Surface complexation of DNA with insoluble monolayers. Influence of divalent counterions

Electrostatic interactions control the adsorption of extracellular vesicles onto supported lipid bilayers

Communication between cells located in different parts of an organism is often mediated by membrane-enveloped nanoparticles, such as extracellular vesicles (EVs). EV binding and cell uptake mechanisms depend on the heterogeneous composition of the EV membrane. From a colloidal perspective, the EV membrane interacts with other biological interfaces via both specific and non-specific interactions, where the latter include long-ranged electrostatic and van der Waals forces, and short-ranged repulsive "steric-hydration" forces. While electrostatic forces are generally exploited in most EV immobilization protocols, the roles played by various colloidal forces in controlling EV adsorption on surfaces have not yet been thoroughly addressed. In the present work, we study the adsorption of EVs onto supported lipid bilayers (SLBs) carrying different surface charge densities using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and confocal laser scanning microscopy (CLSM). We demonstrate that EV adsorption onto lipid membranes can be controlled by varying the strength of electrostatic forces and we theoretically describe the observed phenomena within the framework of nonlinear Poisson-Boltzmann theory. Our modelling results confirm the experimental observations and highlight the crucial role played by attractive electrostatics in EV adsorption onto lipid membranes. They furthermore show that simplified theories developed for model lipid systems can be successfully applied to the study of their biological analogues and provide new fundamental insights into EV-membrane interactions with potential use in developing novel EV separation and immobilization strategies.

Ionic Liquid-Induced Assembly of DNA at Air-Water Interface

DNA nanotechnology is the future of many products in the pharmaceutical and cosmetic industries. Self-assembly of this negatively charged biopolymer at surfaces and interfaces is an essential step to elaborate its field of applications. In this study, the ionic liquid (IL) monolayer-assisted self-assembly of DNA macromolecules at the air-water interface has been closely monitored by employing various quantitative techniques, namely, surface pressure-area (π-A) isotherms, surface potential, interfacial rheology, and X-ray reflectivity (XRR). The π-A isotherms reveal that the IL 1,3-didecyl 3-methyl imidazolium chloride induces DNA self-assembly at the interface, leading to a thick viscoelastic film. The interfacial rheology exhibits a notable rise in the viscoelastic modulus as the surface pressure increases. The values of storage and loss moduli measured as a function of strain frequency suggest a relaxation frequency that depends on the length of the macromolecule. The XRR measurements indicate a considerable increase in DNA layer thickness at the elevated surface pressures depending on the number of base pairs of the DNA. The results are considered in terms of the electrostatic and hydrophobic interactions, allowing a quantitative conclusion about the arrangement of DNA strands underneath the monolayer of the ILs at the air-water interface.

Cation and buffer specific effects on the DNA-lipid interaction

Knowledge of DNA - lipid layer interactions is key for the development of biosensors, synthetic nanopores, scaffolds, and gene-delivery systems. These interactions are strongly affected by the ionic composition of the solvent. We have combined quartz crystal microbalance (QCM) and ellipsometry measurements to reveal how pH, buffers and alkali metal chloride salts affect the interaction of DNA with lipid bilayers (DOTAP/DOPC 30:70 in moles). We found that the thickness of the DNA layer adsorbed onto the lipid bilayer decreased in the order citrate > phosphate > Tris > HEPES. The effect of cations on the thickness of the DNA layer decreased in the order (K⁺ > Na⁺ > Cs⁺ ∼ Li⁺). Rationalization of the experimental results requires that adsorption, due to cation specific charge screening, is driven by the simultaneous action of two mechanisms namely, the law of matching water affinities for kosmotropes (Li⁺) and ion dispersion forces for chaotropes (Cs⁺). The outcome of these two opposing mechanisms is a ^"bell-shaped" specific cations sequence. Moreover, a superimposed buffer specificity, which goes beyond the simple effect of pH regulation, further modulated cation specificity. In summary, DNA-lipid bilayer interactions are maximized if citrate buffer (50 mM, pH 7.4) and KCl (100 mM) are used.

Lipid membranes modulate the activity of RNA through sequence-dependent interactions

RNA is a ubiquitous biomolecule that can serve as both catalyst and information carrier. Understanding how RNA bioactivity is controlled is crucial for elucidating its physiological roles and potential applications in synthetic biology. Here, we show that lipid membranes can act as RNA organization platforms, introducing a mechanism for riboregulation. The activity of R3C ribozyme can be modified by the presence of lipid membranes, with direct RNA-lipid interactions dependent on RNA nucleotide content, base pairing, and length. In particular, the presence of guanine in short RNAs is crucial for RNA-lipid interactions, and G-quadruplex formation further promotes lipid binding. Lastly, by artificially modifying the R3C substrate sequence to enhance membrane binding, we generated a lipid-sensitive ribozyme reaction with riboswitch-like behavior. These findings introduce RNA-lipid interactions as a tool for developing synthetic riboswitches and RNA-based lipid biosensors and bear significant implications for RNA world scenarios for the origin of life.

Hydration and Charge-Transfer Effects of Alkaline Earth Metal Ions Binding to a Carboxylate Anion, Phosphate Anion, and Guanine Nucleobase

To investigate the ability of alkaline earth metal ions to tune ion-mediated DNA adsorption, hydrated Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ ions bound to a carboxylate anion, phosphate anion, and guanine nucleobase were modeled using density functional theory (DFT) and a combined explicit and continuum solvent model. The large first solvation shell of Ba²⁺ requires a larger solute cavity defined by a solvent-accessible surface, which is used to model all hydrated ions. Alkaline earth metal ions bind indirectly or directly to each binding site. DFT binding energies decrease with increasing ion size, which is likely due to ion size and hydration structure, rather than quantum effects such as charge transfer. However, charge transfer explains weaker ion binding to guanine compared to phosphate or carboxylate. Overall, carboxylate and phosphate anions are expected to compete equally for hydrated Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ ions and larger alkaline earth metal ions may induce weaker ion-mediated adsorption. The ion size and hydration structure of alkaline earth metal ions may effectively tune ion-mediated adsorption processes, such as DNA adsorption to functionalized surfaces.

