DNA Delivery Systems Based on Peptide-Mimicking Cationic Lipids-The Effect of the Co-Lipid on the Structure and DNA Binding Capacity.

In continuation of previous work, we present a new promising DNA carrier, OO4, a highly effective peptide-mimicking lysine-based cationic lipid. The structural characteristics of the polynucleotide carrier system OO4 mixed with the commonly used co-lipid DOPE and the saturated phospholipid DPPE have been studied in two-dimensional and three-dimensional model systems to understand their influence on the physical-chemical properties. The phase behavior of pure OO4 and its mixtures with DOPE and DPPE was studied at the air-water interface using a Langmuir film balance combined with infrared reflection-absorption spectroscopy. In bulk, the self-assembling structures in the presence and absence of DNA were determined by small-angle and wide-angle X-ray scattering. The amount of adsorbed DNA to cationic lipid bilayers was measured using a quartz crystal microbalance. The choice of the co-lipid has an enormous influence on the structure and capability of binding DNA. DOPE promotes the formation of nonlamellar lipoplexes (cubic and hexagonal structures), whereas DPPE promotes the formation of lamellar lipoplexes. The correlation of the observed structures with the transfection efficiency and serum stability indicates that OO4/DOPE 1:3 lipoplexes with a DNA-containing cubic phase encapsulated in multilamellar structures seem to be most promising.

Introduction

The concept of gene therapy was first proposed in 1972 by Friedman and Roblin[1] to cure monogenetic diseases such as cystic fibrosis and sickle-cell anaemia.[2] This innovative therapy requires efficient nucleic acid transfer into cells. This challenge took 40 years until the first in vivo therapy against lipoprotein lipase deficiency, Glybera, was approved by the European Medicine Agency. In 2016, an ex vivo therapy against ADA-SCID, called Strimvelis, was made available on the European market. Both therapeutics rely on viral vectors which carry several disadvantages, such as the immunogenic potential and expensive as well as difficult manufacturing.[3,4] The costs for a treatment with Glybera and Strimvelis are approximately 1 million $ and 665 000$, respectively.[5,6] Taking into account the decrease of follow-up costs if the causal treatment of these diseases occurs justifies the price, but cheaper alternatives can push forward a broader clinical use of gene therapeutics for diseases with a higher prevalence.

Nonviral vectors, mainly realized by cationic polymers or cationic lipids, are an alternative approach because they can be easier and cheaply produced. However, despite the many advantages of nonviral vectors based on cationic lipids such as high loading capacity, biodegradability, lower immunogenic potential, and their comparatively simple, large-scale, and low-cost manufacturing, their low efficiency is the main drawback. Especially in vivo, these so-called cytofectins show a weak performance compared with viral vectors.[7,8] The transfection efficiency depends on many parameters such as lipoplex size and structure, temperature, pH value, cell type, ionic strength, N/P-ratio, and resulting charge density. Up to now, there is still a great interest in uncovering the structure–activity-correlation of lipid-based nonviral gene carriers.[9−11] One key parameter is the choice of the co-lipid and its influence on the resulting lipoplex formation.[12,13] In general, neutral or zwitterionic co-lipids are added to cationic lipids to adjust liposome charge density and to tune the lipoplex stability. If the charge density is too low, the lipoplex remains trapped in the endosome.[14] Therefore, the charge density has to be high enough to enable an escape from the endosome through active fusion. A too tight packing of DNA (due to very high charge density) reduces the lipoplex dissociation and only a low amount of DNA will be set free for effective gene expression. An optimum charged density would be high enough to avoid endosomal entrapment and low enough to efficiently release the DNA cargo.[15] Additionally, the loading degree of the vector, often termed as the N/P-ratio of the lipoplex, is crucial. The lipoplex needs to remain overall positively charged to interact with the negatively charged cell membrane to induce endocytosis.[16,17] Furthermore, the complex structure can be adjusted by the choice of helper lipids. DOPE adopts the inverted hexagonal phase at room temperature because of its conical molecule shape. The gel to liquid-crystalline phase transition occurs around −8 °C, while the Lα to HII transition takes place around 10 °C.[18,19] Adding the zwitterionic helper lipid DOPE promotes the formation of inverted hexagonal structures, which can rapidly fuse with the endosomal bilayer and allow cytoplasmic release of the DNA in high efficiency.[20] In contrast, DPPE has a cylinder-like molecular architecture and favors the formation of stable lamellar structures. The gel to liquid-crystalline phase transition occurs at around 64 °C, whereas the Lα to HII transition takes place at around 123 °C.[21] Also, the miscibility of cationic lipid and helper lipid plays an important role for an efficient gene transfer.[22] One key parameter to ensure miscibility is the adjustment of alkyl chain length of the cationic and the helper lipids. Similar chain length and phase state enhance miscibility. Up to now, nonlamellar phases (hexagonal and cubic) are believed to result in higher transfection rates because of their fusogenic properties with membranes (cell and endosome).[20,23,24] Additionally, the curvature of liposomes can be tuned by co-lipids because the vesicle size is expected to depend on the composition. The size or curvature of the equilibrium vesicles is undoubtedly linked to their stability, which affects transfection efficiency.[25,26]

In continuation of our structure–properties-activity study on lipid-based nonviral DNA carries for enhanced gene transfection, we investigate the interaction of calf thymus DNA and OO4 in mixtures with either DOPE or DPPE. The OO4/DOPE (1:3, n/n) mixture is more effective and has a higher stability in physiological conditions compared with the OO4/DPPE (1:3, n/n) mixture.[27]OO4/DOPE (1:3, n/n) can be efficiently loaded with DNA and has several beneficial properties in biological media which help to overcome the polycation dilemma.[27] This work will enlighten the physical–chemical properties of both mixtures. OO4 belongs to our second generation of malonic acid-based cationic lipids[28,29] and has been proven to be strongly internalized in NIH3T3 cells.[30] This work focuses on the influence of the chain pattern of co-lipids on the physical–chemical properties of OO4 in two-dimensional (2D) and three-dimensional (3D) model systems. For the 2D model systems, experiments were performed with Langmuir monolayers [compression and adsorption isotherms, infrared reflection-absorption spectroscopy (IRRAS)]. The bulk (3D) systems are studied in small- and wide-angle X-ray scattering and quartz crystal microbalance (QCM) experiments.

Materials and Methods

Material

If not stated otherwise, all materials were purchased from Sigma-Aldrich. Milli-Q Millipore water with a specific resistance of 18.2 MΩ·cm was used for all measurements and sample preparations.

