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Influence of Proton and Salt Concentration on the Chromonic Liquid Crystal Phase Diagram of Disodium Cromoglycate Solutions: Prospects and Limitations of a Host for DNA Nanostructures Bingru Zhang, and Heinz-Siegfried Kitzerow J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b01644 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 15, 2016

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The Journal of Physical Chemistry

Influence of Proton and Salt Concentration on the Chromonic Liquid Crystal Phase Diagram of Disodium Cromoglycate Solutions: Prospects and Limitations of a Host for DNA Nanostructures Bingru Zhang and Heinz-S. Kitzerow* Department of Chemistry, University of Paderborn, Warburger Str. 100, 33098 Paderborn, Germany

ABSTRACT. Lyotropic chromonic liquid crystals have recently been suggested to be used as a self-organized host for dispersing and aligning self-organized DNA origami nanostructures. However, an appropriate pH-value and a suitable cation concentration are necessary to stabilize such nanostructures and to avoid unfolding of the DNA. The present study shows that the nematic and the columnar liquid crystal phases appearing in aqueous solutions of disodium cromoglycate are robust against the replacement of de-ionized water by a neutral or alkaline buffer solution. However, disodium cromoglycate precipitates, when an acidic buffer is used or when the concentration of magnesium cations exceeds a critical concentration of about 0.6 – 0.7 millimoles per liter.

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INTRODUCTION Lyotropic chromonic liquid crystals (LCLCs)1-4 form a special class of liquid crystals (LCs), which has been extensively studied during the last years. Chromonic LC phases appear frequently in aqueous solutions of dyes, drugs, nucleic acids and similar water-soluble, aromatic compounds. While the more common lyotropic mesophases that appear in surfactant solutions are composed of micelles or double layers of amphiphilic molecules, LCLCs consist of columnar aggregates, which are held together by non-covalent interactions, such as π-π stacking of polyaromatic molecular cores.1-4 Some LCLCs show a relatively large birefringence (∆n ≈ 0.02) in comparison with micellar lyotropic liquid crystals,5, 6 but still a lower birefringence than thermotropic liquid crystals. While the latter are widely used in flat panel liquid crystals displays, LCLCs offer quite different opportunities. Non-toxic compounds forming LCLCs can be found as food additives or drugs.7 Their water-based liquid crystalline solutions are often biocompatible8 and have been suggested, for example, to be applied for biosensors,9 which make microorganisms visible through changes of the LC director field. Apart from pharmaceutical,7 biological or biosensing9 and organic electronic10 or optical applications,11 LCLCs are fundamentally interesting model systems for studying the aggregation and self-assembly of ionic aromatic molecules1-3, 6, 12-19 and topological aspects of ordering processes in general.20 The variability of material properties – for example, elastic coefficients or viscosity – as a function of temperature and concentration has been found to be larger than in thermotropic liquid crystals.20-23 In most cases, this can be attributed to the fact that the size and shape of the aggregates forming chromonic mesophases may vary to a large extent – unlike the size and shape of single molecules, which are the building blocks of thermotropic liquid crystals. The puzzling influences of the pH value, salts and other additives on the aggregation and


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mesophase formation have been extensively studied in solutions of the dye sunset yellow (SSY, an azo compound permitted as a food additive in some countries),24-32 but also in solutions of di sodium cromoglycate (DSCG, a compound used in asthma drugs)32-36 and – for example – of an indanthrone derivative.37 In a recent work, LCLCs have been proposed as mesogenic solvents for DNA origami nanostructures.38 The latter are sophisticated functional nanostructures that are formed by folding of single stranded DNA scaffolds by adding a well-designed mixture of DNA oligomer staples, each binding highly selective to a very specific site of the scaffold owing to the self-recognition of DNA base pairs.39-43 These nanostructures can not only be designed to exhibit a very precise size and shape, but also be functionalized by binding functional molecular moieties or nanoparticles with certain chemical or optical properties at specific positions, thereby offering wide opportunities in biology,44-45 nanophotonics,45-47 nanometrology,48 nanomechanics,49 or nanoelectronics.50 Embedding anisometric DNA nanostructures in a liquid crystal may provide a way to direct and control their alignment, thereby making anisotropic metamaterials with unusual properties feasible. However, this ambitious dream imposes some requirements on the LC solvent in order to ensure the stability of the DNA nanostructures. It is well known51 that the latter are only stable in a gently alkaline TAE (Tris / acetic acid / EDTA) buffer solution (pH = 8.0), which contains the base 2-amino-2-hydroxymethyl-propane-1,3-diol [also known as tris(hydroxymethyl)aminomethane, Tris], acetic acid, and the chelating agent 2-({2[bis(carboxymethyl)amino]ethyl}(carboxymethyl)amino) acetic acid [also known as ethylenediaminetetraacetic acid, EDTA]. In addition, a sufficient concentration of metal cations is required in order to compensate the repulsive interaction of the negatively charged phosphate groups of the polyelectrolyte DNA.51-52 Typically, magnesium chloride is added for this purpose