DNA Interaction with a Polyelectrolyte Monolayer at Solution-Air Interface

The formation of ordered 2D nanostructures of double stranded DNA molecules at various interfaces attracts more and more focus in medical and engineering research, but the underlying intermolecular interactions still require elucidation. Recently, it has been revealed that mixtures of DNA with a series of hydrophobic cationic polyelectrolytes including poly(N,N-diallyl-N-hexyl-N-methylammonium) chloride (PDAHMAC) form a network of ribbonlike or threadlike aggregates at the solution—air interface. In the present work, we adopt a novel approach to confine the same polyelectrolyte at the solution—air interface by spreading it on a subphase with elevated ionic strength. A suite of techniques–rheology, microscopy, ellipsometry, and spectroscopy–are applied to gain insight into main steps of the adsorption layer formation, which results in non-monotonic kinetic dependencies of various surface properties. A long induction period of the kinetic dependencies after DNA is exposed to the surface film results only if the initial surface pressure corresponds to a quasiplateau region of the compression isotherm of a PDAHMAC monolayer. Despite the different aggregation mechanisms, the micromorphology of the mixed PDAHMAC/DNA does not depend noticeably on the initial surface pressure. The results provide new perspective on nanostructure formation involving nucleic acids building blocks.

Morphologies and Structure of Brain Lipid Membrane Dispersions

This study aims to explore the variety of previously unknown morphologies that brain lipids form in aqueous solutions. We study how these structures are dependent on cholesterol content, salt solution composition, and temperature. For this purpose, dispersions of porcine sphingomyelin with varying amounts of cholesterol as well as dispersions of porcine brain lipid extracts were investigated. We used cryo-TEM to investigate the dispersions at high-salt solution content together with small-angle (SAXD) and wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) for dispersions in the corresponding salt solution at high lipid content. Sphingomyelin forms multilamellar vesicles in large excess of aqueous salt solution. These vesicles appear as double rippled bilayers in the images and as split Bragg peaks in SAXD together with a very distinct lamellar phase pattern. These features disappear with increasing temperature, and addition of cholesterol as the WAXD data shows that the peak corresponding to the chain crystallinity disappears. The dispersions of sphingomyelin at high cholesterol content form large vesicular type of structures with smooth bilayers. The repeat distance of the lamellar phase depends on temperature, salt solution composition, and slightly with cholesterol content. The brain lipid extracts form large multilamellar vesicles often attached to assemblies of higher electron density. We think that this is probably an example of supra self-assembly with a multiple-layered vesicle surrounding an interior cubic microphase. This is challenging to resolve. DSC shows the presence of different kinds of water bound to the lipid aggregates as a function of the lipid content. Comparison with the effect of lithium, sodium, and calcium salts on the structural parameters of the sphingomyelin and the morphologies of brain lipid extract morphologies demonstrate that lithium has remarkable effects also at low content.

Kelvin probe force microscopy to study electrostatic interactions of DNA with lipid-gemini surfactant monolayers for gene delivery

In novel gene therapy mechanisms utilising gemini surfactants, electrostatic interactions of the surfactant molecules with the DNA strands is a primary mechanism by which the two components of the delivery vehicle bind. In this work, we show for the first time direct evidence of electrostatic interactions of these compounds visualised with Kelvin probe force microscopy (KPFM) and correlated to their topography from atomic force microscopy (AFM). We construct monolayers of lipids and gemini surfactant to simulate interactions on a cellular level, using lipids commonly found in cell membranes, and allow DNA to bind to the monolayer as it is formed on a Langmuir-Blodgett trough. The difference in topography and electrical surface potential between monolayers with and without DNA is striking. In fact, KPFM reveals a strongly positive relative electrical surface potential in between where we identify a background lipid and the DNA strands, evidenced by the height profiles of the domains. Such identification is not possible without KPFM. We conclude that it is likely we are seeing cationic surfactant molecules surrounding DNA strands within a sea of background lipid.

Interaction of DNA with likely-charged lipid monolayers: An experimental study

Anionic lipids are increasingly being used in lipoplexes for synthetic gene vectors as an alternative to cationic lipids. This is primarily due to their lower toxicity, which makes them biocompatible and adaptable to be tissue specific. However, anionic lipoplexes require the presence of multivalent cations to promote the electrostatic attraction between DNA and anionic lipid mono- and bilayers. In this work we provide for the first time experimental results of the adsorption of linear DNA onto anionic/zwitterionic lipid monolayers without any addition of cations. This is demonstrated experimentally by means of Langmuir monolayers of DOPE/DOPG (1:1) lipids spread on a water subphase that contains calf thymus DNA. The adsorption of DNA onto anionic/zwitterionic lipid monolayers is discussed in terms of the surface pressure-molecular area isotherms recorded in the absence and in the presence of different electrolytes. Measurements of the surface potential provide additional evidence of the different interaction of DNA anionic/zwitterionic lipid monolayers depending on the presence and nature of electrolyte. These experimental results are further analysed in terms of the overall dipole moment normal to the monolayers providing new insight into the behaviour of anionic lipoplexes and the role of zwitterionic lipids.

A comparative study on intrinsic fluorescence of BSA and lysozyme proteins in presence of different divalent ions from their solution and thin film conformations

Optical emission behaviours of lysozyme and bovine serum albumin, from bulk and thin film geometry, were studied in the presence of three different divalent ions (Mg²⁺ , Ca²⁺ or Ba²⁺ ) using different spectroscopic [steady-state fluorescence, UV-Vis and Fourier transform infra-red (FTIR)] techniques. Additionally, protein thin films on silicon surfaces were prepared and morphological studies were carried out using atomic force microscopy. Dynamic quenching was mainly identified for both proteins in the presence of Mg²⁺ , Ca²⁺ and Ba²⁺ ions. The molecular conformation of the proteins was modified in thin films compared with that in solution, consequently quenching efficiencies also varied. ATR-FTIR studies confirmed the conformational changes of proteins in the presence of all divalent ions. All metal ions used were divalent in nature and belonged to the same group of the periodic table but, depending on their individual characteristics such as electron affinity, ionic radius, etc., the magnitude of the protein and hydrated ion interaction varied and accordingly the quenching efficiency was modified. Quenching was maximum for Ca²⁺ ions, followed by the other two ions. Our study clearly illustrates the geometry-dependent physical and biological functions of proteins.