Methods

Sample Preparation

For the experiments, a 1 mM stock solution of OO4 (C51H101N7O3, 860.39 g/mol) was prepared in chloroform/methanol in a ratio of 8:2 (v/v) (CHCl3: J. T. Baker, Netherlands; stabilized with 0.75% of ethanol, CH3OH: Merck, Germany; purity > 99.9%). The general synthesis of the malonic acid-based lipids and the analytical data of OO4 have been already described.[28,30] The zwitterionic phospholipids DOPE (C41H78NO8P, 744.03 g/mol) and DPPE (C37H74NO8P, 691.96 g/mol) were purchased from Avanti (Avanti Polar Lipids, Inc., Alabaster, USA) and used without further purification.

The deoxyribonucleic acid sodium salt from calf thymus (>13 kbp) was purchased from Sigma-Aldrich (Type 1, CAS: 73049-39-5). A solution of 1 mMnucleotides ct-DNA was freshly prepared in 1 mM NaCl solution by gently stirring at 5 °C overnight. The molar mass of DNA refers to a monomer containing one charge per phosphate moiety with 10% of hydration (M ≈ 370 g/mol).[31] Because of the large amounts of DNA needed for this experiment, we choose calf thymus DNA instead of the biologic active plasmid used in other publications,[27] because comparative studies in our laboratory show no difference in the structure (unpublished results).

The bromide-containing buffers had a constant concentration of bromide anions (2 mM). The pH 3 was adjusted with 1,4-diazabicyclo(2,2,2)octane (pKa = 4.2, pKa = 8.2) and pH 10 was obtained by using piperazine (pKa = 5.7, pKa = 9.8). These buffers were also used in the experiments performed for the determination of the apparent pKa values.[32] The 2-(N-morpholino)ethansulfonic acid (MES) buffer (C6H13NO4S, 195.2 g/mol) had a constant concentration of either 10 mM [experiments except for small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS)] or 100 mM (SAXS and WAXS), and pH 6.5 was adjusted with hydrochloric acid (pKa = −3). This buffer was also used in the biological experiments.[27]

Liposomes for QCM or zeta-potential measurements were prepared with the film hydration procedure. Briefly, OO4 and the co-lipids were solved in chloroform/methanol (v/v 8:2) to obtain 1 mM solutions. Later on these solutions were mixed in the desired molar ratio of 1:3 (v/v OO4/co-lipid). The resulting solutions were dried under nitrogen flow for 2 h and stayed in a desiccator under vacuum for 12 h. Afterward, the lipid film was hydrated with a sterile aqueous medium [bromide-containing buffer (2 mM, pH 3) for QCM or pH adjusted water for zeta potential measurements]. The liposome solution had a final concentration of 1 mg/mL. Because small unilamellar vesicles were desired for the QCM experiments, the lipid dispersions were sonicated (37 kHz) for 3 min at room temperature.

Langmuir Film Balance

The lipid monolayers were examined on a computer-interfaced Langmuir trough equipped with a Wilhelmy balance to measure the surface tension with an accuracy of ±0.1 mN/m. The temperature was fixed at 20 °C with a precision of ±0.1 °C by a thermostat. The lipid monolayer was compressed with a moveable barrier with a velocity of 5 Å2/molecule/min to the maximum surface pressure (approximately 45 mN/m). Afterward, the lipid monolayer was expanded immediately to record the hysteresis. The 1 mM lipid solution was spread carefully onto the aqueous subphases (bromide-containing buffers, 2 mM, different pH values) using a microsyringe (Hamilton, Switzerland). Before compression, 10 min were given for evaporation of the organic solvent. In case of DNA adsorbing to the lipid monolayer, 60 min were given. All isotherms were measured at least twice for reproducibility.

Infrared Reflection-Absorption Spectroscopy

The infrared reflection-absorption spectra were collected with a Vertex 70 FT-IR spectrometer (Bruker Optics, Ettlingen, Germany). The set-up includes a film balance (R&K, Potsdam, Germany) inside a container (external air–water reflection unit XA-511, Bruker). A sample trough with two movable barriers and a reference trough (only water or buffer) allow the fast recording of the sample and reference spectra using a shuttle technique. The infrared beam is focused on the liquid surface by a set of mirrors. The IR-beam angle of incidence (ϕ) normal to the surface has been adjusted to 40°. A KRS-5 wire grid polarizer is used to polarize the infrared radiation either in the parallel (p) or perpendicular (s) direction. After reflection from the surface, the beam is directed to a narrow band mercury–cadmium–telluride detector cooled with liquid nitrogen. The final reflectance–absorbance spectra were obtained using −log(R/R0), with R being the reflectance of the film-covered surface and R0 being the reflectance from the pure subphase. For each spectrum of s-polarized light, 200 scans were performed with a scanning velocity of 20 kHz and resolution of 8 cm–1, apodized using the Blackman–Harris 3-term function, and fast Fourier transformed after one level of zero filling.[33] All spectra were corrected for atmospheric interference using the OPUS software and baseline-corrected applying the spectra-subtraction software. The spectra are not smoothed.[34] The maximum of the antisymmetric CH2-band (Lorentzian fit with ±0.2 cm–1) was taken to determine the lipid phase state, while the intensity of the antisymmetric PO2– band was used to estimate the amount of DNA coupled to the monolayers.

ζ-Potential Measurements

The electrophoretic mobility was measured using LASER Doppler electrophoresis technique with a ZetasizerNano ZS ZEN3600 (Malvern Instruments, Worcestershire, UK). Three measurements consisting of 30 runs with a voltage of 60 V were performed at 25 °C. For the calculations, a viscosity of η = 0.8872 mPa·s, dielectric constant of ε = 78.5 F/m, and refractive index of 1.33 were assumed. The mobility μ of the migrating particles was converted into the ζ-potential using the Smoluchowski relation (eq ) (Zetasizer Software 6.34).

The presented ζ potential titration curve contains three individual measurements on different days.

Small- and Wide-Angle X-ray Scattering

The SAXS/WAXS data were recorded at the High Brilliance Beamline ID02 (ESRF, France) with an energy of the incident X-ray beam equal 12.5 keV (λ = 0.995 Å). The beam size was about 100 × 100 μm2 and the sample-to-detector distance was 1.2 m. The small-angle diffraction patterns were collected by a 4 FT-CCD detector (Rayonix MX-170HS). For SAXS, a q range from 0.006 to 0.65 Å–1 with a detector resolution of 3 × 10–4 Å–1 (full width at half-maximum) was used. WAXS data were obtained in a q range from 0.72 to 5.1 Å–1. To avoid sample damage by radiation, each sample was measured with 10 frames with an exposure time of 0.05 s per frame. For data analysis, the average of all 10 frames was used. The collected 2D powder diffraction spectra were reduced and background subtracted using SAXSutilities and analyzed in Origin. The SAXS/WAXS data were corrected for the empty sample holder with pure buffer solution at the corresponding temperature. The angular calibration of the SAXS detectors was performed using silver behenate powder as reference. For WAXS, para bromobenzoic acid was used. The temperature was adjusted with a Huber Unistat with an accuracy of ±0.1 °C. Experiments have been performed at 20, 25, and 37 °C. The sealed glass capillaries containing the lipid dispersions were well-positioned in a Peltier controlled automatic sample changer. The real-space repeating distance d of the lattice planes was calculated from the position of the first diffraction peak by eq . Lorentzian-functions were fitted to the diffraction peaks.