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at concentrations of 10 mM to 20 mM MgCl2. Unfortunately and surprisingly, it was found that a stable homogeneous LCLC mixture containing the well-investigated compound disodium cromoglycate (DSCG, Fig. 1a)32-36 could not be prepared at such high concentrations of MgCl2.38 While the influence of monovalent cations on LCLC phase diagrams have been studied previously,32-36 there is a lack of studies about the influence of pH buffer solutions and of salts with divalent cations on the LCLC phase diagram of DSCG. Thus, the LCLC phase diagram of aqueous mixtures of DSCG is studied here at different pH-values and at different Mg2+ concentrations.

EXPERIMENTS AND RESULTS Disodium cromoglycate (DSCG) was purchased from Sigma Aldrich and used as received. It was thoroughly mixed with either de-ionized water or one of five different buffer solutions by repeated heating, agitating and exposing to ultrasound until a homogeneous mixture was obtained. The latter was sandwiched between two glass slides separated by 11 µm mylar spacers (Fig. 1b) and sealed with an epoxy glue from all sides to prevent water evaporation. The respective cell was placed in a temperature controlled microscope stage (model LTS350/TMS94, Linkam). The liquid crystalline mesophases and their transition temperatures were determined by observation in a polarizing optical microscope (model DM4500 P, Leica) using crossed polarizers. The influence of proton concentration on the phase diagram of aqueous DSCG solutions was studied in the concentration range from 11 % by weight DSCG to 33 % by weight DSCG. The pH value was controlled by common sodium hydroxide based buffer solutions (from Fluka), which contain either citric acid (pH 5.0 and pH 6.0), sodium tetraborate (pH 10.0), or di-


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sodium hydrogen phosphate (pH 12.0), respectively. For pH 8.0, a TAE (Tris / acetic acid / EDTA) buffer (‘Rotiphorese’, Carl Roth) was used, which is a usual solvent for working with DNA. For DSCG dissolved in TAE buffer, different amounts of MgCl2 were added to test the influence of Mg2+ ions on the mesophase formation. Aqueous DSCG solutions in deionized water show the well-known binary phase diagram, which have been explored in many previous works.1-2, 6, 12, 17, 33-34 For DSCG concentrations up to 22% by weight, a nematic (N) phase appears at room temperature (Fig. 2a), which shows a characteristic schlieren texture (Fig. 2c). Previous X-ray studies have shown that the N phase is formed by uniformly aligned, short columnar stacks of the DSCG molecules (length ≈ 8 nm, ≈ 23 molecules).17 For concentrations of 25% by weight and higher, a columnar or ‘middle’ (M) phase appears at room temperature, which shows a ribbon- or fan-like texture (Fig. 2f). The M phase was previously found to consist of larger columnar stacks, which are assembled in a twodimensional hexagonal array.17 The entire phase diagram contains large ranges with coexistence of two phases. At room temperature, the N and M phase coexist in the range of DSCG concentrations between 22% and 25% by weight (Fig. 2d). Close to the clearing temperature, broad coexistence ranges between the isotropic (I) phase and the nematic (N) phase (Fig. 2b) or between the isotropic (I) phase and the columnar (M) phase (Fig. 2e) appear, respectively. The temperature of the transition from the respective mesophase to the isotropic liquid phase increases monotonously with increasing DSCG concentration. These observations are in agreement with earlier findings.1-2, 6, 12, 17, 33-34 Not surprisingly, the binary phase diagram remains unchanged, when de-ionized water is substituted by a neutral (pH 7.0) buffer solution (Fig. 3a). However, using an acidic buffer solution can alter the phase behavior considerably. For example, Fig. 3a shows a comparison of