Applications of Brewster angle microscopy from biological materials to biological systems

Brewster angle microscopy (BAM) is a powerful technique that allows for real-time visualization of Langmuir monolayers. The lateral organization of these films can be investigated, including phase separation and the formation of domains, which may be of different sizes and shapes depending on the properties of the monolayer. Different molecules or small changes within a molecule such as the molecule's length or presence of a double bond can alter the monolayer's lateral organization that is usually undetected using surface pressure-area isotherms. The effect of such changes can be clearly observed using BAM in real-time, under full hydration, which is an experimental advantage in many cases. While previous BAM reviews focused more on selected compounds or compared the impact of structural variations on the lateral domain formation, this review provided a broader overview of BAM application using biological materials and systems including the visualization of amphiphilic molecules, proteins, drugs, extracts, DNA, and nanoparticles at the air-water interface.

Atomic force microscopy as a tool to study the adsorption of DNA onto lipid interfaces

The Atomic Force Microscopy (AFM) technique appears as a central tool for the characterization of DNA adsorption onto lipid interfaces. Regardless of the huge number of surveys devoted to this issue, there are still fascinating phenomena in this field that have not been explored in detail by AFM. For instance, adsorption of DNA onto like-charged lipid surfaces mediated by cations is still not fully understood even though it is gaining popularity nowadays in gene therapy and nanotechnology. Studies related to the complexation of DNA with anionic lipids as a non-viral gene delivery vehicle as well as the formation of self-assembled nanoscale DNA constructs (DNA origami) are two of the most attractive systems. Unfortunately, molecular mechanisms underlying the adsorption of DNA onto anionic lipid interfaces remain unclear so far. In view of that, AFM becomes an appropriate technique to provide valuable information to understand the adsorption of DNA to anionic lipid surfaces. As a second part of this review we provide an illustrative example of application of the AFM technique to probe the DNA adsorption onto a model lipid monolayer negatively charged. Microsc. Res. Tech. 80:11-17, 2017. © 2016 Wiley Periodicals, Inc.

Interactions between DNA and gemini surfactant: impact on gene therapy: part II

Nonviral gene delivery, provides distinct treatment modalities for the inherited and acquired diseases, relies upon the encapsulation of a gene of interest, which is then ideally delivered to the target cells. Variations in the chemical structure of gemini surfactants and subsequent physicochemical characteristics of the gemini-based lipoplexes and their impact on efficient gene transfection were assessed in part I, which was published in first March 2016 issue of Nanomedicine (1103). In order to design an efficient vector using gemini surfactants, the interaction of the surfactant with DNA and other components of the delivery system must be characterized, and more critically, well understood. Such studies will help to understand how nonviral transfection complexes, in general, overcome various cellular barriers. The Langmuir-Blodgett monolayer studies, atomic force microscopy, differential scanning calorimetry, isothermal titration calorimetry, small-angle x-ray scattering, are extensively used to evaluate the interaction behavior of gemini surfactants with DNA and other vector components. Part II of this review focuses on the use of these unique techniques to understand their interaction with DNA.

FRET study of G-quadruplex forming fluorescent oligonucleotide probes at the lipid monolayer interface

Spectral properties and G-quadruplex folding ability of fluorescent oligonucleotide probes at the cationic dioctadecyldimethylammonium bromide (DODAB) monolayer interface are reported. Two oligonucleotides, a 19-mer bearing thrombin binding aptamer sequence and a 21-mer with human telomeric sequence, were end-labeled with fluorescent groups (FAM and TAMRA) to give FRET probes F19T and F21T, respectively. The probes exhibited abilities to fold into a quadruplex structure and to bind metal cations (Na(+) and K(+)). Fluorescence spectra of G-quadruplex FRET probes at the monolayer interface are reported for the first time. Investigations included film balance measurements (π-A isotherms) and fluorescence spectra recording using a fiber optic accessory interfaced with a spectrofluorimeter. The effect of the presence of DODAB monolayer, metal cations and the surface pressure of monolayer on spectral behavior of FRET probes were examined. Adsorption of probe at the cationic monolayer interface resulted in the FRET signal enhancement even in the absence of metal cations. Variation in the monolayer surface pressure exerted rather modest effect on the spectral properties of probes. The fluorescence energy transfer efficiency of monolayer adsorbed probes increased significantly in the presence of sodium or potassium ion in subphase, which indicated that the probes retained their cation binding properties when adsorbed at the monolayer interface.

Changes in Interfacial Tension of a Lipid Membrane Formed at the Water/Chloroform Interface upon DNA Complex Formation

Changes in the interfacial tension of a lipid monolayer membrane formed at the water/chloroform interface upon DNA addition were measured using the quasi-elastic laser scattering (QELS) method. A cationic lipid, N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium chloride (DOTAP), as well as zwitterionic lipids, 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), were used to form lipid monolayer membranes at different calcium ion concentrations. A rapid decrease of the interfacial tension resulting from electrostatic interactions between DOTAP and DNA was observed within 10 s. However, such rapid decreases were not observed for DOPE or DOPC. A decrease in the interfacial tension was exhibited by DOPE after 1000 s from the addition of DNA, which may be due to an overall structural change in the DOPE membrane. A DOTAP/DOPE complex system showed behaviors attributable to both DOTAP and DOPE, whereas the behavior of the DOTAP/DOPC system resembled that of DOPC alone. The current results provide a model for the so-called lipoplex carriers used in gene therapy.