Dissipation-Enhanced QCM Experiments

The dissipation-enhanced QCM (QCM-D) experiments[35] were carried out using a QCM-D E1 (Q-Sense, Gothenburg, Sweden) to quantify the adsorbed amount of DNA to a lipid bilayer. The liposomes were prepared as stated above to a final concentration of 1 mg/mL in sterile filtrated bromide-containing buffer (2 mM, pH 3). The used DNA solution had a concentration of 0.1 mM in sterile filtrated bromide-containing buffer. The AT-cut quartz crystals with a diameter of 14 mm and a frequency of (4.95 ± 0.05) MHz were coated with SiO2 (Q-Sense QSX 303). The flow rate of sterile filtrated aqueous solutions was 20 μL/min. The QCM-chamber was immersed in a temperature-insulating box with a constant temperature of (20 ± 0.1) °C. The liposomes and the ct-DNA solutions were injected with a flow rate of 20 μL/min. Each sample injection was stopped, when no change in the frequency occurred after 5 min. After each injection, the sample was flushed with buffer solution to remove dispensable molecules. To evaluate the real time recorded QCM data, the third overtone has been used. The mass deposition has been analyzed according to the Sauerbrey equation (eq ).[36]with f0 being the natural frequency of the quartz crystal (eq ), ρq being the density of the quartz material (ρq = 2.648 g/cm3) and μq being the shear modulus (μq = 2.947 × 1011 g/cm/s2), and dq being the thickness of the quartz crystal. The material constant C (C = 17.7 ng/cm2·s) refers to a mass deposition of 17.7 ng onto an area of 1 cm2 causing a frequency shift of 1 Hz for a 5 MHz quartz crystal. The calculation of the theoretical mass depositions is given in Supporting Information chapter 1.

Results and Discussion

Structural Properties of the Used Lipids

OO4 (Figure ) is a lysine-based amino-functionalized peptide-mimicking lipid, which was synthesized for polynucleotide delivery. The term peptide-mimicking is based rather on the special aggregation behavior which includes the formation of hydrogen bond networks in a peptide-like manner, than on the fact that it contains l-lysine (see Figure ).[37,38] The hydrophobic domain is unsaturated and composed of two oleyl chains (C18:1) because they are known for high transfection efficiency. OO4 has a branched tris(2-aminoethyl)amine spacer and three primary amine groups leading to a pKa value of 6.[32] The transfection capacity of OO4 alone is moderate, but the mixing with a co-lipid increases the transfection efficiency. As co-lipids, the unsaturated DOPE and the saturated DPPE were used. The OO4/co-lipid ratio of 1:3 (n/n) was used in the following experiments because this was found to be the best ratio in transfection experiments.[27] The pKa values of the used lipids as well as DNA are given in Table .

Figure 1

Chemical structures of N-{6-amino-1-[N-(9Z)-octadec-9-enylamino]-1-oxohexan-(2S)-2-yl}-N′-{2-[N,N-bis(2-aminoethyl)amino]ethyl}-2-[(9Z)-octadec-9-enyl]propandiamide OO4, 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine DOPE, and 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine DPPE. The structural features of OO4 are indicated. The primary amines are neglected in the H-bond donor/acceptor indication because the characteristic depends on the protonation state, and they are also interacting with the DNA during lipoplex formation.

Table 1

pKa Values

	(apparent) pKa value
lysine (conjugated)	10.4 (ε-amino),[39,40] 6.1–9.1 (α-amino)[40]
OO4	6[32]
DOPE	zwitterionic (PO4– and NH3+)
DPPE	zwitterionic (PO4– and NH3+)
DNA	9.6 (guanine),[41] 10.5 (thymine)[41]

Monolayer Experiments

Langmuir Isotherms

Measuring compression isotherms of Langmuir monolayers gives first information about area requirements. OO4, DOPE, DPPE, OO4/DOPE (1:3), and OO4/DPPE (1:3) monolayers were spread on buffer (pH 3) and on the same buffer containing 0.1 mM calf thymus DNA at 20 °C. The π–A-isotherms are presented in Figure A,B. The buffer and the pH were chosen because the lipid OO4 is fully protonated at this pH value, and hence, we have no additional effects of different protonated lipid species. Although pH 3 is far from biological pH values, it is very useful to understand interactions of OO4 with the co-lipids and/or DNA.

Figure 2

π–A-isotherms of (A) OO4 (black), DOPE (red), and DPPE (green) on bromide-containing buffer (pH 3) at 20 °C. (B) OO4 (black), OO4/DOPE (1:3), (blue) and OO4/DPPE (1:3) (orange) on bromide-containing buffer (straight lines) and the corresponding pressure/area isotherms on calf thymus DNA (1 mMnucleotides) in bromide-containing buffer (pH 3) at 20 °C (dashed-dotted lines). (C) Adsorption isotherms of ct-DNA to OO4 monolayers at pH 3 (black), 7 (red), and 10 (blue) (bromide-containing buffer, 2 mM) at 20 °C.

OO4 is in a liquid-expanded state along the π–A-isotherm (Figure A) because of its unsaturated oleyl chains (C18:1). The results are in agreement with IRRAS (Figure A). DOPE contains also two oleyl chains and is therefore also in the liquid-expanded phase state. Since the head group of DOPE is smaller than the one of OO4, DOPE occupies a smaller in-plane area. Unlike the zwitterionic DOPE, the phase behavior of OO4 depends strongly on the pH value. At pH 3, the OO4 molecules have three charged amine groups leading to electrostatic repulsions,[32] which result in large molecular areas. DPPE is in the liquid-condensed phase state already at close to zero pressures (re-sublimation). Because of strong van-der-Waals interactions of the saturated hexadecyl chains (C16:0), DPPE is tightly packed and occupies the smallest area per molecule (Figure A). With the area per molecule at 30 mN/m we can calculate the form factor of DPPE resulting in a value of 1.09 [using an A (30 mN/m) = 40.9 Å2 and the formulas presented in the Supporting Information of an already published article].[65] This value proves the assumed cylindrical shape of DPPE.

