Phase evolution of lamellar cationic lipid-DNA complex: steric effect of an electrolyte

The complexation isotherms of DNA plus lipids of a symmetric lamellar cationic lipid-DNA system were determined within a mean field free energy. The free energy incorporates the ion's finite size of NaCl simple electrolyte in solution and makes use of known structure data on this complex. The results for the predicted isotherms are in qualitative agreement with the trends of the experimental data for this property.     

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.     

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.     

Molecular mechanism and thermodynamics study of plasmid DNA and cationic surfactants interactions

The molecular mechanism and thermodynamics of the interactions between plasmid DNA and cationic surfactants were investigated by isothermal titration calorimetry (ITC), dynamic light scattering, surface tension measurements, and UV spectroscopy. The cationic surfactants studied include benzyldimethyldodecylammonium chloride, benzyldimethyltetradecylammonium chloride, cetylpyridinium chloride, and cetyltrimethylammonium chloride. The results indicate a critical aggregation concentration (cac) of a surfactant: above the cac the surfactant forms aggregates with plasmid DNA; below the cac, however, there is no detectable interaction between DNA and surfactant. Surfactants with longer hydrocarbon chains have smaller cac, indicating that hydrophobic interaction plays a key role in DNA-surfactant complexation. Moreover, an increase in ionic strength (I) increases the cac but decreases the critical micellization concentration (cmc). These opposite effects lead to a critical ionic strength (I(c)) at which cac = cmc; when I < I(c), cac < cmc; when I > I(c), DNA does not form complexes with surfactant micelles. In the interaction DNA exhibits a pseudophase property as the cac is a constant over a wide range of DNA concentrations. ITC data showed that the reaction is solely driven by entropy because both deltaH(o) (approximately 2-6 kJ mol(-1)) and deltaS(o) (approximately 70-110 J K(-1) mol(-1)) have positive values. In the complex, the molar ratio of DNA phosphate to surfactant is in the range of 0.63-1.05. The reaction forms sub-micrometer-sized primary particles; those aggregate at high surfactant concentrations. Taken together, the results led to an inference that there is no interaction between surfactant monomers and DNA molecules and demonstrated that DNA-cationic surfactant interactions are mediated by the hydrophobic interactions of surfactant molecules and counterion binding of DNA phosphates to the cationic surfactant aggregates.     

Complexes of cationic block copolymer micelles with DNA: histone/DNA complex mimetics

The formation of complexes between cationic polymeric micelles of PS-b-PQ2VP amphiphilic block copolymers and DNA molecules in aqueous solutions is investigated at pH = 7. The physicochemical characteristics of the "polyplexes" at different DNA/polymer ratios were characterized in terms of mass, size and charge using static, dynamic and electrophoretic light scattering and AFM. The complexes are spherical and assume their maximum size and mass around the charge stoichiometric ratio. After addition of increased amounts of salt in the solutions, partial dissociation of the systems was observed. The present systems can be considered as mimetics of histone/DNA complexes formed under physiological conditions in living cells.     

Spectroscopic investigation of cationic comb-type copolymers/DNA interaction: interpolyelectrolyte complex enhancement synchronized with DNA hybridization

We have demonstrated that cationic comb-type copolymers consisting of a polycation backbone and abundant grafts of water-soluble polymers stabilize DNA hybrids. Furthermore, the copolymers were found to accelerate strand exchange reaction between a double-stranded DNA and its complementary single-stranded DNA. In this article, we investigated the effects of PLL-g-Dex on base pairs of a self-complementary DNA octamer, d(GGAATTCC). The soluble interpolyelectrolyte complex (IPEC) between the DNA and copolymer allowed us to characterize the complex by using spectroscopic methods under physiological ionic condition. Chemical shifts of nucleobase proton signals were not changed by PLL-g-Dex. Furthermore, the copolymer slightly changed the von't Hoff DeltaH accompanying the helix-coil transition of the octamer. These results indicated that the base pairs of the duplex DNA in the IPEC were not perturbed by the polycationic copolymer. It was obviously shown by temperature dependencies of proton and phosphorus NMR spectra that DNA/copolymer interaction was considerably enhanced in response to ds DNA formation. An increase in the density and total number of DNA negative charges upon hybrid formation likely caused the higher affinity of the copolymer with the ds form over that of the copolymer with the ss form. The IPEC formation of CCCs with DNA, however, seems highly sensitive to the coil-helix transition of the DNA.     

