DNA Condensation by Peptide-Conjugated PAMAM Dendrimers. Influence of Peptide Charge.

Nucleic acid delivery to cells is an important therapeutic strategy that requires the transport of nucleic acids to intracellular compartments and their protection from enzymatic degradation. This can be achieved through the complexation of the nucleic acids with polycations. Poly(amidoamine) (PAMAM) dendrimers and peptide-conjugated dendrimers have been investigated as delivery vectors. Inspired by these studies and the role of flexible peptide domains in protein-DNA interactions, we studied the impact of conjugating two peptides (tails) to generation 2 (G2) PAMAM dendrimers on DNA condensation and polyplex formation. Using gel electrophoresis, dye exclusion assays, atomic force microscopy, and Monte Carlo simulations, it is shown that the steric impact of neutral peptide tails is to hinder the formation of DNA-G2 polyplexes composed of multiple DNA chains. If the tails are negatively charged, which results in overall neutral G2 conjugates, then the interaction of G2 with DNA is hindered. Increasing the net positive charge of the tails resulted in the complexation capacity of G2 with the DNA being restored. While DNA complexation is obtained for a similar net charge balance for G2 and G2 conjugates with positive tails, fewer of the latter are required to achieve a comparable condensation degree. Furthermore, it is shown that about 40% of the DNA remains accessible to binding by small molecules. Overall, this shows that tuning the net charge of peptide tails conjugated to PAMAM dendrimers offers a handle to control the complexation capacity of DNA, which can be explored as a novel route for optimization as gene delivery vehicles.

Efficient DNA Condensation Induced by Chiral ß-Amino Acid-Based Cationic Surfactants.

Four cationic chiral amino acid-based surfactants, cis- and trans-1 and cis- and trans-2, have been studied as DNA-condensing agents with enhanced properties and the absence of cell toxicity. The polar head of the surfactant is made of a cyclobutane ß-amino acid in which the amino group is a hydrochloride salt and the carboxyl group is involved in an amide bond, allowing the link with hydrophobic C12 (surfactant 1) or C16 (surfactant 2) chains. The ability of these surfactants to condense DNA was investigated using a dye exclusion assay, gel electrophoresis, and circular dichroism and compared with the well-studied dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB). The surfactant with the longest chain length and the trans stereochemistry (trans-2) was found to be the most efficient in condensing the DNA, including CTAB. Surfactant cis-2 was found to be less efficient, probably due to its poorer solubility. The ß-amino acid surfactants with the shorter chain length behaved similarly, such that the cis/trans stereochemistry does not seem to play a role in this case. Interestingly, these were also found to induce DNA condensation for the same concentration as trans-2 and CTAB but showed a lower binding cooperativity. Therefore, a longer alkyl chain only slightly improved the effectiveness of these surfactants. Further, atomic force microscopy revealed that they compact DNA into small complexes of about 55-110 nm in diameter.

Influence of macromolecular crowding on the charge regulation of intrinsically disordered proteins.

In this work we study the coupling between ionization and conformational properties of two IDPs, histatin-5 and ß-amyloid 42, in the presence of neutral and charged crowders. The latter is modeled to resemble bovine serum albumin (BSA). With this aim, semi-grand canonical Monte Carlo simulations are performed, so that the IDP charge is a dynamic property, undergoing protonation/deprotonation processes. Both ionization properties (global and specific amino acid charge and binding capacitance) and radius of gyration are analyzed in a large range of pH values and salt concentrations. Without crowder agents, the titration curve of histatin-5, a polycation, is salt-dependent while that of ß-amyloid 42, a polyampholyte, is almost unaffected. The salt concentration is found to be particularly relevant at pH values where the protein binding capacitance (directly linked with charge fluctuation) is larger. Upon addition of neutral crowders, charge regulation is observed in histatin-5, while for ß-amyloid 42 this effect is very small. The main mechanism for charge regulation is found to be the effective increase in the ionic strength due to the excluded volume. In the presence of charged crowders, a significant increase in the charge of both IDPs is observed in almost all the pH range. In this case, the IDP charge is altered not only by the increase in the effective ionic strength but also by its direct electrostatic interaction with the charged crowders.

Role of Protein Self-Association on DNA Condensation and Nucleoid Stability in a Bacterial Cell Model.