Figure 4

(A) Phase state of OO4 on bromide-containing buffer (filled symbols) and 0.1 mM calf thymus DNA-containing bromide buffer (2 mM) (empty symbols) at different pH values (pH 3—black, pH 7—red, and pH 10—blue)—Lorentzian fit maximum as position of the νasym CH2 band (20 °C, s-polarized light, incidence angle of 40°) vs lateral pressure and (B) attached amount of calf thymus DNA to a Langmuir monolayer of OO4—integral of νasym PO2– band versus area per molecule from Langmuir isotherms. Dashed lines are linear fits.

The binary mixture OO4/DOPE (1:3) is in the liquid-expanded phase state, but requires smaller molecular areas than OO4 alone. The isotherm of OO4/DOPE (1:3) looks similar to the isotherm of DOPE (compare Figure A,B). The addition of DPPE to OO4 changes the phase state. The stiff DPPE enhances the rigidity of the OO4/DPPE (1:3) monolayer (compare Figure A,B). OO4/DPPE (1:3) is in the liquid-condensed phase state with smaller molecular area than OO4 and OO4/DOPE (1:3).

The addition of DNA to the subphase changes the behavior. Surface pressure and IRRAS experiment with the DNA-containing subphase in the absence of cationic lipids show that the DNA will not appear at the air–water interface within 12 h. However, DNA adsorbs at lipid monolayers. The adsorbed DNA has different effects on the lipid monolayers. In case of OO4 + DNA (at pressures above 13 mN/m) and OO4/DOPE (1:3) + DNA, the area per molecule even decreases compared with the same monolayers in the absence of DNA. Furthermore, the slope of the isotherms changes drastically, and a surface pressure of 25 mN/m is hardly reachable. This indicates that there is no stable Langmuir monolayer anymore and most likely some material dives into the subphase (hypothesis: lipoplex formation in bulk) upon compression. A comparable behavior was described for the assembling of lipoplexes at the air–water interface.[42] In contrast, OO4/DPPE (1:3) + DNA and OO4 + DNA (at pressures below 13 mN/m) show an increase in the required area per molecule compared with OO4/DPPE (1:3) and OO4. At 30 mN/m (lateral pressure in biological membranes),[43] the difference in area for OO4/DPPE (1:3) in the presence and absence of DNA is about 29 Å2 per molecule, which should be the area occupied by DNA segments adsorbing to the air–water interface and penetrating the OO4/DPPE monolayer. Furthermore, there is a kink in the π–A-isotherms of OO4 + DNA and OO4/DPPE (1:3) + DNA at 10.6 mN/m and 31.5 mN/m, respectively. IR-spectra clearly indicate that OO4 + DNA remains in the fluid phase state as characterized by high wavenumbers (asym-CH2 stretching vibrations ∼2927–2926 cm–1). This hints that DNA and/or lipoplexes, respectively, are squeezed out from the monolayer at high surface pressures and dive into the subphase.

pH Dependence of OO4 at the Air–Water Interface

As mentioned above, the cationic lipid OO4 is sensitive to the pH; therefore, its behavior in the absence of the co-lipids was studied at different pH values. Figure A illustrates the isotherms of OO4 at three different pH values. The inset (Figure A) shows the methylene stretching modes for a pH titration in IRRAS experiments. Because of the two unsaturated oleyl chains (C18:1), OO4 remains in the fluid phase (gauche conformation indicated by the typical large wavenumbers) even at high pH values (uncharged head group), whereas the head group protonation changes drastically with the pH values (Figure B). At pH 3, OO4 is fully protonated, and at pH 8, OO4 is completely unprotonated.

Figure 3

pH dependence of OO4 on bromide-containing buffer (2 mM, different pH values) at 20 °C. (A) Molecular area and position of the symmetric and asymmetric methylene stretching bands at π = 30 mN/m and (B) protonation state of OO4 at π = 5 mN/m (○) and π = 30 mN/m (■). For experimental details, see Tassler et al.[32]

Because the protonation state is a key parameter for the interaction between DNA and cationic lipids, the pH value must have a huge influence on the adsorption of DNA to OO4 monolayers. The adsorption of ct-DNA to OO4 monolayers has been measured at pH 3, 7, and 10 (Figure C). In fact, DNA is a polyelectrolyte and its protonation state also strongly depends on the pH value. At physiological pH 7.4, DNA is negatively charged because of the phosphate backbone (PO2–). In contrast, the cationic lipid is weakly protonated at pH 7.4 (only 59% of the molecules carry one positive charge). At the investigated pH of 7, 94% of the lipids have only a single protonated amine group (theoretically three amine groups can be protonated if the tertiary amine is excluded) available to interact electrostatically with DNA (see Figure B). At pH 10, the lipids are completely unprotonated, whereas DNA is close to strand separation. At higher pH values, guanine and thymine will be deprotonated and exist as negatively charged conjugated bases.[44] On the contrary, DNA is weakly charged at pH 3 because its (PO2–) groups will be balanced by protons (H+). Lowering the pH value will induce hydrolysis and result in denaturation of DNA because the hydrogen bonds between the corresponding bases (A=T and G≡C) are going to break because of electrostatic repulsion of the positive charges. At pH 3, OO4 is almost fully protonated (see Figure B).

The resulting change in the surface pressure (Δπ = π60min – πinitial) due to the adsorption of ct-DNA after 60 min is given in Table . At pH 7, the negatively charged DNA is attracted by the weakly protonated OO4 because of electrostatic interactions between the phosphate diester backbone (PO2–) of DNA and the amine groups (R-NH3+) in the lipid head group. The surface pressure increases strongly during adsorption until an adsorption–desorption equilibrium is established. Interestingly, even at pH 10, at which the OO4 is unprotonated, a Δπ comparable to that at pH 7 was observed (Table ), indicating an interaction with DNA. The increasing surface pressure can be explained by two scenarios: (i) the DNA partially penetrates into the lipid monolayer[37] or (ii) the DNA triggers the protonation of the lipids at the interface, resulting in increased space requirements for the lipid head groups.[45] Surprisingly, at pH 3 only a small increase in the surface pressure is detected (Δπ = 1.7 mN/m). Therefore, it seems that DNA does not occupy additional space at the surface or trigger conformational changes in the lipid leading to increased area requirement per molecule. A change in the protonation state of the lipids can also be excluded because the lipid is at pH 3 nearly completely positively charged. Most-likely, DNA segments are aligned underneath the monolayer by the protonated amine groups of OO4.

Table 2

Increase in Surface Pressure (Δπ) Because of Calf Thymus DNA Adsorption to OO4 Monolayers on Buffers with Different pH Values at 20 °Ca

ct-DNA containing bromide ion-based buffer	πinitial [mN/m]	Δπ [mN/m]
pH 3	8.1	1.7
pH 7	9.2	11.8
pH 10	9.1	11.2

The adsorption isotherms are given in Figure .

