Characterization of single- and double-stranded DNA on gold surfaces

Single- and double-stranded deoxy ribonucleic acid (DNA) molecules attached to self-assembled monolayers (SAMs) on gold surfaces were characterized by a number of optical and electronic spectroscopic techniques. The DNA-modified gold surfaces were prepared through the self-assembly of 6-mercapto-1-hexanol and 5'-C(6)H(12)SH -modified single-stranded DNA (ssDNA). Upon hybridization of the surface-bound probe ssDNA with its complimentary target, formation of double-stranded DNA (dsDNA) on the gold surface is observed and in a competing process, probe ssDNA is desorbed from the gold surface. The competition between hybridization of ssDNA with its complimentary target and ssDNA probe desorption from the gold surface has been investigated in this paper using X-ray photoelectron spectroscopy, chronocoulometry, fluorescence, and polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS). The formation of dsDNA on the surface was identified by PM-IRRAS by a dsDNA IR signature at approximately 1678 cm(-)(1) that was confirmed by density functional theory calculations of the nucleotides and the nucleotides' base pairs. The presence of dsDNA through the specific DNA hybridization was additionally confirmed by atomic force microscopy through colloidal gold nanoparticle labeling of the target ssDNA. Using these methods, strand loss was observed even for DNA hybridization performed at 25 degrees C for the DNA monolayers studied here consisting of attachment to the gold surfaces by single Au-S bonds. This finding has significant consequence for the application of SAM technology in the detection of oligonucleotide hybridization on gold surfaces.     

Excited-state intramolecular proton transfer distinguishes microenvironments in single- and double-stranded DNA

Herein, the efficient interaction of an environment-sensitive fluorophore that undergoes excited-state intramolecular proton transfer (ESIPT) with DNA has been realized by conjugation of a 3-hydroxychromone (3HC) with polycationic spermine. On binding to a double-stranded DNA (dsDNA), the ratio of the two emission bands of the 3HC conjugates changes up to 16-fold, so that emission of the ESIPT product increases dramatically. This suggests an efficient screening of the 3HC fluorophore from the water molecules in the DNA complex, which is probably realized by its intercalation into dsDNA. In sharp contrast, the 3HC conjugates show only moderate changes in the dual emission on binding to a single-stranded DNA (ssDNA), indicating a much higher fluorophore exposure to water at the binding site. Thus, the 3-hydroxychromone fluorophore being conjugated to spermine discriminates the binding of this polycation to dsDNA from that to ssDNA. Consequently, ESIPT-based dyes are promising for monitoring the interaction of polycationic molecules with DNA and probing the microenvironment of their DNA binding sites.     

DNA oxidation in anionic reverse micelles: ruthenium-mediated damage at Guanine in single- and double-stranded DNA

One-electron guanine oxidation in DNA has been investigated in anionic reverse micelles (RMs). A photochemical method for generating Ru3+ from the ruthenium polypyridyl complex tris(2-2'-bipyridine)ruthenium(II) chloride ([Ru(bpy)3]Cl2) is combined with high-resolution polyacrylamide gel electrophoresis (PAGE) to quantify piperidine-labile guanine oxidation products. As characterized by emission spectroscopy of Ru(bpy)3(2+), the addition of DNA to RMs containing Ru(bpy)3(2+) does not perturb the environment of Ru(bpy)3(2+). The steady-state quenching efficiency of Ru(bpy)3(2+) with K3[Fe(CN)6] in buffer solution is approximately 2-fold higher than that observed in RMs. Consistent with the difference in quenching efficiency in the two media, a 1.5-fold higher yield of piperidine-labile damage products as monitored by PAGE is observed for duplex oligonucleotide in buffer vs RMs. In contrast, a 13-fold difference in the yield of PAGE-detected G oxidation products is observed when single-stranded DNA is the substrate. Circular dichroism spectra showed that single-stranded DNA undergoes a structural change in anionic RMs. This structural change is potentially due to cation-mediated adsorption of the DNA phosphates on the anionic headgroups of the RMs, leading to protection of the guanine from oxidatively generated damage.     