Bacterial cells do not have a nuclear membrane that encompasses and isolates the genetic material. In addition, they do not possess histone proteins, which are responsible for the first levels of genome condensation in eukaryotes. Instead, there is a number of more or less specific nucleoid-associated proteins that induce DNA bridging, wrapping and bending. Many of these proteins self-assemble into oligomers. The crowded environment of cells is also believed to contribute to DNA condensation due to excluded volume effects. Ribosomes are protein-RNA complexes found in large concentrations in the cytosol of cells. They are overall negatively charged and some DNA-binding proteins have been reported to also bind to ribosomes. Here the effect of protein self-association on DNA condensation and stability of DNA-protein complexes is explored using Monte Carlo simulations and a simple coarse-grained model. The DNA-binding proteins are described as positively charged dimers with the same linear charge density as the DNA, described using a bead and spring model. The crowding molecules are simply described as hard-spheres with varying charge density. It was found that applying a weak attractive potential between protein dimers leads to their association in the vicinity of the DNA (but not in its absence), which greatly enhances the condensation of the model DNA. The presence of neutral crowding agents does not affect the DNA conformation in the presence or absence of protein dimers. For weakly self-associating proteins, the presence of negatively charged crowding particles induces the dissociation of the DNA-protein complex due to the partition of the proteins between the DNA and the crowders. Protein dimers with stronger association potentials, on the other hand, stabilize the nucleoid, even in the presence of highly charged crowders. The interactions between protein dimers and crowding agents are not completely prevented and a few crowding molecules typically bind to the nucleoid.

Nanoparticle-Hydrogel Composites: From Molecular Interactions to Macroscopic Behavior.

Hydrogels are materials used in a variety of applications, ranging from tissue engineering to drug delivery. The incorporation of nanoparticles to yield composite hydrogels has gained substantial momentum over the years since these afford tailor-making and extend material mechanical properties far beyond those achievable through molecular design of the network component. Here, we review different procedures that have been used to integrate nanoparticles into hydrogels; the types of interactions acting between polymers and nanoparticles; and how these underpin the improved mechanical and optical properties of the gels, including the self-healing ability of these composite gels, as well as serving as the basis for future development. In a less explored approach, hydrogels have been used as dispersants of nanomaterials, allowing a larger exposure of the surface of the nanomaterial and thus a better performance in catalytic and sensor applications. Furthermore, the reporting capacity of integrated nanoparticles in hydrogels to assess hydrogel properties, such as equilibrium swelling and elasticity, is highlighted.

pH-Dependent Polyelectrolyte Bridging of Charged Nanoparticles.

Systems comprised of polyelectrolytes and charged nanoparticles are of great technological interest, being common components in formulations among other uses. The colloidal stability of formulations is an important issue, and thus a lot of effort has been made to study the interactions of individual components in these systems. Here, the complexation and adsorption of an annealed (pH-dependent) polyelectrolyte to two spherical nanoparticles has been studied using coarse-grained Monte Carlo simulations. This has been done mainly by varying the solution pH and separation distance (concentration) between the nanoparticles. The polyelectrolyte charge distribution is seen to vary with nanoparticle separation distance, and its ability to bridge both nanoparticles changes with pH. The flexible polyelectrolyte creates compact, multilink bridges at short nanoparticle separation distances and evolves to a stretched single-link bridge at longer distances, where a larger fraction of the polyelectrolyte wraps around the nanoparticles. The annealed polyelectrolyte is also compared with a quenched polyelectrolyte of similar fixed fractional charge. Here, a difference is found in the adsorption ability at low pH/ionization due to the ability of the annealed polyelectrolytes to concentrate charges in the vicinity of the nanoparticle. At intermediate polyelectrolyte charge fractions and with increasing nanoparticle separation distances, the annealed system is able to link nanoparticles at larger distances as compared to the quenched, in good agreement with experimental observations. The results in this work contribute to the understanding of the effect of annealed polyelectrolytes and pH variations in the phase behavior of polyelectrolyte-nanoparticle systems, potentially aiding in the design and optimization of pH-responsive systems.

Adsorption of charged macromolecules upon multicomponent responsive surfaces.