The position of the methylene stretching vibrational modes gives information about the phase state of the lipids. OO4 is in the fluid phase with most oleyl chain segments in gauche conformation on the bromide-containing buffers at pH 3, 7, and 10 as can be seen in Figure A (asymmetric methylene stretching vibration above 2924 cm–1). Selected IRRA-spectra can be found in the Supporting Information, Figure S1A. Upon compression, the wavenumbers decrease only slightly, meaning that OO4 remains in the fluid phase state even at high surface pressures. The adsorption of DNA does not influence the phase state of OO4 strongly. At pH 3, the wavenumbers increase by about 1 cm–1, indicating a slight increase in fluidity of the OO4 monolayer. At pH 7 and 10, the wavenumbers are marginally lower than those for OO4 without adsorbed DNA but within the error of ±0.2 cm–1. We also get information about the amount of adsorbed DNA from IRRAS evaluating the phosphate modes. For OO4 on all DNA-containing buffers, the νasym PO2– band is at 1220 cm–1 which implies strong hydration of the DNA phosphate groups and the presence of hydrogen bonds (see the Supporting Information, Figure S1B).[46] The relative amount of attached calf thymus DNA to the lipid monolayers of OO4 on the bromide ion-based buffers at pH 3, 7, and 10 has been estimated by the integration of the antisymmetric PO2– stretching vibration band after subtraction of the spectra of DNA solutions at the corresponding pH values. The intensity plots of the νasym PO2– band versus the area per molecule of the corresponding lipids are given in Figure B. The relative amount of attached DNA to the OO4 monolayer strongly depends on the pH values of the subphase. As expected, the largest amount of DNA is attached to OO4 at pH 3, at which OO4 has the highest protonation state. The increase of the pH value results in a lower protonation degree of OO4 and therewith the amount of attracted DNA decreases. At pH 10, the relative amount of attached DNA is very small. At pH 3 and 7, the amount of adsorbed DNA increases with decreasing molecular areas (increasing charge density) upon compression.

In summary, the IRRAS experiments demonstrate that OO4 can bind DNA efficiently in a wide pH range from pH 3 to 7. The adsorption of DNA at monolayers composed of the lipid mixtures was not investigated because the phospholipids (co-lipids) contain also phosphate in the head group what makes it more difficult to quantify the amount of DNA. Therefore, quartz microbalance experiments have been performed to answer this open question in bulk experiments.

Bulk Experiments

Quantification of Adsorbed DNA by QCM Experiments

The QCM-D technique has been used to quantify the amount of DNA, which attaches to mixed lipid bilayers. The experiments have been performed at pH 3. The obtained results and calculated theoretical mass depositions are summarized in Table . The zwitterionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC, C14:0 alkyl chains, Tm = 24 °C) was used as a reference. The obtained frequency shift of DMPC amounts to 26 Hz (see Figure S2, the Supporting Information), which clearly indicates the formation of a supported lipid bilayer (SLB).[47,48] As already described in the literature, the formation of supported DMPC bilayers at pH 3 takes place without the observation of a critical coverage.[48] The resulting mass deposition is higher than the expected theoretical mass. The difference (19 ng/cm2) is most-likely based on 3–4 adsorbed water molecules (strongly hydrated phosphocholine head group). From NMR measurements it is known, that one DMPC molecule binds 4–5 water molecules in the Lβ′ phase state.[49]

Table 3

Frequency Shift (Δf) and Resulting Mass Deposition (Δm) Compared with Theoretical Mass

sample	remarks	Δf [Hz]	Δm [ng/cm2]	theoretical mass [ng/cm2]
DMPC	reference	26	460	441
ct-DNA	1 base pair			25.7
OO4/DOPE (1:3)		28	496	469
OO4/DOPE (1:3) + ct-DNA		16	283
OO4/DPPE (1:3)		16.5	292	601
OO4/DPPE (1:3) + ct-DNA		15.5	274

The supported OO4/DOPE 1:3 (n/n) bilayer is also formed without observing a critical packing density of vesicles on the surface (Θc) (see Figure A). Its theoretical mass amounts to 469 ng/cm2. However, the measured frequency shift (Δf = 28 Hz) results in a mass deposition of 496 ng/cm2. The mass difference of 27 ng/cm2 is due to 5 water molecules attached to the SLB. Specular X-ray reflectivity (XRR) data of OO4 monolayers support the assumption of hydrated amino head groups (see XRR curves Figure S3 and Table S1, the Supporting Information). Also, DOPE can bind up to 12 water molecules in the liquid-crystalline phase state.[50] Adding DNA to the OO4/DOPE SLBs causes a frequency shift of 16 Hz. The mass of adsorbed DNA amounts to 283 ng/cm2 (∼2.58 × 1011 kbp DNA per cm2OO4/DOPE bilayer equivalent to 2.5 base pairs per 100 Å2). The Langmuir isotherm of the OO4/DOPE mixture at 30 mN/m indicates that one molecule needs 60 Å2 (isotherm on the pure buffer without DNA). This value translates into 1.5 base pairs per lipid molecule.

Figure 5

Δf(t) of (A) OO4/DOPE 1:3 and (B) OO4/DPPE 1:3 (1 mg/mL) in bromide-containing buffer (pH 3) and calf thymus DNA (0.1 mM)-containing bromide buffer (pH 3) at 20 °C. The 3rd overtone is shown.

A significantly lower mass deposition was observed for OO4/DPPE (see Figure B) with only a small shift in frequency of 16.5 Hz. The resulting mass deposition amounts to 292 ng/cm2. Compared with the theoretical value (mtheoretical = 601 ng/cm2), the small Δm-value implies a lipid monolayer or rather lipid bilayer patches of OO4/DPPE on the quartz crystal instead of a SLB. If we assume the latter case, roughly half of the area is covered with lipid bilayer patches. The adsorption of calf thymus DNA to this system caused a frequency shift of 15.5 Hz. The measured mass deposition of DNA to the OO4/DPPE bilayer patches (Δm = 274 ng/cm2) is marginally smaller than that observed for OO4/DOPE (Δm = 283 ng/cm2). Assuming, that the cationic charged lipid patches bind the highest proportion of the DNA, ∼5.16 × 1011 kbp DNA per cm2 is attached to OO4/DPPE bilayers and therewith roughly 2 base pairs per lipid molecule (using a molecular area of 40 Å2 at 30 mN/m). This assumption is justified by the work of Vandeventer et al.; they demonstrate that only small amounts of DNA attach to the untreated silica-covered quartz crystal.[66] Consequently, ct-DNA more likely attaches to positively charged OO4/DPPE patches than to negatively charged silica.

