPE/DNA lipoplexes has been investigated by energy dispersive X-ray diffraction (EDXD) technique, at different cationic lipid/DNA weight ratios (ρ). The role of serum on the size of lipoplexes has also been studied by dynamic light scattering. Lipoplex transfection efficiency (TE) as a function of ρ, and lipoplex toxicity to C6 rat glioma cells have been evaluated in Dulbecco's Modified Eagle Medium (DMEM) with and without serum. A multi-parametric analysis concerning the role of size, structure and cytotoxicity on transfection efficiency contributes to explain the experimental observation that 3β-[N-(N′,N′-dimethylaminoethane)carbamoyl]-cholesterol (DC-Chol)–DOPE/DNA transfect C6 cells better than DDAB–DOPE/DNA lipoplexes.     

Protonation of lipids impacts the supramolecular and biological properties of their self-assembly

We assessed in this work how a chemical structure difference could influence a supramolecular organization and then its biological properties. In our case study, we considered two amphiphilic lipidic gene vectors. The chemical difference was situated on their hydrophilic part which was either a pure neutral thiourea head or a mixture of three thiourea function derivatives, thiourea, iminothiol, and charged iminothiol. This small difference was obtained thanks to the last chemical deprotection conditions of the polar head hydroxyl groups. Light, neutron, and X-ray scattering techniques have been used to investigate the spatial structure of the liposomes and lipoplexes formed by the lipids. The chemical structure difference impacts the supramolecular assemblies of the lipids and with DNA as shown by fluorescence correlation spectroscopy (FCS), X-ray, and neutron scattering. Hence the structures formed were found to be highly different in terms of liposomes to DNA ratio and size and polydispersity of the aggregates. Finally, the transfection and internalization results proved that the differences in the structure of the lipid aggregates fully affect the biological properties of the lipopolythiourea compounds. The lipid containing three functions is a better gene transfection agent than the lipid which only contains one thiourea moiety. As a conclusion, we showed that the conditions of the last chemical step can influence the lipidic supramolecular structure which in turn strongly impacts their biological properties.     

Protein binding onto surfactant-based synthetic vesicles

Synthetic vesicles were prepared by mixing anionic and cationic surfactants, aqueous sodium dodecylsulfate with didodecyltrimethylammonium or cetyltrimethylammonium bromide. The overall surfactant content and the (anionic/cationic) mole ratios allow one to obtain negatively charged vesicles. In the phase diagram, the vesicular region is located between a solution phase, a lamellar liquid crystalline dispersion, and a precipitate area. Characterization of the vesicles was performed by electrophoretic mobility, NMR, TEM, and DLS and we determined their uni-lamellar character, size, stability, and charge density. Negatively charged vesicular dispersions, made of sodium dodecylsulfate/didodecyltrimethylammonium bromide or sodium dodecylsulfate/cetyltrimethylammonium bromide, were mixed with lysozyme, to form lipoplexes. Depending on the protein/vesicle charge ratio, binding, surface saturation, and lipoplexes flocculation, or precipitation, occurs. The free protein in excess remains in solution, after binding saturation. The systems were investigated by thermodynamic (surface tension and solution calorimetry), DLS, CD, TEM, 1H NMR, transport properties, electrophoretic mobility, and dielectric relaxation. The latter two methods give information on the vesicle charge neutralization by adsorbed protein. Binding is concomitant to modifications in the double layer thickness of vesicles and in the surface charge density of the resulting lipoplexes. This is also confirmed by developing the electrophoretic mobility results in terms of a Langmuir-like adsorption isotherm. Charges in excess with respect to the amount required to neutralize the vesicle surface promote lipoplexes clustering and/or flocculation. Protein-vesicle interactions were observed by DLS, indicating changes in particle size (and in their distribution functions) upon addition of LYSO. According to CD, the bound protein retains its native conformation, at least in the SDS/CTAB vesicular system. In fact, changes in the alpha-helix and beta-sheet conformations are moderate, if any. Calorimetric methods indicate that the maximum heat effect for LYSO binding occurs at charge neutralization. They also indicate that enthalpic are by far the dominant contributions to the system stability. Accordingly, energy effects associated with charge neutralization and double-layer contributions are much higher than counterion exchange and dehydration terms.     