The binding of ethidium bromide with DNA: interaction with single- and double-stranded structures

The pH-induced helix-coil transition of DNA and its complexes with EtBr is carried out at acidic pH in a wide interval of change of concentration ratio of EtBr/DNA. The binding isotherms of EtBr on double and single-stranded DNA at pH = 7.0 and pH = 3.0 (t = 25(o)C) are obtained by absorption and fluorimetric methods. Binding constants (K) and number of bases (n), corresponding to one binding site were determined. Non fluorescent "strong" complex with ds-DNA at pH = 7.0 and t = 25(o)C as well as "strong" and "weak" complexes with ss-DNA at pH = 3.0 and t = 25(o)C are revealed.     

Selective covalent binding of a positively charged water-soluble benzoheterocycle triosmium cluster to single- and double-stranded DNA

The water-soluble triosmium cluster [Os₃(CO)₉(μ-η²-(4-CHO)C₉H₅N)(μ-H)(P(OCH₂CH₂N(CH₃)₃I)₃)] (4) was tested for its reactivity with plasmid DNA. In contrast to the band retardation previously observed with a related series of positively charged clusters, an intensification and retardation of three discrete bands was observed with increasing cluster concentration. In order to further investigate the apparent modification of DNA by 4, its interaction with a 22-oligomer (sequence 5′-AGT TGT GGT GAC TTT CCC AGG C-3′) was examined. Incubation with this oligonucleotide (pH 7.4 in Tris-HCl buffer and 100 mM NaCl) followed by HPLC analysis revealed the formation of three dose dependent products assigned as covalent modifications at three sites of the oligonucleotide. Incubation of 4 with ³²P-ATP labeled oligonucleotide at the 5′-end followed by treatment with piperidine and comparison with the standard Maxam-Gilbert sequencing protocol products revealed only general background cleavage, indicating that the modification products are piperidine labile and suggesting that the modification involved formation of a Schiff base. An alternative approach was then pursued which involved annealing the 4-oligonucleotide products with their complementary strand and treatment of the resulting duplex DNAwith the exonuclease, Exo III. This assay indicated three exonuclease stops, consistent with the three products observed by HPLC whose electrophoretic mobility approximately matched guanine containing fragments when compared with the Maxam-Gilbert sequencing lanes. Reduction of the 4-oligonucleotide products with borohydride reducing agents, followed by treatment with piperidine, resulted in the formation of one product (by HPLC) with the same electrophoretic mobility as the AGTT fragment based on comparison with the Maxam-Gilbert sequencing lanes. This product most likely results from reduction of an initially formed Schiff base adduct (to the corresponding amine) with the guanine of the TGT fragment of the oligonucleotide, and corresponds to the most stable of the three Schiff base adducts detected by HPLC and by incubation with the exonuclease. The other two products are less stable and competitive reduction of the free aldehyde functionality on the cluster in equilibrium with these adducts precludes their detection after treatment with the reducing agents. The formation of the Schiff base adduct is further corroborated by the model reaction of [Os₃(CO)₁₀(μ-η²-(4-CHO)C₉H₅N)(μ-H)] (4′) with acetylated guanine in nonaqueous solvents where disappearance of the aldehyde resonance and the appearance of several new resonances in the 6-9 ppm region of the ¹H NMR of the reaction mixture is noted.     

A theoretical and experimental study of two thiazole orange derivatives with single- and double-stranded oligonucleotides, polydeoxyribonucleotides and DNA

The effect of interaction with DNA and oligonucleotides on the photophysical properties of two thiazole orange (TO) derivatives, with different side chains (-(CH2)3-N+(CH3)3 and -(CH2)6-I)) linked to the nitrogen of the quinoline ring of the thiazole orange, is presented here. The first one called TO-PRO1 is a commercially available dye, whereas the second one called TO-MET has been specially synthesized for further covalent binding to oligonucleotides with the aim of being used for specific in situ detection of biomolecular interactions. Both photophysical measurements and molecular calculations have been done to assess their possible mode of interaction with DNA. When dissolved in buffered aqueous solutions both derivatives exhibit very low fluorescence quantum yields of 8 x 10(-5) and 2 x 10(-4), respectively. However, upon binding to double-stranded DNA, large spectroscopic changes result and the quantum yield of fluorescence is enhanced by four orders of magnitude, reaching values up to phi F = 0.2 and 0.3, respectively, as a result of an intercalation mechanism between DNA base pairs. A modulation of the quantum yield is observed as a function of the base sequence. The two derivatives also bind with single-stranded oligonucleotides, but the fluorescence quantum yield is not so great as that when bound to double-stranded samples. Typical fluorescence quantum yields of 7 x 10(-3) to 3 x 10(-2) are observed when the dyes interact with short oligonucleotides, whereas the fluorescence quantum yield remains below 10(-2) when interacting with single-stranded oligonucleotides. This slight but significant quantum-yield increase is interpreted as a folding of the single strand around the dye, which reduces the internal rotation of the two heterocycles around the central methine bridge that links the two moieties of the dye. From these properties, it is proposed to link monomer covalently to oligonucleotides for the subsequent detection of target sequences within cells.     