Although we investigated the adsorption of DNA at pH 3 and the OO4/DPPE bilayer did not cover the whole area of the crystal, we can assume that OO4/DPPE bilayers can bind a higher amount of DNA compared with OO4/DOPE bilayer. If we assume that OO4 carries the same charge in both mixtures, the charge density is higher for the OO4/DPPE 1:3 mixture because the lipids need less space compared with the OO4/DOPE 1:3 mixture. A surface with a higher charge density should attract more DNA.

Protonation State of OO4 in Bulk

The protonation state of the cationic lipid OO4 in vesicles (bulk system) has been estimated by pH-dependent ζ-potential measurements. The obtained titration curve is presented in Figure . The zeta potential is the potential at the shear plane of the ion cloud surrounding a particle, consequently the real surface charge (Nernst potential) is not accessible. Furthermore, the zeta potential is used to estimate the colloidal stability of a system. For liposomes, a ζ-potential above ±30 mV has been reported for colloidal stable systems.[51]OO4 has a positive ζ-potential (>30 mV) below pH 10. The isoelectric point at pH 11 leads to the first assumption that the lipids in the vesicle are partially charged up to this pH-value. The ζ-potential is constant between pH 3 and 7, whereas the protonation state in the monolayer system decreases from pH 3 to 7 (compare Figures B and6). An increased ζ-potential is observed between pH 8 and pH 9. However, it is very unlikely that the protonation state (positive charge) increases. Above pH 9, the ζ-potential decreases continuously. Consequently, the monolayer experiments (especially total reflection X-ray fluorescence)[32] give exact information about the protonation state of OO4 in contrast to the ζ-potential. Nevertheless, the zeta potential curve is needed to better understand the structural investigations of the 3D systems.

Figure 6

ζ-potential titration curve of OO4 in 20 mM NaCl at different pH values. The pH was adjusted by adding NaOH or HCl. Dashed lines are for guiding the eyes only.

Small- and Wide-Angle X-ray Scattering in Nonphysiological HBr Buffer

At 25 °C, OO4 arranges in multilamellar bilayers in the bromide-containing buffer pH 10 (d = 55.9 Å, Figure A). The alkyl chains are in the liquid-crystalline phase (Figure B). The lamellar phase has a high correlation length, and the reflexes are visible up to the 4th order. In contrast, the SAXS pattern of OO4 in bromide-containing buffer pH 3 is poorly resolved. A broad peak is observed indicating noncorrelated lamellar bilayers (d = 55.1 Å), an effect of the electrostatic repulsion (ζ-potential ∼ 43 mV, see Figure ). The correlation length is extremely small. Additionally, the small peaks at low q values indicate the coexistence of a cubic mesophase. The Bragg peaks are in the ratio √3:√4:√8:√11:√12:√16:√19:√20:√24:√27 and characterize a micellar cubic phase with Fm3m symmetry (Qα225, see Supporting Information Table S2). The cubic Fm3m lattice parameter amounts to 283.3 Å. The characteristic cubic lattice parameter a is the slope of the linear function passing through the origin (0, 0). Here, h, k, and l are Miller indices of the cubic lattice, and the slope is equal to 1/a.[52] The appearance of the micellar cubic phase indicates that OO4 has a conical shape, meaning that the highly charged head group needs more space than the fluid alkyl chains.

Figure 7

(A) SAXS and (B) WAXS of OO4 in bromide-containing buffer pH 3 (black line) and pH 10 (red line) at 25 °C.

OO4 was mixed with either DOPE or DPPE to adjust the liposome properties such as charge density and phase behavior. DOPE forms inverted hexagonal cylinders because of the inverse conical molecular shape (the fluid chains require more space than the head group). DPPE forms multilamellar vesicles. For the mixtures with calf thymus DNA, the N/P-ratio of 4 was chosen because it showed the best performance in the transfection experiments.[27]

At pH 3 and 25 °C, DOPE has a hexagonal lattice parameter a equal to 74.4 Å (Supporting Information Figure S4A). The mixing of DOPE and OO4 in the ratio 1:3 results in phase separation as can be seen in Figure A. The lamellar phase LOO4 (d = 53.4 Å) corresponds most likely to phase-separated OO4 because pure OO4 arranges in multilamellar bilayers with a similar d-value (55.1 Å). Additionally, there are Bragg peaks in the SAXS pattern in a ratio of √6:√8:√14:√16:√20:√22:√24:√34:√41, indicating an inverted cubic Ia3d phase (Qα230, so-called Gyroid (G) minimal surface, see Table S3).[53,54] The cubic Ia3d lattice parameter amounts to 187.6 Å. The cubic phase contains most likely DOPE and OO4 for three reasons: First, because the Ia3d phase is usually located between lamellar bilayers with chains in the liquid-crystalline state (Lα like OO4) and inverted hexagonal phases (HII like DOPE) in the phase diagram.[55] Second, no phase-separated DOPE can be detected. Third, for OO4 without co-lipid a cubic phase of the Fm3m symmetry coexisting with the lamellar phase was found. Adding DNA to the system OO4/DOPE 1:3 (n/n) leads to further coexisting phases (Figure B). The cubic phase disappears, while the lamellar phase remains, and two hexagonal phases appear (peak ratio √1:√3:√4:√7:√9:√12:√13:√16:√21). The lamellar phase LOO4 (d = 51.5 Å) is most probably formed by phase-separated OO4 (d = 55.1 Å) or it is an OO4-rich phase, while the hexagonal phase HDOPE (a = 68.2 Å) contains most likely phase-separated DOPE (a = 74.4 Å) or is a DOPE-rich phase. Therefore, an inverse hexagonal structure is very likely. Nevertheless, DNA, with a double helix diameter of 20 Å,[56] seems not to be incorporated in these two phases. The DNA is most probably complexed inside the tubes of a hexagonal lipoplex indexed as HDNA. Because the lattice parameter of HDNA is 138.1 Å and therewith much larger as the a-value determined for HDOPE, it is reasonable to assume that the DNA is completely incorporated in the HDNA phase. If the HDNA phase is an inverted hexagonal or a honeycomb arranged lipoplex structure cannot be determined by the pattern. A DOPE-rich lipid mixture could lead to the first described structure because of the inverse conical shape of DOPE,[20] whereas an OO4-rich mixture would lead to the honeycomb hexagonal lipoplex because of the conical shape of OO4.[57] The alkyl chains in all three phases are in the liquid-crystalline phase state.