Correlation of the physicochemical properties of symmetric 1,3-dialkoylamidopropane-based cationic lipids containing single primary and tertiary amine polar head groups with in vitro transfection activity

The physicochemical properties of a novel series of symmetric 1,3-dialkylamidopropane-based cationic amphiphiles [M. Sheikh, J. Feig, B. Gee, S. Li, M. Savva, In vitro lipofection with novel series of symmetric 1,3-dialkoylamidopropane-based cationic surfactants containing single primary and tertiary amine polar head groups, Chem. Phys. Lipids 124 (2003) 49-61] were studied by several techniques, in an effort to correlate cationic lipid structure with transfection efficacy. It was found that only the unsubstituted amine and tertiary amine dioleoyl derivatives 1,3lmp5 and 1,3lmt5, respectively, mediated in vitro transfection activity in the absence of helper lipids. This activity pattern was consistent with ethidium bromide fluorescence quenching studies, which indicated that only these two derivatives bound to and efficiently condense plasmid DNA at physiological pH. Dynamic light scattering indicated that lipoplexes made by these two cationic lipids were relatively small particles below 1 microm, in sharp contrast to lipoplexes bigger than 3 microm composed of saturated cationic derivatives. Transmission electron microscopy studies clearly indicated that cationic lipid dispersions made by saturated derivatives form multilamellar tubules at physiological pH. Calorimetric studies showed that cationic amphiphiles with saturated acyl chains longer than 12 carbons exhibit solid-to-liquid crystalline phase transitions above 37 degrees C. In agreement with the microscopy and calorimetry studies, Langmuir film balance experiments indicated that saturated derivatives with hydrophobic chains longer that 12 carbons are not well hydrated and exist at a chain-ordered state at ambient temperature. Calculation of compressibility moduli from monolayer compression isotherms at 23 degrees C suggested that monolayers made by cationic lipids bearing saturated acyl chains are less compressible relative to those of the dioleoyl derivatives 1,3lmp5 and 1,3lmt5. In conclusion, high hydration, increased fluidity and high elasticity of cationic lipid assemblies in isolation, all correlate with high in vitro transfection activity.     

Physicochemical properties affecting lipofection potency of a new series of 1,2-dialkoylamidopropane-based cationic lipids

The in vitro transfection activity of a novel series of N,N'-diacyl-1,2-diaminopropyl-3-carbamoyl-(aminoethane) derivatives was evaluated against a mouse melanoma cell line at different +/- charge ratios, in the presence and absence of helper lipids. Only the unsaturated derivative N,N'-dioleoyl-1,2-diaminopropyl-3-carbamoyl-(aminoethane), (1,2lmp[5]) mediated significant increase in the reporter gene level which was significantly boosted in the presence of DOPE peaking at +/- charge ratio of 2. The electrostatic interactions between the cationic liposomes and plasmid DNA were investigated by gel electrophoresis, fluorescence spectroscopy, dynamic light scattering and electrophoretic mobility techniques. In agreement with the transfection results, 1,2lmp[5]/DOPE formulation was most efficient in associating with and retarding DNA migration. The improved association between the dioleoyl derivative and DNA was further confirmed by ethidium bromide displacement assay and particle size distribution analysis of the lipoplexes. Differential scanning calorimetry studies showed that 1,2lmp[5] was the only lipid that exhibited a main phase transition below 37 degrees C. Likewise, 1,2lmp[5] was the only lipid found to form all liquid expanded monolayers at 23 degrees C. In conclusion, the current findings suggest that high in vitro transfection activity is mediated by cationic lipids characterized by increased acyl chain fluidity and high interfacial elasticity.     