Using AFM to probe the complexation of DNA with anionic lipids mediated by Ca(2+): the role of surface pressure

Complexation of DNA with lipids is currently being developed as an alternative to classical vectors based on viruses. Most of the research to date focuses on cationic lipids owing to their spontaneous complexation with DNA. Nonetheless, recent investigations have revealed that cationic lipids induce a large number of adverse effects on DNA delivery. Precisely, the lower cytotoxicity of anionic lipids accounts for their use as a promising alternative. However, the complexation of DNA with anionic lipids (mediated by cations) is still in early stages and is not yet well understood. In order to explore the molecular mechanisms underlying the complexation of anionic lipids and DNA we proposed a combined methodology based on the surface pressure-area isotherms, Gibbs elasticity and Atomic Force Microscopy (AFM). These techniques allow elucidation of the role of the surface pressure in the complexation and visualization of the interfacial aggregates for the first time. We demonstrate that the DNA complexes with negatively charged model monolayers (DPPC/DPPS 4 : 1) only in the presence of Ca(2+), but is expelled at very high surface pressures. Also, according to the Gibbs elasticity plot, the complexation of lipids and DNA implies a whole fluidisation of the monolayer and a completely different phase transition map in the presence of DNA and Ca(2+). AFM imaging allows identification for the first time of specific morphologies associated with different packing densities. At low surface coverage, a branched net like structure is observed whereas at high surface pressure fibers formed of interfacial aggregates appear. In summary, Ca(2+) mediates the interaction between DNA and negatively charged lipids and also the conformation of the ternary system depends on the surface pressure. Such observations are important new generic features of the interaction between DNA and anionic lipids.

The interaction between the outer layer of a mixed ion pair amphiphile/double-chained cationic surfactant vesicle and DNA: a Langmuir monolayer study

The charge density of vesicular bilayers plays an important role in the structure characteristic of the vesicle-DNA complex for gene delivery. In this work, the charge density effect of catanionic vesicle surfaces on the association behavior of the vesicle with DNA was explored with the model Langmuir monolayer approach. The interaction of negatively charged DNA with positively charged Langmuir monolayers composed of catanionic vesicle-forming materials, hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS) and dihexadecyldimethylammonium bromide (DHDAB), was investigated with surface pressure-area isotherms, area-time relaxation curves and Brewster angle microscope images. The results showed that the adsorption of DNA molecules onto the monolayers was enhanced with an increased DHDAB molar fraction (XDHDAB), which was apparently related to the increased charge density of the monolayers. With XDHDAB being increased up to 0.5, the mixed monolayers with a higher XDHDAB, or higher charge density, possessed a more stable characteristic at high surface pressures, at which the molecular status was close to that in a corresponding vesicular bilayer, due to the DHDAB-improved molecular packing/interaction. It was found that the composition of the mixed HTMA-DS-DHDAB monolayers at high surface pressures would be affected by the adsorbed DNA with the extent depending on XDHDAB. For the formation of stable HTMA-DS-DHDAB monolayer-DNA complexes, a strong electrostatic interaction of DNA with a monolayer of high charge density and a high monolayer stability characteristic resulting from DHDAB-improved molecular packing/interaction were thus required. The finding has an implication for the formulation of catanionic vesicles composed of an ion pair amphiphile, HTMA-DS, with DHDAB in gene delivery applications.

Calcium-mediated binding of DNA to 1,2-distearoyl-sn-glycero-3-phosphocholine-containing mixed lipid monolayers

The calcium-mediated interaction of DNA with monolayers of the non-toxic, zwitterionic phospholipid, 1,2-distearoyl-sn-glycero-3-phosphocholine when mixed with 50 mol% of a second lipid, either the zwitteronic 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine or neutral cholesterol was investigated using a combination of surface pressure-area isotherms, Brewster angle microscopy, external reflectance Fourier transform infrared spectroscopy and specular neutron reflectivity in combination with contrast variation. When calcium and DNA were both present in the aqueous subphase, changes were observed in the compression isotherms as well as the surface morphologies of the mixed lipid monolayers. In the presence of calcium and DNA, specular neutron reflectivity showed that directly underneath the head groups of the lipids comprising the monolayers, DNA occupied a layer comprising approximately 13 and 18% v/v DNA for the 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine and cholesterol-containing monolayers, respectively. The volume of the corresponding layer for 1,2-distearoyl-sn-glycero-3-phosphocholine only containing monolayers was ∼15% v/v DNA. Furthermore regardless of the presence and nature of the second lipid and the surface pressure of the monolayer, the specular neutron reflectivity experiments showed that the DNA-containing layer was 20-27 Å thick, suggesting the presence of a well-hydrated layer of double-stranded DNA. External reflectance Fourier transform infrared studies confirmed the presence of double stranded DNA, and indicated that the strands are in the B-form conformation. The results shed light on the interaction between lipids and nucleic acid cargo as well as the role of a second lipid in lipid-based carriers for drug delivery.

Structure and kinetics of lipid-nucleic acid complexes

The structure and function of lipid-based complexes (lipoplexes) have been widely investigated as cellular delivery vehicles for nucleic acids-DNA and siRNA. Transfection efficiency in applications such as gene therapy and gene silencing has been clearly linked to the local, nano-scale organization of the nucleic acid in the vehicle, as well as to the global properties (e.g. size) of the carriers. This review focuses on both the structure of DNA and siRNA complexes with cationic lipids, and the kinetics of structure evolution during complex formation. The local organization of the lipoplexes is largely set by thermodynamic, equilibrium forces, dominated by the lipid preferred phase. As a result, complexation of linear lambda-phage DNA, circular plasmid DNA, or siRNA with lamellae-favoring lipids (or lipid mixtures) forms multi-lamellar L(α)(C) liquid crystalline arrays. Complexes created with lipids that have bulky tail groups may form inverted hexagonal HII(C) phases, or bicontinuous cubic Q(II)(C) phases. The kinetics of complex formation dominates the large-scale, global structure and the properties of lipoplexes. Furthermore, the time-scales required for the evolution of the equilibrium structure may be much longer than expected. In general, the process may be divided into three distinct stages: An initial binding, or adsorption step, where the nucleic acid binds onto the surface of the cationic vesicles. This step is relatively rapid, occurring on time scales of order of milliseconds, and largely insensitive to system parameters. In the second step, vesicles carrying adsorbed nucleic acid aggregate to form larger complexes. This step is sensitive to the lipid characteristics, in particular the bilayer rigidity and propensity to rupture, and to the lipid to nucleic acid (L/D) charge ratio, and is characterized by time scales of order seconds. The last and final step is that of internal rearrangement, where the overall global structure remains constant while local adjustment of the nucleic acid/lipid organization takes place. This step may occur on unusually long time scales of order hours or longer. This rate, as well, is highly sensitive to lipid characteristics, including membrane fluidity and rigidity. While the three step process is consistent with many experimental observations to date, improving the performance of these non-viral vectors requires better understanding of the correlations between the parameters that influence lipoplexes' formation and stability and the specific rate constants i.e., the timescales required to obtain the equilibrium structures. Moreover, new types of cellular delivery agents are now emerging, such as antimicrobial peptide complexes with anionic lipids, and other proteins and small-molecule lipid carriers, suggesting that better understanding of lipoplex kinetics would apply to a variety of new systems in biotechnology and nanomedicine.