Figure 8

SAXS pattern of (A) OO4/DOPE (black line), (B) OO4/DOPE + DNA (red line) in comparison with OO4/DOPE (black line), (C) OO4/DPPE (black line) and (D) OO4/DPPE + DNA (red line) in comparison with OO4/DPPE (black line) at 25 °C in bromide-containing buffer pH 3. All lipids were mixed in the molar ratio OO4/co-lipid 1:3. The samples with DNA were complexed in a N/P-ratio of 4.

The mixtures with DPPE show a different behavior. At pH 3 and 25 °C, the multilamellar DPPE is in the gel phase state with chains in all-trans conformation (d = 54.9 Å) (Supporting Information Figure S4). The 1:3 mixture of DPPE and OO4 arranges in multilamellar bilayers (d = 54.8 Å, see Figure C). The shoulder at the first reflex might indicate a phase separation into two lamellar phases (OO4-rich and DPPE-rich), but the higher order peaks are missing to support this assumption. Adding DNA to the OO4/DPPE mixture leads to the appearance of two lamellar phases which can be evaluated separately as shown in Figure D. At least one phase has tilted chains in gel state (orthorhombic packing of the chains indicated by two peaks in the WAXS region). One lamellar phase (named as LOO4/DPPE, d = 54.8 Å) is comparable with the structure found for the OO4/DPPE mixture. The second phase (named as LDNA) has a higher d-value (d = 64.6 Å) and incorporates most probably the DNA. The broad peak at q = 2.47 nm–1 corresponds to a DNA–DNA in-plane distance of 25.5 Å. In the LDNA lipoplex the DNA is complexed sandwich-like between the lipid bilayers and the DNA rods align in a 1D lattice.[58] With the assumption that the DNA rod diameter is 20 Å,[56] the observed DNA–DNA in-plane distance of 25.5 Å indicates a tight packing. Obviously, the high positive charge density of the lipid composition compensates the repulsive forces between the DNA strands. This tight packing means also that the OO4/DPPE mixture has a high DNA loading capacity, even with the coexisting empty lamellar phase. This observation is in good agreement with the QCM experiments presented above.

Small- and Wide-angle X-ray Scattering in MES Buffer

The X-ray experiments were additionally performed in the MES buffer to allow the comparison with the biological experiments.[27] The SAXS and WAXS patterns of OO4, DOPE, and DPPE at 25 °C and pH 6.5 are presented in Figure A,B, respectively. The cationic lipid OO4 is most probably in the lamellar mesophase with its oleyl chains in the liquid-crystalline phase state (Lα). The Bragg peaks are very broad, indicating a structure with many defects. Most likely, the electrostatic repulsions between the charged head groups are responsible for the small correlation length (ζ-potential ∼ 43 mV, Figure ). The estimated d-value amounts to 53.1 Å. Comparable to the results at pH 3, DOPE arranges in inverted hexagonal cylinders with a lattice parameter of 74.2 Å. DPPE forms multilamellar vesicles with tilted chains in the gel state (Lβ′, d = 56 Å). The Bragg peak at qH = 13 nm–1 can be associated with intermolecular hydrogen bonds.

Figure 9

(A) SAXS and (B) WAXS of the single lipids OO4 (black line), DOPE (red line) and DPPE (green line), (C) SAXS and (D) WAXS of OO4/DOPE (black line) and OO4/DOPE + DNA (red line), (E) SAXS and (F) WAXS of OO4/DPPE (black line) and OO4/DPPE + DNA (red line) in MES buffer (pH 6.5) at 25 °C. The samples with DNA were complexed in a N/P-ratio of 4. At 20 and 37 °C, comparable diffraction patterns were measured (Supporting Information Figures S5–S7). (F) Observed peaks can be indexed as (10), (01), and (1–1).

The 1:3 mixture of OO4 and DOPE gives a broad signal in the SAXS region (Figure C) and a halo in the WAXS region (Figure D). Because no distinct peaks corresponding to DOPE can be seen in the SAXS pattern, OO4 and DOPE seemed to mix well arranging in a Lα mesophase. Adding DNA to the OO4/DOPE mixture leads to the formation of two phases: a micellar cubic phase (Qα223) and lamellar phase (Lα). Because of the large Pm3n lattice parameter (a = 353.4 Å, reflections at √13 and √17 are missing), DNA is most likely complexed within the cubic mesophase. The lamellar phase consists of phase-separated OO4/DOPE (LαOO4/DOPE, d = 67.3 Å), but we cannot safely exclude that it does not contain DNA. Lipids with a comparable backbone to OO4 can form cubic lipoplex structures. In a recently published study, TT10 and OT10 were found to form cubic Im3m lipoplexes with DOPE in the ratio 1:4 complexed with calf thymus DNA (N/P = 4).[38] Furthermore, lipid 6, a malonic acid diamide of the first generation, forms cubic Im3m lipoplexes after mixing with DOPE.[64]

At 25 °C and pH 6.5, the cationic lipid OO4 and the zwitterionic lipid DPPE form multilamellar phases in 1:3 mixtures. The repeating distance equals 56.1 Å (Figure E). The three Bragg peaks at q10 = 15 nm–1, q01 = 15.4 nm–1, and q1–1 = 16.5 nm–1 in the WAXS region (Figure F) characterize an oblique chain lattice. The additional peak at qH = 13 nm–1 can be assigned to intermolecular hydrogen bonds. Adding DNA to the OO4/DPPE liposomes leads to the appearance of two phases. The Bragg peaks in the SAXS region imply a coexistence of two lamellar mesophases, namely, LOO4/DPPE (d = 56.1 Å) and LDNA (d = 75.7 Å). The latter one describes the lamellar lipoplex with DNA entrapped between lipid bilayers. The difference in the spacing amounts to ∼20 Å,[56] which is the diameter of the DNA double helix. The WAXS region does not change. The three Bragg peaks indicating an oblique unit cell and the peak characterizing H-bonds are still present. The signals seem not to be superimposed by a halo of lipids in the liquid-crystalline phase, what implies that the mixture OO4/DPPE forms rarely described lamellar lipoplexes with lipids in the gel state and not the typical Lαc phase.[58]