Adsorption of polyions onto charged surfaces has long been recognized as a crucial phenomenon in biological and technological applications. An intuitive model relating polyelectrolyte adsorption with the imposed features of polarizable surfaces of different compositions and charges is proposed based on Monte Carlo simulations using a coarse-grained approach. The excellent performance of the equation allows simultaneously describing a wide range of adsorption regimes and accounting for specific non-monotonic trends. For a constant surface charge density, the surface composition governs adsorption, promoting variations exceeding 100%. Adsorption increases with the number of attractive charges in the surface until reaching a maximum, decreasing thereafter due to the presence of polyanion-like charged particles. The presence of crowders hampers adsorption. These results can be used to efficiently predict and modulate the interaction between charged macromolecules and different substrates with direct implications in de novo designs of vehicles and biomedical devices.

In vitro studies of DNA condensation by bridging protein in a crowding environment.

The macromolecules of the bacterial cell occupy 20-40% of the total cytosol volume, and crowded environments have long been known to compact and stabilize DNA. Nevertheless, investigations on DNA-protein binding are generally performed in the absence of crowding, which may yield an incomplete understanding of how nucleoid-assembling proteins work. A family of such proteins, abundant in Gram-negative bacteria, is the histone-like nucleoid structuring proteins (H-NS). Herein, the synergistic role of macromolecular crowding (mimicked using polyethylene glycol, PEG) and H-NS was investigated using fluorescence correlation spectroscopy (FCS) and enzyme protection assays. We show that crowding enhances the binding of H-NS to the AT-rich tracks of the DNA, where it preferentially binds to, protecting these tracks towards enzyme digestion, inducing some DNA condensation, and inhibiting the biological function of DNA. We further suggest that the looping of DNA chains, induced by H-NS, contributes to the synergistic effect of DNA-binding protein and crowding agents, on DNA condensation.

Combining polyethylenimine and Fe(III) for mediating pDNA transfection.

BACKGROUND: The potential use of Fe(III) ions in biomedical applications may predict the interest of its combination with pDNA-PEI polyplexes. The present work aims at assessing the impact of this metal on pDNA complex properties. METHODS: Variations in the formation of complexes were imposed by using two types of biological buffers at different salt conditions. The incorporation of pDNA in complexes was characterised by gel electrophoresis and dynamic light scattering. Transfection efficiency and cytotoxicity were evaluated in HeLa and HUH-7 cell lines, supported by flow cytometry assays. RESULTS: Fe(III) enhances pDNA incorporation in the complex, irrespective of the buffer used. Transfection studies reveal that the addition of Fe(III) to complexes at low ionic strength reduces gene transfection, while those prepared under high salt content do not affect or, in a specific case, increase gene transfection up to 5 times. This increase may be a consequence of a favoured interaction of polyplexes with cell membrane and uptake. At low salt conditions, results attained with chloroquine indicate that the metal may inhibit polyplex endosomal escape. A reduction on the amount of PEI (N/P 5) formed at intermediary ionic strength, complemented by Fe(III), reduces the size of complexes while maintaining a transfection efficiency similar to that obtained to N/P 6. CONCLUSIONS: Fe(III) emerges as a good supporting condensing agent to modulate pDNA-PEI properties, including condensation, size and cytotoxicity, without a large penalty on gene transfection. GENERAL SIGNIFICANCE: This study highlights important aspects that govern pDNA transfection and elucidates the benefits of incorporating the versatile Fe(III) in a gene delivery system.

Complexes formed between DNA and poly(amido amine) dendrimers of different generations--modelling DNA wrapping and penetration.

This study deals with the build-up of biomaterials consisting of biopolymers, namely DNA, and soft particles, poly(amido amine) (PAMAM) dendrimers, and how to model their interactions. We adopted and applied an analytical model to provide further insight into the complexation between DNA (4331 bp) and positively charged PAMAM dendrimers of generations 1, 2, 4, 6 and 8, previously studied experimentally. The theoretical models applied describe the DNA as a semiflexible polyelectrolyte that interacts with dendrimers considered as either hard (impenetrable) spheres or as penetrable and soft spheres. We found that the number of DNA turns around one dendrimer, thus forming a complex, increases with the dendrimer size or generation. The DNA penetration required for the complex to become charge neutral depends on dendrimer generation, where lower generation dendrimers require little penetration to give charge neutral complexes. High generation dendrimers display charge inversion for all considered dendrimer sizes and degrees of penetration. Consistent with the morphologies observed experimentally for dendrimer/DNA aggregates, where highly ordered rods and toroids are found for low generation dendrimers, the DNA wraps less than one turn around the dendrimer. Disordered globular structures appear for high generation dendrimers, where the DNA wraps several turns around the dendrimer. Particularly noteworthy is that the dendrimer generation 4 complexes, where the DNA wraps about one turn around the dendrimers, are borderline cases and can form all types of morphologies. The net-charges of the aggregate have been estimated using zeta potential measurements and are discussed within the theoretical framework.