From self-assembly fundamental knowledge to nanomedicine developments

This review highlights the key role of NMR techniques in demonstrating the molecular aspects of the self-assembly of surfactant molecules that nowadays constitute the basic knowledge which modern nanoscience relies on. The aim is to provide a tutorial overview. The story of a rigorous scientific approach to understand self-assembly in surfactant systems and biological membranes starts in the early seventies when the progresses of SAXRD and NMR technological facilities allowed to demonstrate the existence of ordered soft matter, and the validity of Tanford approach concerning self-assembly at a molecular level. Particularly, NMR quadrupolar splittings, NMR chemical shift anisotropy, and NMR relaxation of dipolar and quadrupolar nuclei in micellar solutions, microemulsions, and liquid crystals proved the existence of an ordered polar-apolar interface, on the NMR time scale. NMR data, rationalized in terms of the two-step model of relaxation, allowed to quantify the dynamic aspects of the supramolecular aggregates in different soft matter systems. In addition, NMR techniques allowed to obtain important information on counterion binding as well as on size of the aggregate through molecular self-diffusion. Indeed NMR self-diffusion proved without any doubt the existence of bicontinuous microemulsions and bicontinuous cubic liquid crystals, suggested by pioneering and brilliant interpretation of SAXRD investigations. Moreover, NMR self-diffusion played a fundamental role in the understanding of microemulsion and emulsion nanostructures, phase transitions in phase diagrams, and particularly percolation phenomena in microemulsions. Since the nineties, globalization of the knowledge along with many other technical facilities such as electron microscopy, particularly cryo-EM, produced huge progresses in surfactant and colloid science. Actually we refer to nanoscience: bottom up/top down strategies allow to build nanodevices with applications spanning from ICT to food technology. Developments in the applied fields have also been addressed by important progresses in theoretical skills aimed to understand intermolecular forces, and specific ion interactions. Nevertheless, this is still an open question. Our predictive ability has however increased, hence more ambitious targets can be planned. Nanomedicine represents a major challenging field with its main aims: targeted drug delivery, diagnostic, theranostics, tissue engineering, and personalized medicine. Few recent examples will be mentioned. Although the real applications of these systems still need major work, nevertheless new challenges are open, and perspectives based on integrated multidisciplinary approaches would enable both a deeper basic knowledge and the expected advances in biomedical field.

RNA and DNA association to zwitterionic and charged monolayers at the air-liquid interface

The objective of this work is to establish under which conditions short RNA molecules (similar to miRNA) associate with zwitterionic phospholipids and how this differs from the association with cationic surfactants. We study how the base pairing (i.e., single stranded versus double stranded nucleic acids) and the length of the nucleic acid and the charge of the lipid/surfactant monolayer affect the association behavior. For this purpose, we study the adsorption of nucleic acids to monolayers composed of dipalmitoyl phosphatidylcholine (DPPC) or dioctadecyl-dimethyl-ammoniumbromide (DODAB) using the surface film balance, neutron reflectometry, and fluorescence microscopy. The monolayer studies with the surface film balance suggested that short single-stranded ssRNA associates with liquid expanded zwitterionic phospholipid monolayers, whereas less or no association is detected for double-stranded dsRNA and dsDNA. In order to quantify the interaction and to determine the location of the nucleic acid in the lipid/surfactant monolayer we performed neutron reflectometry measurements. It was shown that ssRNA adsorbs to and penetrates the liquid expanded monolayers, whereas there is no penetration of nucleic acids into the liquid condensed monolayer. No adsorption was detected for dsDNA to zwitterionic monolayers. On the basis of these results, we propose that the association is driven by the hydrophobic interactions between the exposed hydrophobic bases of the ssRNA and the hydrocarbon chains of the phospholipids. The addition of ssRNA also influences domain formation in the DPPC monolayer, leading to fractal-like interconnected domains. The experimental results are discussed in terms of the implication for biological processes and new leads for applications in medicine and biotechnology.

Structure of the complex monolayer of gemini surfactant and DNA at the air/water interface

The properties of the complex monolayers composed of cationic gemini surfactants, [C(18)H(37)(CH(3))(2)N(+)-(CH(2))(s)-N(+)(CH(3))(2)C(18)H(37)],2Br(-) (18-s-18 with s = 3, 4, 6, 8, 10 and 12), and ds-DNA or ss-DNA at the air/water interface were in situ studied by the surface pressure-area per molecule (π-A) isotherm measurement and the infrared reflection absorption spectroscopy (IRRAS). The corresponding Langmuir-Blodgett (LB) films were also investigated by the atomic force microscopy (AFM), the Fourier transform infrared spectroscopy (FT-IR), and the circular dichroism spectroscopy (CD). The π-A isotherms and AFM images reveal that the spacer of gemini surfactant has a significant effect on the surface properties of the complex monolayers. As s ≤ 6, the gemini/ds-DNA complex monolayers can both laterally and normally aggregate to form fibril structures with heights of 2.0-7.0 nm and widths of from several tens to ~300 nm. As s > 6, they can laterally condense to form the platform structure with about 1.4 nm height. Nevertheless, FT-IR, IRRAS, and CD spectra, as well as AFM images, suggest that DNA retains its double-stranded character when complexed. This is very important and meaningful for gene therapy because it is crucial to maintain the extracellular genes undamaged to obtain a high transfection efficiency. In addition, when s ≤ 6, the gemini/ds-DNA complex monolayers can experience a transition of DNA molecule from the double-stranded helical structure to a typical ψ-phase with a supramolecular chiral order.