Discussion

The presented work helps to understand the characteristics of both lipid mixtures in biological systems. OO4/DOPE 1:3 is the more effective nucleic acid carrier, which is even stable in the presence of biological relevant substances (serum proteins, salt, glucosaminoglycans).[27] The structural approach made in this study leads to the assumption, that the structures formed by the lipid formulations after complexing DNA determine the properties rather than charge density and capacity to bind DNA. If we assume that the charge state of OO4 in the OO4/DOPE 1:3 and the OO4/DPPE 1:3 mixtures is the same, the space requirement of the second mixture is smaller (see monolayer experiments) and therewith the charge density is higher. Therefore, the DNA binding capacity of the OO4/DPPE 1:3 mixture must be higher compared with the OO4/DOPE 1:3 mixture. A higher charge density would also imply a more stable complexation of DNA and therewith a higher stability against disassembling promoted by salts, anionic proteins, and anionic polycarbonates. In reality, we have observed the opposite behavior.[27] The reason seems to be the different structures of both lipid mixtures complexed with DNA. OO4/DOPE 1:3 forms cubic Pm3n structures at pH 6.5 which complex DNA, a lipoplex structure known to be fusiogenic. Such structures promote cellular entry and endosomal escape.[59−61] However, there is a higher risk for disassembling by anionic molecules compared with the lamellar lipoplex structures. The SAXS pattern also proves a coexisting lamellar phase in OO4/DOPE 1:3 lipoplexes at pH 6.5. If the lamellar phase surrounds the cubic lipoplex structures, a highly protective effect against the anion-induced lipoplex disassembling would occur. This effect was observed in biological experiments supporting the assumption. Also freeze-fracture TEM images show only spherical structures,[27] and dried samples of negatively stained TEM exhibit only aggregated spherical structures with a certain substructure (Figure S8, the Supporting Information). Therefore, we propose a cubic complexing DNA mesophase which is surrounded and therewith protected by multilamellar structures (Figure ). Lowering the pH to 3 results in hexagonal lipoplexes and additional lamellar as well as hexagonal structures. Hexagonal mesophases are highly fusiogenic if they adopt the inverse hexagonal arrangement because of the high content of DOPE.[11] The stable lipoplex structure, preformed at pH 6.5, becomes unstable during the endosomal maturation to the lysosome (pH 6.5–4.5).[62] This model can explain the high serum stability and the high DNA transfer efficiency of OO4/DOPE 1:3 and further how this lipoplex formulation is able to overcome the polycationic dilemma.[27,63]

Figure 10

Proposed structures and uptake of DNA complexed with the two investigated lipid mixtures.

The OO4/DPPE 1:3 mixture forms lamellar complexes with DNA both at pH 6.5 and 3. Lamellar lipoplexes are known to be stable in the presence of serum and other polyanionic biological molecules, but because of the low interaction potential with cellular membranes, the efficiency of DNA transfer is often lower compared with nonlamellar phases.[11] Our investigations demonstrate that both the efficiency and stability in the presence of salts and polyanions are lower compared with the OO4/DOPE 1:3 mixture.[27] Consequently, the interaction between the lipid mixture and DNA is weak although the DNA binding capacity is higher in the DPPE-containing mixture compared with the DOPE-containing mixture.

Conclusion

In continuation of our previous work, the peptide-mimicking lysine-based amino-functionalized lipid OO4 was investigated in mixtures with the commonly used unsaturated co-lipid DOPE and the saturated DPPE in 2D and 3D model systems. The choice of the zwitterionic co-lipid has an enormous influence on the cationic lipid OO4 in terms of phase behavior, interaction with calf thymus DNA, and biological behavior.[27]

At the air–liquid interface, OO4 is in the liquid-expanded phase state because of its two oleyl chains independent of the pH value. Because OO4 contains three primary amino groups in the head group, its protonation state is pH-sensitive. As a consequence, OO4 is fully protonated at pH 3 and completely uncharged above pH 8. Therefore, different scenarios of DNA coupling to OO4 monolayers have been proposed for different pH values. The saturated co-lipid DPPE (C16:0) strongly increases the packing density in the OO4/DPPE monolayer in comparison to the OO4/DOPE monolayer containing two unsaturated lipids.

In bulk, the structures of different lipid mixtures in the presence and absence of DNA and at different pH values have been investigated (Table ). OO4 arranges in multilamellar bilayers with fluid chains at pH 10. Interestingly, OO4 mixes better with DPPE because both lipids form lamellar mesophases, than with DOPE, because the phase structures are different due to drastically different molecule shapes even with the same chain composition. OO4/DPPE liposomes and OO4/DPPE/DNA lipoplexes are multilamellar. The tightly complexed DNA aligns in a 1D pattern between the sandwich-like OO4/DPPE bilayers. Furthermore, OO4/DPPE bilayers have a higher capacity to bind DNA compared with OO4/DOPE.

Table 4

Structures of Liposomes and Lipoplexes in HBr-Containing Buffers (pH 3 and 10) and MES Buffer (pH 6.5) at 25 °C

	pH	mesophase	d/a [Å]	additional phase
OO4	3	cubic Fm3m (Qα225)/lamellar (Lα)	283.3/55.1
OO4	10	lamellar (Lα)	55.9
DOPE	3	hexagonal (Hα)	74.4
OO4/DOPE		cubic Ia3d (Qα230)	187.6	LOO4
OO4/DOPE/DNA		hexagonal (Hα)	138.1	HDOPE, LOO4
DPPE		lamellar (Lβ′)	54.9
OO4/DPPE		lamellar	54.8
OO4/DPPE/DNA		lamellar with 1D alignment of DNA	64.6	LOO4/DPPE
OO4	6.5	lamellar (Lα)	53.1
DOPE		hexagonal (Hα)	74.2
OO4/DOPE		lamellar (Lα)
OO4/DOPE/DNA		cubic Pm3n (Qα223)	353.4	LOO4/DOPE
DPPE		lamellar (Lβ′)	56
OO4/DPPE		lamellar	56.1
OO4/DPPE/DNA		lamellar	75.7	LOO4/DPPE

The mixing of DOPE and OO4 results in phase separation and the formation of nonlamellar mesophases. At pH 3, OO4/DOPE forms bicontinuous cubosomes (Qα230), whereas phase-separated OO4 remains in the Lα mesophase. DNA is complexed by OO4/DOPE in an inverted hexagonal lipoplex at pH 3. At pH 6.5, it is assumed that OO4/DOPE arranges in lamellar bilayers. However, the adding of DNA causes phase separation and the formation of a cubic lipoplex (Qα223). Phase-separated OO4/DOPE arranges in the lamellar mesophase with molten chains. Cubic lipoplexes encapsulated in lamellar structures have been proposed. This is the most reasonable explanation for the high stability and good efficiency of this lipoplex formulation.

The exchange of the co-lipid (DOPE → DPPE) has two major consequences for the gene transfection because of changed lipoplex structure and DNA binding capacity. The lipoplexes with DOPE are hexagonal (pH 3) and cubic (pH 6.5), whereas the lipoplexes with DPPE remain lamellar at both pH values. Nonlamellar lipoplexes are supposed to rapidly fuse with the endosomal bilayer and allow the release of DNA with high efficiency. In the lamellar lipoplex, DNA is complexed between the OO4/DPPE bilayers. DPPE increases the charge density in the mixed monolayer. However, the fusiogenic potential of the lamellar lipoplex is low, which results in poor DNA release performance.