On the stability of single- and double-stranded DNA helices: The application of the PPT-MCF method on large fragments of DNA

The pseudo-polarization tensor mutually consistent field (PPT -MCF ) method recently introduced [1] has been applied to study the stacking interactions between the nucleotide bases in large periodic B-DNA fragments. The effects on the global and local binding properties caused by replacing one base in the periodic sequence by another base are investigated. The increase in the stability for comparable fragments owing to this base substitution is further enforced in the case of periodic alternating helices. The most important results are that the stacking interaction between two bases is slowly converging with the interbase distance and that the average contribution per base to the binding energy is repulsive. Furthermore, the energetical properties of double helix models in B- and Z-DNA configurations, respectively, consisting of up to five base pairs have been compared. It turns out that the G C G C sequence in Z-DNA is significantly more stable than either in periodic or periodic alternating B-DNA. In these cases the average energy contribution of a single Watson–Crick-type base pair is predicted also to be positive. From the calculations it follows that the double helix is not stabilized owing to the hydrogen bonding between the bases belonging to both strands, in contradiction to most other investigations.     

Enzymatic kinetics of cellulose hydrolysis: a QCM-D study

The interactions between films of cellulose and cellulase enzymes were monitored using a quartz crystal microbalance (QCM). Real-time measurements of the coupled contributions of enzyme binding and hydrolytic reactions were fitted to a kinetic model that described closely significant cellulase activities. The proposed model combines simple Boltzmann sigmoidal and 1 - exp expressions. The obtained kinetics parameters were proven to be useful to discriminate the effects of incubation variables and also to perform enzyme screening. Furthermore, it is proposed that the energy dissipation of a film subject to enzymatic hydrolysis brings to light its structural changes. Overall, it is demonstrated that the variations registered in QCM frequency and dissipation of the film are indicative of mass and morphological transformations due to enzyme activities; these include binding phenomena, progressive degradation of the cellulose film, existence of residual, recalcitrant cellulose fragments, and the occurrence of other less apparent changes throughout the course of incubation.     

Photophysical properties of fluorescent DNA-dyes bound to single- and double-stranded DNA in aqueous buffered solution.

The absorption and fluorescence spectra, fluorescence quantum yields, lifetimes and time-resolved fluorescence spectra are reported for nine different fluorescent DNA-dyes. The work was initiated in search of a quantitative method to detect the ratio of single-to-double stranded DNA (ssDNA/dsDNA) in solution based on the photophysics of dye-DNA complexes; the result is a comprehensive study providing a vast amount of information for users of DNA strains. The dyes examined were the bisbenzimide or indole-derived stains (Hoechst 33342, Hoechst 33258 and 4',6-diamidino-2-phenylindole), phenanthridinium stains (ethidium bromide and propidium iodide) and cyanine dyes (PicoGreen, YOYO-1 iodide, SYBR Green I and SYBR Gold). All were evaluated under the same experimental conditions in terms of ionic strength, pH and dye-DNA ratio. Among the photophysical properties evaluated only fluorescence lifetimes for the cyanine stilbene dyes allowed a convenient differentiation between ssDNA and dsDNA. The bisbenzimide dyes showed multiexponential decays when bound to either form of DNA, making lifetime-based analysis cumbersome with inherent errors. These dyes also presented biexponential decay when free in aqueous buffered solutions at different pH. A mechanism for their deactivation is proposed based on two different conformers decaying with different kinetics. The phenanthridinium dyes showed monoexponential decays with ssDNA and dsDNA, but there was no discrimination between them. High dye-DNA ratios (e.g. 1:1) resulted in multiexponential decays for cyanine dyes, resulting from energy transfer or self-quenching deactivation. Shifts in both absorption and fluorescence maxima for both ssDNA and dsDNA DNA-cyanine dye complexes were small. Broadening of dye-ssDNA absorption and fluorescence bands for the cyanine dyes relative to dye-dsDNA bands was detected and attributed to higher degrees of rotational freedom in the former.     