The effect of lattice compressibility on the thermodynamics of gas sorption in polymers

A compressible lattice model with holes, the glassy polymer lattice sorption model (GPLSM), was used to model the sorption of carbon dioxide, methane, and ethylene in glassy polycarbonate and carbon dioxide in glassy tetramethyl polycarbonate. For glassy polymers, an incompressible lattice model, such as the Flory–Huggins theory, requires concentration-dependent and physically unrealistic values for the lattice site volumes in order to satisfy lattice incompressibility. Rather than forcing lattice incompressibility, GPLSM was used and reasonable parameter values were obtained. The effect of conditioning on gas sorption in glassy polymers was analyzed quantitatively with GPLSM. The Henry's law constant decreases significantly upon gas conditioning, reflecting changes in the polymer matrix at infinite dilution. Treating the Henry's law constant as a hypothetical vapor pressure at infinite dilution, gas molecules in the conditioned polymer are less “volatile” than those in the unconditioned polymer. Flory–Huggins theory was used to model the sorption of carbon dioxide, methane, and ethylene in silicone rubber. Above the glass transition temperature, the criterion of lattice incompressibility for Flory-Huggins theory was satisfied with physically realistic and constant values for the lattice site volumes. © 1992 John Wiley & Sons, Inc.     

DNA strand exchange stimulated by spontaneous complex formation with cationic comb-type copolymer

Cationic comb-type copolymers (CCCs) composed of a polycation backbone and water-soluble side chains accelerate by 4-5 orders the DNA strand exchange reaction (SER) between double helical DNA and its homologous single-strand DNA. The accelerating effect is considered due to alleviation of counterion association during transitional intermediate formation in sequential displacement pathway. CCCs stabilize not only matured hybrids but also the nucleation complex to accelerate hybridization.     

Structural studies of lamellar surfactant systems under shear

Recent experimental studies on concentrated surfactant systems are reviewed. Particular attention is focused on the transformation from planar lamellar sheets to multilamellar vesicles. It is discussed whether both of these states are thermodynamic stable, or if the MLV is an artifact of shear induced factors. Recent studies includes the dependence on shear, and dependence on salt and cosurfactants, and thereby related lamellar defects. The review include moreover the demonstration that polymeric amphiphiles dramatically enhance the quality of classical surfactants.     

Structural properties and thermodynamics of water clusters: a Wang-Landau study

The temperature dependence of structural properties and thermodynamic behavior of water clusters has been studied using Wang-Landau sampling. Four potential models, simple point charge/extended (SPC/E), transferable intermolecular potential 3 point (TIP3P), transferable intermolecular potential 4 point (TIP4P), and Gaussian charge polarizable (GCP), are compared for ground states and properties at finite temperatures. Although the hydrogen bond energy and the distance of the nearest-neighbor oxygen pair are significantly different for TIP4P and GCP models, they approach to similar ground state structures and melting transition temperatures in cluster sizes we considered. Comparing with TIP3P, SPC/E model provides properties closer to that of TIP4P and GCP.     

The use of capillary electrophoresis with entangled polymer matrix to analyse plasmid DNA and a cationic lipid/cholesterol liposome: DNA complex

Summary A liposome based gene therapeutic product is being developed for the treatment of cystic fibrosis. The product comprises a complex of plasmid DNA and a cationic lipid/cholesterol liposome. Using CE with an entangled polymer matrix, routine separations of linear, supercoiled and open-circle conformers of DNA in plasmid DNA and the liposome complex have been performed. The CE method has been used to support novel quality control and process validation for the manufacture of plasmid DNA and to monitor the degradation of DNA in the liposome complex. Significant features of the method are simple sample preparation and the use of direct UV detection, avoiding the use of potentially mutagenic reagents.     

Effect of polydispersity on the tensile modulus of diblock copolymers in a lamellar phase

We have calculated the tensile moduli of nanomaterials having lamellar microstructures prepared through the self-assembly of a polydisperse diblock copolymer. We observed that the extensional moduli K33 provided the major contribution to the tensile modulus and that the value of K33 depended mainly on the contribution of K33(U) (the internal energy contribution to K33). We found that a larger polydispersity index (PDI) weakens the material for our polydisperse model; we attribute this phenomenon to the larger lamellar domain size at equilibrium when the polydispersity of the block increases and to the competition between short and long chains. We found that longer chains in this system strengthen the material, but shorter chains weaken it as a result of the influence of the PDI. The shear modulus contributed negligibly to the extensional moduli.     

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.     

Phase behavior and thermodynamics of a mixture of cationic gemini and anionic surfactant

We present the phase behavior and thermodynamics of the catanionic mixture of the gemini surfactant hexanediyl-alpha,omega-bis(dodecyldimethylammonium bromide), designated here as 12-6-12Br(2), and sodium dodecyl sulfate (SDS) over the full range of composition, at the water-rich corner. Visual and turbidity measurements of the mixtures provide some basic macroscopic information on phase behavior. The structure of the aggregates formed spontaneously in the mixtures has been observed with TEM. As the molar fraction of SDS, X(SDS), is increased, at constant total surfactant concentration, the aggregation morphologies change gradually from gemini-rich micelles, through multiphase regions containing a precipitate (catanionic surfactant) and a vesicle region, to SDS-rich micelles. From isothermal titration calorimetry measurements, the phase boundaries and corresponding enthalpy changes for phase transitions have been obtained. The formation of the different microstructures, in particular, the spontaneously formed vesicles in the SDS-rich side, is discussed on the basis of geometric and electrostatic effects occurring in the SDS-gemini mixture.     