Dramatic influence of the orientation of linker between hydrophilic and hydrophobic lipid moiety in liposomal gene delivery

A number of prior studies have demonstrated that the DNA-binding and gene transfection efficacies of cationic amphiphiles crucially depend on their various structural parameters including hydrophobic chain lengths, headgroup functionalities, and the nature of the linker-functionality used in tethering the polar headgroup and hydrophobic tails. However, to date addressing the issue of linker orientation remains unexplored in liposomal gene delivery. Toward probing the influence of linker orientation in cationic lipid mediated gene delivery, we have designed and synthesized two structurally isomeric remarkably similar cationic amphiphiles 1 and 2 bearing the same hydrophobic tails and the same polar headgroups connected by the same ester linker group. The only structural difference between the cationic amphiphiles 1 and 2 is the orientation of their linker ester functionality. While lipid 1 showed high gene transfer efficacies in multiple cultured animal cells, lipid 2 was essentially transfection incompetent. Findings in both transmission electron microscopic and dynamic laser light scattering studies revealed no significant size difference between the lipoplexes of lipids 1 and 2. Findings in confocal microscopic and fluorescence resonance energy transfer (FRET) experiments, taken together, support the notion that the remarkably higher gene transfer efficacies of lipid 1 compared to those of lipid 2 presumably originate from higher biomembrane fusogenicity of lipid 1 liposomes. Differential scanning calorimetry (DSC) and fluorescence anisotropy studies revealed a significantly higher gel-to-liquid crystalline temperature for the lipid 2 liposomes than that for lipid 1 liposomes. Findings in the dye entrapment experiment were also consistent with the higher rigidity of lipid 2/cholesterol (1:1 mole ratio) liposomes. Thus, the higher biomembrane fusibility of lipid 1 liposomes than that of lipid 2 liposomes presumably originates from the more rigid nature of lipid 2 cationic liposomes. Taken together, the present findings demonstrate for the first time that even as minor a structural variation as linker orientation reversal in cationic amphiphiles can profoundly influence DNA-binding characteristics, membrane rigidity, membrane fusibility, cellular uptake, and consequently gene delivery efficacies of cationic liposomes.     

Cationic lipo-thiophosphoramidates for gene delivery: synthesis, physico-chemical characterization and gene transfection activity--comparison with lipo-phosphoramidates

The synthesis of cationic lipo-thiophosphoramidates, a new family of cationic lipids designed for gene delivery, is reported herein. This new class of lipids is less polar than its oxygenated equivalent the lipo-phosphoramidates. Fluorescence anisotropy and FRET were used to determine the fluidity and fusogenicity of the lipo-phosphoramidates 3a-b and lipo-thiophosphoramidates 7a-b. The determination of both the size and the zeta potential of the nano-objects (liposomes and lipoplexes) and the determination of the DNA binding ability of the liposomes have completed the physico-chemical characterizations of the cationic lipids studied. Finally, the cationic lipids 3a-b and 7a-c have been evaluated as synthetic vectors for gene transfection into a variety of mammalian cell lines. The lipo-thiophosphoramidate 7a proved to be an efficient and low toxicity synthetic vector even when used at low lipid to DNA charge ratios.     

Polyadenylic acid binding on cationic liposomes doped with the non-ionic nucleolipid Lauroyl Uridine

In this work unilamellar liposomes doped with a novel non-ionic 5′-Uridine-head nucleolipid, Lauroyl Uridine (LU), were prepared and characterized for their ability to interact with the polynucleotide polyadenylic acid (poly-A). Vesicles, were made up of the cationic lipid DOTAP (1,2-Dioleoyl-3-Trimethylammonium-Propane), the zwitterionic lipid DOPE (1,2-Dioleoyl-sn-Glycero-3-Phosphoethanolamine), and the novel amphiphile Lauroyl Uridine. The influence of the non-ionic nucleolipid on essential liposomes properties, such as the structure and net charge was first investigated by a comparative analysis performed on the different lipoplex preparations by means of ζ-potential and size measurements. Both structure and net charge of liposomes were shown to be not modified by the presence of the non-ionic nucleolipid.The role of the synthetic lipid inserted as anchor in the liposome bilayer in the condensation process between vesicles and the polynucleotide poly-A was then analyzed by UV–vis, Circular Dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopies. The data presented comparative UV–vis analyses that evidenced the occurrence of staking interactions in the poly-A only in LU containing lipoplexes. CD and NMR studies indicated the presence of H-bonding interaction between Lauroyl Uridine containing vesicles and the polynucleotide poly-A. The results presented in this work support a role for Lauroyl Uridine in A-U molecular recognition, thus, suggesting that cationic liposomes doped with the non-ionic nucleolipid Lauroyl Uridine could represent a model system to study molecular interactions among single stranded polynucleotides and lipid anchor bearing the complementary bases.     