DNA compaction induced by a cationic polymer or surfactant impact gene expression and DNA degradation.

There is an increasing interest in achieving gene regulation in biotechnological and biomedical applications by using synthetic DNA-binding agents. Most studies have so far focused on synthetic sequence-specific DNA-binding agents. Such approaches are relatively complicated and cost intensive and their level of sophistication is not always required, in particular for biotechnological application. Our study is inspired by in vivo data that suggest that DNA compaction might contribute to gene regulation. This study exploits the potential of using synthetic DNA compacting agents that are not sequence-specific to achieve gene regulation for in vitro systems. The semi-synthetic in vitro system we use include common cationic DNA-compacting agents, poly(amido amine) (PAMAM) dendrimers and the surfactant hexadecyltrimethylammonium bromide (CTAB), which we apply to linearized plasmid DNA encoding for the luciferase reporter gene. We show that complexing the DNA with either of the cationic agents leads to gene expression inhibition in a manner that depends on the extent of compaction. This is demonstrated by using a coupled in vitro transcription-translation system. We show that compaction can also protect DNA against degradation in a dose-dependent manner. Furthermore, our study shows that these effects are reversible and DNA can be released from the complexes. Release of DNA leads to restoration of gene expression and makes the DNA susceptible to degradation by Dnase. A highly charged polyelectrolyte, heparin, is needed to release DNA from dendrimers, while DNA complexed with CTAB dissociates with the non-ionic surfactant C12E5. Our results demonstrate the relation between DNA compaction by non-specific DNA-binding agents and gene expression and gene regulation can be achieved in vitro systems in a reliable dose-dependent and reversible manner.

Interpreting the rich behavior of ternary DNA-PEI-Fe(III) complexes.

This work aims to shed light on the mechanism of interaction between components of ternary DNA-PEI-Fe(III) complexes, using experimental and theoretical approaches. In the experimental part, the chelation between PEI-Fe(III) was inspected by potentiometry and electrical conductance measurements and the respective importance for the condensation of DNA analyzed. To this end, three different mixing protocols for the components were imposed using different PEIs, branched (bPEI1.2 and bPEI10) and linear (lPEI2.5 and lPEI25). A delay in DNA condensation was observed when PEI and Fe(III) were premixed and then added to DNA. The set of observations was complemented by determination of the amount of Fe(III) included in the polyplexes, which was found to be dependent on the order of mixture and on the type of PEI used, decreasing with intrinsic PEI condensation efficiency. Overall, a coherent picture in which Fe(III) compensates PEI, probably modulating the respective charge, emerges. Some points arisen from the experimental part were rationalized using Monte Carlo simulations. Different architectured polycation (PC) chains were modeled and an interaction between PC and multivalent ions, mimicking the chelation of Fe(III) by the PEI, was imposed. It was found that chelation enhances polyanion (PA) compaction, irrespective of the PC architecture and charge density. The amount of multivalent ions in each polyplex compensates the negative charge unbalanced by the PC. The charge density and the ability of chelation of each PC dictate the disposition of each condensing agent along the PA backbone, and their coexistence strengthens PA compaction. The deep understanding of these ternary mixtures is a step forward in the optimization of such systems for application in gene delivery.

Enhanced condensation and facilitated release of DNA using mixed cationic agents: a combined experimental and Monte Carlo study.

Efficient DNA condensation and decondensation, as well as low toxicity, are required for an efficient gene delivery vehicle. We report on the condensation of DNA by a mixture of cationic agents, low-molecular-weight polyethylenimine (PEI, 1.2 KDa) and Fe(III) ions, and respective decondensation, using experimental and theoretical methods. It was found that a significant reduction in the amount of PEI necessary to induce DNA condensation is achieved by the addition of the trivalent ions, which are very inefficient on their own. In addition, the mixture makes DNA decompaction by heparin easier, starting from similar degrees of condensation. The results obtained using simulations of coarse-grain models are coherent with those obtained experimentally. It was also found that the improved effect of the multivalent ions is related to the preferred positioning of the trivalent ions in the DNA areas less populated by the polycation chains, in between the polycation chains and at the ends of the DNA, which facilitates the overall condensation.