Ferrocenylethylene cationic complex of π-cyclopentadienyliron dicarbonyl

Conclusions We obtained the ferrocenylethylene cationic complex of -cyclopentadienyliron diearbonyl, which reacts with PPh₃, NaI, NaOCOCF₃, NaCN, NaOH, and NaBH₄ to give vinylferrocene.Translated from Izvestiya Akademii Nauk SSSR, Seriva Khimicheskaya, No. 11, pp. 2582–2584, November, 1977.     

Structural characterization of polybutadiene synthesized via cationic mechanism

The microstructure of polybutadiene synthesized via cationic polymerization using TiCl₄‐based initiating systems has been investigated using 1D (¹Н, ²Н, and ¹³С) and 2D (HSQC and HMBC) NMR spectroscopy. It was found that trans‐1,4‐unit is predominant structure of unsaturated part of polymer chain. Besides, the small amount of 1,2‐structures was also detected, while cis‐1,4‐units were totally absent. The signals of carbon atoms of three types of head groups (trans‐1,4‐, 1,2‐, and tert‐butyl) and two types of end groups (trans‐1,4‐Cl and 1,2‐Cl) were identified for the first time in macromolecules of cationic polybutadiene. It was showed that tert‐butyl head groups were formed due to the presence in monomer of admixtures of isobutylene. The new methodology for calculation of the content of different structural units in polybutadiene chain as well as the head and end groups was proposed. It was established that main part of 1,2‐units distributed randomly along the polybutadiene chain as separate units between trans‐1,4‐structures. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 387–398     

Interaction between DNA and cationic surfactants: effect of DNA conformation and surfactant headgroup

The interactions between DNA and a number of different cationic surfactants, differing in headgroup polarity, were investigated by electric conductivity measurements and fluorescence microscopy. It was observed that, the critical association concentration (cac), characterizing the onset of surfactant binding to DNA, does not vary significantly with the architecture of the headgroup. However, comparing with the critical micelle concentration (cmc) in the absence of DNA, it can be inferred that the micelles of a surfactant with a simple quaternary ammonium headgroup are much more stabilized by the presence of DNA than those of surfactants with hydroxylated head-groups. In line with previous studies of polymer-surfactant association, the cac does not vary significantly with either the DNA concentration or its chain length. On the other hand, a novel observation is that the cac is much lower when DNA is denaturated and in the single-stranded conformation, than for the double-helix DNA. This is contrary to expectation for a simple electrostatically driven association. Thus previous studies of polyelectrolyte-surfactant systems have shown that the cac decreases strongly with increasing linear charge density of the polyion. Since double-stranded DNA (dsDNA) has twice as large linear charge density as single-stranded DNA (ssDNA), the stronger binding in the latter case indicates an important role of nonelectrostatic effects. Both a higher flexibility of ssDNA and a higher hydrophobicity due to the exposed bases are found to play a role, with the hydrophobic interaction argued to be more important. The significance of hydrophobic DNA-surfactant interaction is in line with other observations. The significance of nonelectrostatic effects is also indicated in significant differences in cac between different surfactants for ssDNA but not for dsDNA. For lower concentrations of DNA, the conductivity measurements presented an "anomalous" feature, i.e., a second inflection point for surfactant concentrations below the cac; this feature was not displayed at higher concentrations of DNA. The effect is attributed to the presence of a mixture of ss- and dsDNA molecules. Thus the stability of dsDNA is dependent on a certain ion atmosphere; at lower ion concentrations the electrostatic repulsions between the DNA strands become too strong compared to the attractive interactions, and there is a dissociation into the individual strands. Fluorescence microscopy studies, performed at much lower DNA concentrations, demonstrated a transformation of dsDNA from an extended "coil" state to a compact "globule" condition, with a broad concentration region of coexistence of coils and globules. The onset of DNA compaction coincides roughly with the cac values obtained from conductivity measurements. This is in line with the observed independence of cac on the DNA concentration, together with the assumption that the onset of binding corresponds to an initiation of DNA compaction. No major changes in either the onset of compaction or complete compaction were observed as the surfactant headgroup was made more polar.     

Hybridization thermodynamics of DNA bound to gold nanoparticles

Isothermal Titration Calorimetry (ITC) was used to study the thermodynamics of hybridization on DNA-functionalized colloidal gold nanoparticles. When compared to the thermodynamics of hybridization of DNA that is free in solution, the differences in the values of the Gibbs free energy of reaction, Δ_rG°, the enthalpy, Δ_rH°, and entropy, Δ_rS°, were small. The change in Δ_rG° between the free and bound states was always positive but with statistical significance outside the 95% confidence interval, implying the free DNA is slightly more stable than when in the bound state. Additionally, ITC was also able to reveal information about the binding stoichiometry of the hybridization reactions on the DNA-functionalized gold nanoparticles, and indicates that there is a significant fraction of the DNA on gold nanoparticle surface that is unavailable for DNA hybridization. Furthermore, the fraction of available DNA is dependent on the spacer group on the DNA that is used to span the gold surface from that to the probe DNA.