Cationic liposomes in mixed didodecyldimethylammonium bromide and dioctadecyldimethylammonium bromide aqueous dispersions studied by differential scanning calorimetry, Nile Red fluorescence, and turbidity

The thermotropic phase behavior of cationic liposomes in mixtures of two of the most investigated liposome-forming double-chain lipids, dioctadecyldimethylammonium bromide (DODAB) and didodecyldimethylammonium bromide (DDAB), was investigated by differential scanning calorimetry (DSC), turbidity, and Nile Red fluorescence. The dispersions were investigated at 1.0 mM total surfactant concentration and varying DODAB and DDAB concentrations. The gel to liquid-crystalline phase transition temperatures (Tm) of neat DDAB and DODAB in aqueous dispersions are around 16 and 43 degrees C, respectively, and we aim to investigate the Tm behavior for mixtures of these cationic lipids. Overall, DDAB reduces the Tm of DODAB, the transition temperature depending on the DDAB content, but the Tm of DDAB is roughly independent of the DODAB concentration. Both DSC and fluorescence measurements show that, within the mixture, at room temperature (ca. 22 degrees C), the DDAB-rich liposomes are in the liquid-crystalline state, whereas the DODAB-rich liposomes are in the gel state. DSC results point to a higher affinity of DDAB for DODAB liposomes than the reverse, resulting in two populations of mixed DDAB/DODAB liposomes with distinctive phase behavior. Fluorescence measurements also show that the presence of a small amount of DODAB in DDAB-rich liposomes causes a pronounced effect in Nile Red emission, due to the increase in liposome size, as inferred from turbidity results.     

Effects of Headgroups and Serum on Gene Transfection of Alkaline Amino Acid Based Cationic Lipids

Three cationic lipids with lysylated(l), histidylated(2), and arginylated(3) headgroups and cholesterol hy-drophobic moiety were synthesized. The average sizes of liposomes and lipoplexes were around 100 and 160 nm, re-spectively. The gene transfection efficiency of the three lipoplexes loaded with pGL3 or pORF-LacZ was compared on 293T cells in the presence or the absence of serum. The transfection efficiency of the three lipoplexes in a se-rum-free medium was 2 to 3-fold higher than that of dioleoyl-trimethylammonium propane(DOTAP). In the presence of serum, however, most of the lipoplexes showed lower transfection activities; only lipoplex 3 retained its high transfection efficiency.     

DNA–DNA electrostatic interactions within cationic lipid/DNA lamellar complexes

We present a simple theoretical analysis of the DNA–DNA electrostatic interactions within charge-neutral lamellar cationic lipid/DNA complexes (lipoplexes). Although always repulsive as a function of the DNA–DNA interaxial distance, the calculated electrostatic force shows a deep minimum for each value of lipid composition corresponding to an equilibrium distance of the system. The excellent agreement between the equilibrium distances predicted by the model and that experimentally observed in charge-neutral complexes as revealed by synchrotron X-ray diffraction, shows that the spatial dimensionality of both the lipids and the DNA may not be a crucial point to capture the essence of the DNA–DNA interactions within charge-neutral lipoplexes.     

Composition dependence of bilayer elasticity

A previously developed molecular level model for homogeneous lipid bilayers [Brannigan and Brown, J. Chem. Phys 120, 1059 (2004)] is extended to allow for multiple lipid species. Monte Carlo simulations (including species exchange moves for efficient sampling) reveal a variety of mixing behaviors in binary systems. Two species are identified that maintain stable, randomly mixed fluid membranes at vanishing tension over all possible binary compositions. The thermal and elastic properties of membranes formed by these lipids are characterized over the full composition range. Equilibrium area at constant tension is nonmonotonic with respect to composition, but consistent with that of a quadratic mixture. In the constant tension ensemble, the bending rigidity of the bilayer is minimized at an intermediate composition. The observed functional form of bending rigidity vs composition is fit to a simple expression motivated by linear elasticity theory; this expression accounts for membrane heterogeneity through a single parameter.