Effect of the architecture on polyelectrolyte adsorption and condensation at responsive surfaces.

Adsorption profiles and conformational properties of negatively charged polyions at responsive surfaces were investigated by Monte Carlo simulations using a simple coarse-grained model. The surface, carrying both negative and positively charged groups, presents different overall charge, ranging from -10 to +50 e, and states, where the surface groups are either in a liquid-like structure (frozen surface) or laterally mobile (fluid surface). Polyions with both linear and ring architectures are considered. We have found that for very attractive surfaces the classical picture of a strongly adsorbed polyion with an extended and flat conformation emerges, independently of the architecture of the polyion or the state of the surface. At weakly attractive surfaces, the ring polyion adsorbs more strongly since it loses less entropy on adsorption than a linear chain. The adsorption of the ring is also enhanced at the fluid surfaces, since its more compact conformation increases the polarization of the surface. However, the linear polyion shows a significant adsorption at a neutral fluid surface, while the ring chains are totally desorbed, suggesting a delicate balance between the entropy of the surface groups and that of the chains. Although ring polyions show a stronger adsorption and a more compact conformation both in- and out-of-plane, at weakly attractive surfaces, no significant influence of the architecture was found on the polyion induced surface polarization (fluid surfaces) or opposite charge patch detection (frozen surfaces), at the monomer level. The adsorption profiles are, however, very different. For linear polyions at weakly attractive surfaces, it was observed a strong predominance of one-tail conformations, which was independent of the state of the surface.

Condensation and decondensation of DNA by cationic surfactant, spermine, or cationic surfactant-cyclodextrin mixtures: macroscopic phase behavior, aggregate properties, and dissolution mechanisms.

The macroscopic phase behavior and other physicochemical properties of dilute aqueous mixtures of DNA and the cationic surfactant hexadecyltrimethylammounium bromide (CTAB), DNA and the polyamine spermine, or DNA, CTAB, and (2-hydroxypropyl)-ß-cyclodextrin (2HPßCD) were investigated. When DNA is mixed with CTAB we found, with increasing surfactant concentration, (1) free DNA coexisting with surfactant unimers, (2) free DNA coexisting with aggregates of condensed DNA and CTAB, (3) a miscibility gap where macroscopic phase separation is observed, and (4) positively overcharged aggregates of condensed DNA and CTAB. The presence of a clear solution beyond the miscibility gap cannot be ascribed to self-screening by the charges from the DNA and/or the surfactant; instead, hydrophobic interactions among the surfactants are instrumental for the observed behavior. It is difficult to judge whether the overcharged mixed aggregates represent an equilibrium situation or not. If the excess surfactant was not initially present, but added to a preformed precipitate, redissolution was, in consistency with previous reports, not observed; thus, kinetic effects have major influence on the behavior. Mixtures of DNA and spermine also displayed a miscibility gap; however, positively overcharged aggregates were not identified, and redissolution with excess spermine can be explained by electrostatics. When 2HPßCD was added to a DNA-CTAB precipitate, redissolution was observed, and when it was added to the overcharged aggregates, the behavior was essentially a reversal of that of the DNA-CTAB system. This is attributed to an effectively quantitative formation of 1:1 2HPßCD-surfactant inclusion complexes, which results in a gradual decrease in the concentration of effectively available surfactant with increasing 2HPßCD concentration.

Kinetic studies of amino acid-based surfactant binding to DNA.

In this work, the binding kinetics of amino acid-based surfactants, presenting different linkers and head groups, with calf thymus (CT)-DNA was studied using stopped-flow fluorescence spectroscopy. The kinetic studies were carried out as a function of Na(+) concentration and surfactant-to-DNA charge ratio. The surfactant binding on DNA took place in two consecutive steps, for which the corresponding first and second relative rate constants (k(1) and k(2)) were determined. The fast step was attributed to the surfactant binding to DNA and micelle formation in its vicinity, the slower step to DNA condensation and possible rearrangement of the surfactant aggregates. In general, both relative rate constants increase with surfactant concentration and decrease with the ionic strength of the medium. The architecture of the surfactant was found to have a significant impact on the kinetics of the DNA-surfactant complexation. Surfactants with amide linkers showed larger relative rate constants than those with ester linkers. The variation of the relative rate constants with the head groups of the surfactants, alanine and proline, was found to be less obvious, being partially dependent on the surfactant concentration.