Label-free detection of nanomolar unmodified single- and double-stranded DNA by using surface-enhanced Raman spectroscopy on Ag and Au colloids

Unlabelled single- and double-stranded DNA (ssDNA and dsDNA, respectively) has been detected at concentrations ≥10(-9) M by surface-enhanced Raman spectroscopy. Under appropriate conditions the sequences spontaneously adsorbed to the surface of both Ag and Au colloids through their nucleobases; this allowed highly reproducible spectra with good signal-to-noise ratios to be recorded on completely unmodified samples. This eliminated the need to promote absorption by introducing external linkers, such as thiols. The spectra of model ssDNA sequences contained bands of all the bases present and showed systematic changes when the overall base composition was altered. Initial tests also showed that small but reproducible changes could be detected between oligonucleotides with the same bases arranged in a different order. The spectra of five ssDNA sequences that correspond to different strains of the Escherichia coli bacterium were found to be sufficiently composition-dependent so that they could be differentiated without the need for any advanced multivariate data analysis techniques.     

Packing structures of single-stranded DNA and double-stranded DNA thiolates on Au(111): a molecular simulation study

The packing structures of single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) thiolates on implicit gold surfaces were studied in explicit aqueous solutions of 1M NaCl using molecular dynamics simulations. The simulations were based on individual DNA chains placed in hexagonal simulation boxes of different sizes, representing various packing densities. The total potential energy per DNA chain was compared. The optimal packing structures were determined based on the minimal potential energy within the limits of the conditions that were evaluated in this study. The optimal packing density of ssDNA was found to be 0.19 DNA chains/nm(2), which is consistent with that determined experimentally. Furthermore, the optimal packing density of dsDNA was shown to be approximately 58% of the packing density for ssDNA, indicating that the packing of ssDNA should be approximately 58% of its optimal packing in order to achieve the best hybridization.     

The nature of the DNA template (single- versus double-stranded) affects the rate of aquation of a dinuclear Pt anticancer drug

The rate of aquation of a dinuclear platinum anticancer agent is altered in the presence of template DNA with enhancement of hydrolysis in the presence of single-stranded over double-stranded DNA, emphasising how the alteration of chemical properties of small molecules in the presence of large host interactions is also dependent on the conformation and nature of that host.     

Adsorption properties of polyelectrolyte-surfactant complexes on hydrophobic surfaces studied by QCM-D

Adsorption and deposition from turbid solutions are common in many industrial processes but notoriously difficult to investigate using standard optical techniques such as ellipsometry and reflectometry. In this report, we have addressed this problem by employing a quartz crystal microbalance with dissipation monitoring ability, QCM-D. The system under investigation consisted of a cationic polyelectrolyte, poly(vinylamine), PVAm, and an anionic surfactant, sodium dodecyl sulfate, SDS, which were mixed together in 10 mM NaCl solution. The polyelectrolyte and the surfactant readily associate in bulk solution, resulting in increased solution turbidity once large aggregates are formed. The solutions were placed in contact with a polystyrene surface, and the adsorption process was monitored by following the changes in the resonance frequency and dissipation factor. The results obtained can in most cases be evaluated using the Sauerbrey relation, but in some cases a more elaborate analysis is necessary. It is found that PVAm adsorbs to polystyrene in the absence of SDS. In the turbid region, deposition is observed, and the sensed mass exceeds the sum of that obtained for each of the components alone. On the other hand, at high SDS concentrations, the surfactant dominates in the adsorbed layer. Adsorption equilibrium is in most cases established within 1-2 h, the exception being found around the solution composition that results in the formation of charge-neutralized aggregates. In this case, a slow deposition of aggregates persists over prolonged times.     