Aligning nanodiscs at the air-water interface, a neutron reflectivity study

Nanodiscs are self-assembled nanostructures composed of a belt protein and a small patch of lipid bilayer, which can solubilize membrane proteins in a lipid bilayer environment. We present a method for the alignment of a well-defined two-dimensional layer of nanodiscs at the air-water interface by careful design of an insoluble surfactant monolayer at the surface. We used neutron reflectivity to demonstrate the feasibility of this approach and to elucidate the structure of the nanodisc layer. The proof of concept is hereby presented with the use of nanodiscs composed of a mixture of two different lipid (DMPC and DMPG) types to obtain a net overall negative charge of the nanodiscs. We find that the nanodisc layer has a thickness or 40.9 ± 2.6 Å with a surface coverage of 66 ± 4%. This layer is located about 15 Å below a cationic surfactant layer at the air-water interface. The high level of organization within the nanodiscs layer is reflected by a low interfacial roughness (~4.5 Å) found. The use of the nanodisc as a biomimetic model of the cell membrane allows for studies of single membrane proteins isolated in a confined lipid environment. The 2D alignment of nanodiscs could therefore enable studies of high-density layers containing membrane proteins that, in contrast to membrane proteins reconstituted in a continuous lipid bilayer, remain isolated from influences of neighboring membrane proteins within the layer.

The interaction between G-quadruplex-forming oligonucleotide and cationic surfactant monolayer at the air/water interface

We report on the interactions between a 21-mer quadruplex-forming oligonucleotide bearing human telomere sequence of dG(3)(T(2)AG(3))(3) (G4 DNA) and a positively charged dioctadecyldimethylammonium bromide (DODAB) monolayer at the air-aqueous interface, studied by surface film balance measurements. In the presence of G4 DNA, the π-A isotherm of the cationic Langmuir film shifted to lower molecular areas when compared with the reference isotherm recorded on the subphase containing only 50 mM triethylamine-acetate (TEAA) buffer. The presence of quadruplex-stabilizing metal cations (K(+) or Na(+)) further affected profiles of π-A isotherms. Further insight into processes related to the G4 DNA-monolayer interactions was provided by recording time profiles of the surface pressure of monolayer at a constant mean molecular area. In these experiments G4 DNA and/or metal ions were sequentially injected under the monolayer surface. Results indicated that multistranded assemblies of G4 DNA were formed at the monolayer interface even in the absence of metal ions, which suggested that the charged cationic surface of Langmuir monolayer induced aggregation of guanine-rich DNA strands. The presence of sodium and potassium ions inhibited formation of multi-stranded assemblies through the competitive G-quadruplex formation but to different extent that might be related to the differences in stability and topology of both quadruplexes.

Immunity by formulation design: induction of high CD8+ T-cell responses by poly(I:C) incorporated into the CAF01 adjuvant via a double emulsion method

The combination of nucleic acid-based Toll-like receptor (TLR)-3 or TLR9 agonists and cationic liposomes constitutes an effective vaccine adjuvant approach for eliciting CD8+ T-cell responses. However, complexing cationic liposomes and oppositely charged oligonucleotides generally results in highly unstable and heterogeneous formulations with limited clinical applicability. The aim of this study was to design, formulate, and carefully characterize a stable CD8-inducing adjuvant based on the TLR3 ligand polyinosinic-polycytidylic acid [poly(I:C)] incorporated into a cationic adjuvant system (CAF01) composed of dimethyldioctadecylammonium (DDA) and trehalose 6,6'-dibehenate (TDB). For this purpose, a modified double emulsion solvent evaporation method was investigated for complexation of high amounts of anionic poly(I:C) to gel-state DDA/TDB liposomes. Addition of a volatile, water-miscible co-solvent (ethanol) to the outer water phase enabled preparation of colloidally stable liposomes, presumably by reducing the poly(I:C)-enhanced rigidity of the lipid bilayer. Cryo-transmission electron microscopy (TEM) revealed the formation of unilamellar as well as multilamellar liposomes, the latter suggesting that poly(I:C) is intercalated between the membrane bilayers in an onion-like structure. Finally, immunization of mice with the model antigen ovalbumin (OVA) and DDA/TDB/poly(I:C) liposomes induced a remarkably strong, antigen-specific CD8+ T-cell response, which was maintained for more than two months. Importantly, whereas injection of soluble poly(I:C) led to rapid production of the pro-inflammatory cytokines tumor necrosis factor (TNF)-α and interleukin (IL)-6 in serum, administration of poly(I:C) in complex with the cationic DDA/TDB liposomes prevented this non-specific systemic pro-inflammatory response. These data emphasize the importance of improving the quality of the vaccine formulation to indeed overcome some of the major obstacles for using CD8-inducing agents such as poly(I:C) in future subunit vaccines.

ON-OFF switching of transcriptional activity of large DNA through a conformational transition in cooperation with phospholipid membrane

We report that structural transitions of DNA cause the ON-OFF switching of transcriptional activity in cooperation with phospholipid membrane in a reconstituted artificial cell. It has been shown that long DNA of more than 20-30 kilo base-pairs exhibits a discrete conformational transition between a coiled state and highly folded states in aqueous solution, depending on the presence of various condensing agents such as polyamine. Recently, we reported a conformational transition of long DNA through interplay with phospholipid membrane, from a folded state in aqueous phase to an extended coil state on a membrane surface, in a cell-sized water-in-oil microdroplet covered by phosphatidylethanolamine monolayer (Kato, A.; Shindo, E.; Sakaue, T.; Tsuji, A.; Yoshikawa, K. Biophys. J. 2009, 97, 1678-1686). In this study, to elucidate the effects of these conformational changes on the biologically important function of DNA, transcription, we investigated the transcriptional activity of DNA in a microdroplet. Transcriptional activity was evaluated at individual DNA molecule level by a method we developed, in which mRNA molecules are labeled with fluorescent oligonucleotide probes. Transcription proceeded on almost all of the DNA molecules with a coiled conformation in the aqueous phase. In the presence of a tetravalent amine, spermine, the DNA had a folded conformation, and transcription was completely inhibited. When the Mg(2+) concentration was increased, DNA was adsorbed onto the inner surface of the membrane and exhibited an extended conformation. The transcription experiments showed that this conformational transition recovered transcriptional activity; transcription occurred on DNA molecules that were on the membrane.