DNA condensation by pH-responsive polycations.

This work addresses the impact of pH variation on DNA-polyethylenimine (PEI) complex formation, in aqueous solution and at constant ionic strength. An initial potentiometric characterization of the acid-base behavior of PEI is carried out to measure the concentration of ionized species in the relevant systems. The characterization of the DNA-PEI complexes is performed by precipitation assays, agarose gel electrophoresis, photon correlation spectroscopy, and zeta potential analysis. It is observed that the variations on the electrophoretic mobility, size, and electrical properties of complexes display nonmonotonic, nontrivial trends with pH, if the same polycation/polyanion charge ratios are used for different values of pH. It is seen that both linear charge density and the relative number of chains of the condensing agent are important factors governing the condensation behavior. Complexes prepared at pH 4, for example, indicate strong binding and a large mean size, while those prepared at pH 8 are smaller, in a more uniform population. Finally, charge inversion was observed for all studied pH values (even below charge neutralization).

Cyclodextrins in DNA decompaction.

Individual T4DNA molecules, previously compacted by using a cationic surfactant (cetyltrimethylammonium bromide, CTAB), were successfully decompacted by the addition of an appropriate concentration of either alpha-cyclodextrin or beta-cyclodextrin (alpha-CD and beta-CD, respectively) due to the formation of inclusion complexes with the surfactant. The process was shown to be a non first-order transition from globules to coils. Density and sound velocity measurements as well as steady state fluorescence spectroscopy have confirmed the approximate CD concentration at which the globule-to-coil transition occurs. Phase maps of the DNA-CTA-CD systems were produced and the CTAB concentration range at which decompaction can be achieved was determined. Evidences for DNA-CD interaction were found, however, its nature and influence on the decompaction process was not yet determined.

Release of DNA from surfactant complexes induced by 2-hydroxypropyl-beta-cyclodextrin.

Decompaction of DNA-CTA self-assembled complexes by 2-hydroxypropyl-beta-cyclodextrin (2-HP-beta-CD) was studied and the results were compared with beta-CD. Different degrees of 2-HP substitution (0.6, 0.8 and 1.0, respectively) were used and the decompaction was successful with all degrees of substitution. Fluorescence microscopy, steady state fluorescence spectroscopy, density and sound velocity measurements, thermal melting and circular dichroism were used. Compared to previous work using alpha- and beta-CD, the fluorescence spectroscopy results showed that the 2-HP-substituted CDs more efficiently released DNA into solution. Furthermore, dissociation of macroscopically phase separated DNA-CTA complexes was achieved upon addition of 2-HP-beta-CD and the results gave strong indications on the non-equilibrium nature of the system. The globule-to-coil transition was not found to proceed through a coexistence region, which seems to be a general phenomenon in DNA decompaction using CDs.

Re-dissolution and de-compaction of DNA-cationic surfactant complexes using non-ionic surfactants.

Addition of a cationic surfactant to a solution of DNA causes the formation of compacted DNA-cationic surfactant complexes which precipitate from aqueous solution. It has been shown previously that addition of anionic surfactant will re-dissolve and de-compact the DNA-cationic surfactant complexes and we find that addition of non-ionic surfactants of the alkylpolyoxyethylene type can be used similarly. In principle, these de-compaction and re-dissolution processes could occur either by stripping of the cationic surfactant from the DNA into mixed micelles with the non-ionic surfactant or by solubilisation of the DNA-cationic surfactant complexes within the non-ionic micelles. Solubility phase-boundary measurements, fluorescence microscopy observations of the de-compaction process and light scattering results indicate that de-compaction and re-dissolution occur by the stripping mechanism, even for non-ionic surfactants where the favourable attractive electrostatic interaction between the two surfactants is absent. Using measurements of critical micelle concentrations and calculations based on regular solution mixed micelle theory, we show that re-dissolution and de-compaction of the DNA-cationic surfactant complexes occurs when the concentration of free monomeric cationic surfactant is reduced (by incorporation into mixed micelles) below a critical value.