QCM-D analysis of the performance of blocking agents on gold and polystyrene surfaces

With today's developments of biosensors and medical implants comes the need for efficient reduction of nonspecific binding. We report on a comparison of the ability of traditionally used blocking agents and poly(ethylene glycol) (PEG) derivatives to prevent protein adsorption on both gold and polystyrene surfaces. The adsorption kinetics of blocking molecules and proteins was monitored gravimetrically using quartz crystal microbalance with dissipation (QCM-D). The resistance to nonspecific adsorption was evaluated on gold and polystyrene surfaces coated with bovine serum albumin (BSA) or casein, gold coated with three different 6-11 ethylene glycol (EG) long hydroxyl- or methoxy-terminated PEG-thiolates and polystyrene blocked with a PLL-g-PEG or three different 12 EG long benzyl-PEG-derivatives. The prevention of protein adsorption on the coated surfaces was evaluated by monitoring the mass uptake at the addition of both pure prostate specific antigen (PSA) and seminal plasma. We demonstrate that on pure gold the PEG-thiols are superior to the other blocking molecules tested, with the end group and length of the PEG-thiols used being of minor importance. On polystyrene surfaces blocking with PLL-g-PEG, BSA and casein gave the best results. These results have an impact on further development of an optimized immunoassay protocol.     

Adsorption of polyelectrolyte multilayers and complexes on silica and cellulose surfaces studied by QCM-D

The adsorption of polyelectrolyte (PE) multilayers and complexes, obtained from both high- and low-charge polyelectrolytes, was studied on silica and on cellulose model surfaces by quartz crystal microbalance with dissipation (QCM-D). The film properties acquired with the different strategies were compared. When polyelectrolytes were added on an oppositely charged surface in sequence to form multilayers both the change in frequency and dissipation increased. The changes in frequency and dissipation were clearly higher if low-charge PEs were used in the multilayer formation. The substrate, silica or cellulose, did not affect the adsorption behaviour of low-charge PEs and only minor differences were seen in the adsorbed amounts and changes in dissipation of high-charge PEs between SiO₂ and cellulose. The complexes formed by low-charge PEs had higher changes in frequency and dissipation at low ionic strength on both surfaces, while the complexes formed from high-charge polyelectrolytes adsorbed more at high salt concentration. The complexes of low-charge polyelectrolytes adsorbed more on silica, while the complexes formed by high-charge PEs formed thicker layers on cellulose. The charge ratio had a significant effect on the adsorption and the highest changes in frequency and dissipation were obtained in the anionic/cationic charge ratio of 0.5–0.6. Generally, the multilayers and complexes formed by low-charge polyacrylamides adsorbed highly and formed rather thick layers on both surfaces, unlike the high-charge PEs which formed thin layers using either one of the addition techniques.     

Fluid supported lipid bilayers containing monosialoganglioside GM1: a QCM-D and FRAP study

In an effort to use model fluid membranes for immunological studies, we compared the formation of planar phospholipid bilayers supported on silicon dioxide surfaces with and without incorporation of glycolipids as the antigen for in situ antibody binding. Dynamic light scattering measurements did not differentiate the hydrodynamic volumes of extruded small unilamellar vesicles (E-SUVs) containing physiologically relevant concentrations (0.5-5 mol%) of monosialoganglioside GM1 (GM1) from exclusive egg yolk L-alpha-phosphatidylcholine (egg PC) E-SUVs. However, quantifiable differences in deposition mass and dissipative energy loss emerged in the transformation of 5 mol% GM1/95 mol% egg PC E-SUVs to planar supported lipid bilayers (PSLBs) by vesicle fusion on thermally evaporated SiO2, as monitored by the quartz crystal microbalance with dissipation (QCM-D) technique. Compared to the 100 mol% egg PC bilayers on the same surface, E-SUVs containing 5 mol% GM1 reached a approximately 12% higher mass and a lower dissipative energy loss during bilayer transformation. PSLBs with 5 mol% GM1 are approximately 18% heavier than 100 mol% egg PC and approximately 11% smaller in projected area per lipid, indicating an increased rigidity and a tighter packing. Subsequent binding of polyclonal immunoglobulin G anti-GM1 to the PSLBs was performed in situ and showed specificity. The anti-GM1 to GM1 ratios at equilibrium were roughly proportional to the concentrations of anti-GM1 administered in the solution. Fluorescence recovery after photobleaching was utilized to verify the retained, albeit reduced lateral fluidity of the supported membranes. Five moles percentage of GM1 membranes (GM1 to PC ratio approximately 1:19) decorated with 1 mol% N-(Texas Red sulfonyl)-1,2-dihexadecanoyl-sn-glycerol-3-phosphoethanolamine (Texas Red DHPE) exhibited an approximately 16% lower diffusion coefficient of 1.32+/-0.06 microm2/s, compared to 1.58+/-0.04 microm2/s for egg PC membranes without GM1 (p<0.01). The changes in vesicle properties and membrane lateral fluidity are attributed to the interactions of GM1 with itself and GM1 with other membrane lipids. This system allows for molecules of interest such as GM1 to exist on a more biologically relevant surface than those used in conventional methods such as ELISA. Our analysis of rabbit serum antibodies binding to GM1 demonstrates this platform can be used to test for the presence of anti-lipid antibodies in serum.     