RNA and DNA interactions with zwitterionic and charged lipid membranes - a DSC and QCM-D study

The aim of the present study is to establish under which conditions tRNA associates with phospholipid bilayers, and to explore how this interaction influences the lipid bilayer. For this purpose we have studied the association of tRNA or DNA of different sizes and degrees of base pairing with a set of model membrane systems with varying charge densities, composed of zwitterionic phosphatidylcholines (PC) in mixtures with anionic phosphatidylserine (PS) or cationic dioctadecyl-dimethyl-ammoniumbromide (DODAB), and with fluid or solid acyl-chains (oleoyl, myristoyl and palmitoyl). To prove and quantify the attractive interaction between tRNA and model-lipid membrane we used quartz crystal microbalance with dissipation (QCM-D) monitoring to study the tRNA adsorption to deposit phospholipid bilayers from solutions containing monovalent (Na(+)) or divalent (Ca(2+)) cations. The influence of the adsorbed polynucleic acids on the lipid phase transitions and lipid segregation was studied by means of differential scanning calorimetry (DSC). The basic findings are: i) tRNA adsorbs to zwitterionic liquid-crystalline and gel-phase phospholipid bilayers. The interaction is weak and reversible, and cannot be explained only on the basis of electrostatic attraction. ii) The adsorbed amount of tRNA is higher for liquid-crystalline bilayers compared to gel-phase bilayers, while the presence of divalent cations show no significant effect on the tRNA adsorption. iii) The adsorption of tRNA can lead to segregation in the mixed 1,2-dimyristoyl-sn-glycerol-3-phosphatidylcholine (DMPC)-1,2-dimyristoyl-sn-glycero-3-phosphatidylserine (DMPS) and DMPC-DODAB bilayers, where tRNA is likely excluded from the anionic DMPS-rich domains in the first system, and associated with the cationic DODAB-rich domains in the second system. iv) The addition of shorter polynucleic acids influence the chain melting transition and induce segregation in a mixed DMPC-DMPS system, while larger polynucleic acids do not influence the melting transition in these system. The results in this study on tRNA-phospholipid interactions can have implications for understanding its biological function in, e.g., the cell nuclei, as well as in applications in biotechnology and medicine.

Conformational transition of giant DNA in a confined space surrounded by a phospholipid membrane

It has been established that a long DNA molecule exhibits a large discrete conformational change from a coiled state to a highly folded state in aqueous solution, depending on the presence of various condensing agents such as polyamines. In this study, T4 DNA labeled with fluorescent dyes was encapsulated in a cell-sized microdroplet covered with a phospholipid membrane to investigate the conformational behavior of a DNA molecule in such a confined space. Fluorescence microscopy showed that the presence of Mg(2+) induced the adsorption of DNA onto the membrane inner-surface of a droplet composed of phosphatidylethanolamine, while no adsorption was observed onto a phosphatidylcholine membrane. Under the presence of spermine (tetravalent amine), DNA had a folded conformation in the bulk solution. However, when these molecules were encapsulated in the microdroplet, DNA adsorbed onto the membrane surface accompanied by unfolding of its structure into an extended coil conformation under high concentrations of Mg(2+). In addition, DNA molecules trapped in large droplets tended not to be adsorbed on the membrane, i.e., no conformational transition occurred. A thermodynamic analysis suggests that the translational entropy loss of a DNA molecule that is accompanied by adsorption is a key factor in these phenomena under micrometer-scale confinement.

Binding of DNA to zwitterionic lipid layers mediated by divalent cations

Divalent cations, i.e., calcium, magnesium, and others, are able to enhance the ability of DNA to interact with membranes that are composed of zwitterionic lipids such as phosphatidylcholine. The resulting condensed complexes offer potential applications as nontoxic gene delivery vehicles. The present study suggests a generic theoretical model to describe the energetics and structural features of a zwitterionic lipid-DNA complex in the presence of divalent cations. Specifically, we consider the adsorption of a single molecule of double-stranded DNA onto a planar zwitterionic lipid layer. Our theoretical model is based on the continuum Poisson-Boltzmann formalisms, which we modified so as to account for the two opposite charges and orientational freedom of the zwitterionic lipid headgroups. We find a substantially more favorable adsorption free energy of the DNA if divalent cations are present. In addition, our model predicts the divalent cations to preferentially interact with the phosphate groups of the zwitterionic lipids, given these lipids are located in close vicinity to the DNA. This is accompanied by a small but notable reorientation of the zwitterionic headgroups toward the DNA. We demonstrate that the binding of DNA onto a zwitterionic lipid layer is not driven by the release of counterions. Instead, the binding leads to a partial redistribution of the divalent cations, from the phosphate groups of the DNA (prior to the binding) to the phosphate groups of the zwitterionic lipids (after the binding). Our results thus suggest a general physical mechanism underlying complex formation between DNA and zwitterionic lipids in terms of mean-field electrostatics, i.e., neither involving correlations nor specific interactions of the divalent cations.

Neutron reflectivity study of the complexation of DNA with lipids and surfactants at the surface of water

Complexation of lipids and surfactants with short DNA fragments at the air-water interface has been studied by neutron reflectivity. Complexation with zwitterionic lipids occurs in the presence of divalent cations, and ion specificity has been demonstrated (binding is less effective with Ba2+ than with Mg2+ or Ca2+). One and two DNA layers have been observed for dilute and more compact lipid monolayers, respectively. Two DNA layers have also been found with the soluble cationic surfactant dodecyltrimethylammonium bromide (DTAB), except close to the precipitation boundary. This result is opposite to that found in ellipsometry where very thick layers are found in this region. It is possible that the ellipsometry signal is due to highly hydrated bulk complexes adsorbing at the surface, not seen by neutrons because of unfavorable contrast conditions. Long DNA was found to be less keen to form surface complexes than short DNA fragments.

Brewster angle microscopy and PMIRRAS study of DNA interactions with BGTC, a cationic lipid used for gene transfer

The lipid bis(guanidinium)-tris(2-aminoethyl)amine-cholesterol (BGTC) is a cationic cholesterol derivative bearing guanidinium polar headgroups which displays high transfection efficiency in vitro and in vivo when used alone or formulated as liposomes with the neutral colipid 1,2-di-[ cis-9-octadecenoyl]- sn-glycero-3-phosphoethanolamine (DOPE). Since transfection may be related to the structural and physicochemical properties of the self-assembled supramolecular lipid-DNA complexes, we used the Langmuir monolayer technique coupled with Brewster angle microscopy (BAM) and polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) to investigate DNA-BGTC and DNA-BGTC/DOPE interactions at the air/water interface. We herein show that BGTC forms stable monolayers at the air/water interface. When DNA is injected into the subphase, it adsorbs to BGTC at 20 mN/m. Whatever the (+/-) charge ratio of the complexes used, defined as the ratio of positive charges of BGTC in the monolayer versus negative charges of DNA injected in the subphase, the DNA interacts with the cationic lipid and forms either an incomplete (no constituent in excess) or a complete (DNA in excess) monolayer of oriented double strands parallel to the lipid monolayer plan. We also show that, under a homogeneous BGTC/DOPE (3/2) monolayer at 20 mN/m, DNA adsorbs homogeneously to form an organized but incomplete layer whatever the charge ratio used (DNA in default or in excess). Compression beyond the collapse of these mixed DNA-BGTC/DOPE systems leads to the formation of dense DNA monolayers under an asymmetric lipid bilayer with a bottom layer of BGTC in contact with DNA and a top layer mainly constituted of DOPE. These results allow a better understanding of the mechanisms underlying the formation of the supramolecular BGTC-DNA complexes efficient for gene transfection.

Adsorption of DNA and dodecyl trimethylammonium bromide mixtures at the air/water interface: a neutron reflectometry study

The interactions between dodecyl trimethylammonium bromide (C12TAB) and two samples of DNA with widely differing molecular weights have been studied using surface tension and neutron reflectometry. Neutron reflection data show that the surfactant and polymer are adsorbed together in a highly cooperative fashion over a 1000-fold change in surfactant concentration. Furthermore, the shorter DNA fragments adsorb with C12TAB as trilayers at higher surfactant concentrations, with overall layer thicknesses of 65-70 A. The high molecular weight DNA, however, shows only approximate monolayer adsorption with thicknesses varying from 19 to 26 A over the entire range of C12TAB concentrations. The difference in behavior between the different samples is believed to be a result of the rigid double helical structure of DNA which makes the formation of bulk phase polymer/micelle aggregates much less favorable for the short fragments. The resulting increase in the critical aggregation concentration (CAC) then leads to the adsorption of additional surfactant/polymer complex to the underside of the initial stable surface active DNA/C12TAB complex. Comparison with previous results obtained for synthetic polyelectrolytes shows that DNA/C12TAB complexes are not capable of reducing the surface tensions to the extent that other mixtures such as the poly(styrene sulfonate)/C12TAB mixtures do. A possible reason for this is the high rigidity of DNA combined with the fact that its hydrophobic moieties are positioned within the double helix so that the external molecule is largely hydrophilic.

Polymer/surfactant interactions at the air/water interface

The development of neutron reflectometry has transformed the study and understanding of polymer/surfactant mixtures at the air/water interface. A critical assessment of the results from this technique is made by comparing them with the information available from other techniques used to investigate adsorption at this interface. In the last few years, detailed information about the structure and composition of adsorbed layers has been obtained for a wide range of polymer/surfactant mixtures, including neutral polymers and synthetic and naturally occurring polyelectrolytes, with single surfactants or mixtures of surfactants. The use of neutron reflectometry together with surface tensiometry, has allowed the surface behaviour of these mixtures to be related directly to the bulk phase behaviour. We review the broad range of systems that have been studied, from neutral polymers with ionic surfactants to oppositely charged polyelectrolyte/ionic surfactant mixtures. A particular emphasis is placed upon the rich pattern of adsorption behaviour that is seen in oppositely charged polyelectrolyte/surfactant mixtures, much of which had not been reported previously. The strong surface interactions resulting from the electrostatic attractions in these systems have a very pronounced effect on both the surface tension behaviour and on adsorbed layers consisting of polymer/surfactant complexes, often giving rise to significant surface ordering.

Self-assembly, DNA complexation, and pH response of amphiphilic dendrimers for gene transfection

Cationic lipids and polymers are routinely used for cell transfection, and a variety of structure-activity relation data have been collected. Few studies, however, focus on the structural aspects of self-assembly as a crucial control parameter for gene delivery. We present here the observations collected for a set of cationic dendritic amphiphiles based on a stiff tolane core (1-4) that are built from identical subunits but differ in the number and balance of their hydrophobic and cationic hydrophilic moieties. We established elsewhere that vectors 3 and 4 have promising transfection properties. Scanning probe microscopy (AFM, STM), cryo-transmission electron microscopy (cryo-TEM), and Langmuir techniques provide insight into the self-assembly properties of the molecules under physiological conditions. Furthermore, we present DNA and pH "jump" experiments where we study the response of Langmuir films to a sudden increase in DNA concentration or a drop in pH. We find that the primary self-assembly of the amphiphile is of paramount importance and influences DNA binding, serum sensitivity, and pH response of the vector system.

DNA condensation and interaction with zwitterionic phospholipids mediated by divalent cations

Artificial viruses are considered to be a promising tool in gene therapy. To find lipid-DNA complexes with high transfection efficiency but without toxicity is a fundamental aim. Although cationic lipids are frequently toxic for cells, neutral lipids are completely nontoxic. Zwitterionic lipids do not interact with DNA directly; however, the interaction can be mediated by divalent cations. Langmuir monolayers represent a well-defined model system to study the DNA-lipid complexes at the air/water interface (quasi-2D systems). In this work, isotherms, infrared reflection absorption spectroscopy (IRRAS), X-ray reflectivity (XR), grazing incidence X-ray diffraction (GIXD), and Brewster angle microscopy (BAM) measurements are used to study the interaction of calf thymus DNA with DMPE (1,2-dimyristoyl-phosphatidylethanolamine) monolayers mediated by Ca2+ or Mg2+ ions. DNA adsorption is observed only in the presence of divalent cations. At low lateral pressure, the DNA partially penetrates into the lipid monolayer but is squeezed out at high pressure. The adsorption layer has a thickness of 18-19 A. Additionally, GIXD provides information about a one-dimensional ordering of adsorbed DNA. The periodic distance between DNA strands depends on the type of the divalent cation.