Nanoscale evaluation of lubricity on well-defined polymer brush surfaces using QCM-D and AFM

For preparing a “highly lubricated biointerface”, which has both excellent lubricity and biocompatibility, we investigated the factors responsible for resistance to friction during polymer grafting. We prepared poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), poly(2-hydroxyethyl methacrylate) (PHEMA), and poly(methyl methacrylate) (PMMA) brush layers with high graft density and well-controlled thickness using atom transfer radical polymerization (ATRP). We measured the water absorptivity in the polymer brush layers and the viscoelasticity of the polymer-hydrated layers using a quartz crystal microbalance with dissipation monitoring (QCM-D) measurements. The PMPC brush layer had the highest water absorptivity, while the PMPC-hydrated layer had the highest fluidity. The friction properties of the polymer brush layers were determined in air, water, and toluene by atomic force microscopy (AFM). The friction on each polymer brush decreased only when a good solvent was chosen for each polymer. In conclusion, the brush layer possessing high water absorptivity and fluidity in water contributes to reduce friction. PMPC grafting is an effective and promising method for obtaining highly lubricated biointerfaces.     

Responsive polymer gels: double-stranded versus single-stranded DNA

Cross-linking of polyelectrolytes such as DNA gives gels that are osmotically highly swollen but contract upon addition of electrolytes and, in particular, upon association of oppositely charged cosolutes with the polyelectrolyte chain. The deswelling behavior of cross-linked DNA gels thus reflects the DNA-cosolute interactions and provides a basis for the development of responsive DNA formulations. Gels of both single- and double-stranded DNA have interesting applications, and a comparison between them provides the basis for understanding mechanisms. Denaturation of cross-linked ds-DNA gels was induced by heating them above the melting temperature and then cooling. This process, studied by fluorescence using ethidium bromide, appeared to be reversible when a heating/cooling cycle was performed. The swelling behavior upon addition of different cosolutes, such as metal ions, polyamines, charged proteins, and surfactants, was investigated for different DNA gel samples, including long and short ds-DNA and long and short ss-DNA. The DNA molecular weight was found to have only a slight effect on the deswelling curves, whereas conformation exhibited a pronounced effect. In general, single-stranded DNA gels exhibited a larger collapse in the presence of cations than did double-stranded DNA. This difference was more pronounced with surfactants than with the other cosolutes investigated. The difference between double- and single-stranded DNA was attributed to differences in linear charge density, chain flexibility, and hydrophobicity. For surfactants with different chain lengths, the swelling behavior displayed by ss-DNA can be interpreted in terms of an interplay between hydrophobic and electrostatic interactions, the latter being influenced by polymer flexibility. Increasing hydrophobicity of the network leads to a decreased critical aggregation concentration (cac) for the surfactant/gel complex, as a result of the strengthened hydrophobic attractive force between the surfactant and the gel chain. The swelling of DNA gels appears to be reversible and to be independent of DNA conformation. Surfactant-induced deswelling of DNA gels under some conditions appears to be quite homogeneous, whereas under other conditions, there is a separation into a collapsed region in the outer parts of the gel sample and an inside swollen part. Such "skin" formation is quite different for ss- and ds-DNA, with ss-DNA giving more pronounced skin formation over a wider range of binding ratio, beta. For example, no macroscopic separation into collapsed and swollen regions was observed at intermediate degrees of binding for ds-DNA gels, whereas a dense surfactant-rich surface phase (skin) was found to coexist with a swollen core network for ss-DNA gels with beta>0.5. One explanation for this difference is the large deformation energy required for the compression of the very stiff ds-DNA chains.