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the phase transition temperatures for pH 7.0 and pH 5.0. At small concentrations of DSCG, the transition temperatures are enhanced. So, obviously short columnar stacks are thermally stable at moderate acidity. However, DSCG precipitates at larger DSCG concentrations and consequently no mesophase is formed. Fig. 3b shows the influence of the pH values in the range 5.0 – 7.0 on the critical concentration at which DSCG starts to precipitate. This critical concentration decreases with increasing proton concentration from > 42% at pH 7.0 to 18% at pH 5.0. At the same time, the transition from the N phase to the M phase is shifted towards higher DSCG concentration with increasing proton concentration (decreasing pH value). The observed precipitation of DSCG with increasing acidity can be explained by the shift of the equilibrium concentration of the acid residue anion. Obviously, an increasing proton concentration supports the formation of the non-dissociated cromoglicic acid. While the acid residue anion and its ionic conglomerates are readily soluble in water, the non-ionic acid formed preferentially at low pH values shows limited solubility and thus precipitates. In contrast to the large influence of acidic buffers, alkaline buffers are found to have little influence on the thermal stability of the mesophases formed by DSCG (Fig. 4). On increasing pH-value from pH 7.0 to pH 12.0, there is no significant change of the transition temperatures from the respective mesophase to the two-phase region and from the two-phase region to the homogeneous isotropic phase (Fig. 4a). Obviously, a decreasing proton concentration below 10-7 M has a small effect on the mesophase formation of DSCG. Only the relative stability of the two mesophases (N versus M) at medium DSCG concentration changes slightly. The maximum DSCG concentration for appearance of the nematic (N) phase shifts from 23% at pH 7.0 to >25% at pH 12, while the minimum DSCG concentration for appearance of the columnar (M) phase shifts from > 25% at pH 7.0 to >28% at pH 12.0 (Fig. 4b). With respect to the relative stability of


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the N and M mesophases, the results presented in Fig. 3b and Fig.4b indicate that a neutral aqueous solution tends to support the formation of regular two-dimensional arrays of the columnar aggregates and thus the formation of the M phase, while any deviation from the neutral environment (pH ≠ 7) tends to stabilize separated columnar stacks of the molecules and thus the formation of the N phase. Finally, the suitability of DSCG solutions as a liquid crystal host for DNA origami nanostructures was tested by studying the influence of di-valent Mg2+ ions on the mesophase formation. DNA nanostructures are commonly prepared and stored in a TAE (Tris / acetic acid / EDTA) buffer solution (pH 8.0) which contains magnesium chloride at concentrations of 10 × 10-3 M to 20 × 10-3 M MgCl2, thereby providing a sufficient ionic strength that is necessary to prevent unfolding of the DNA backbone. In order to test the mesophase stability under similar conditions, TAE buffer was used as a solvent and the phase behavior was studied at different concentrations of DSCG and MgCl2. The results indicate that the addition of any concentration of MgCl2 in the range above 1 mM causes the formation of a DSCG precipitate and thus impedes the fabrication of a homogeneous LC mixture. In order to explore the limits of solubility, a variety of mixtures with MgCl2 concentrations below 10-3 M was prepared. Precipitation was observed at a concentration of 0.725 × 10-3 M MgCl2 and at any larger concentration of magnesium chloride. The largest concentration, at which a homogeneous LC mixture was achieved, was 0.599 × 10-3 M MgCl2. Figure 5 shows the influence of MgCl2 content on the DSCG phase diagram for concentrations smaller than 0.6 × 10-3 M MgCl2. For these small concentrations, the addition of MgCl2 shifts the phase boundary to larger temperatures by a few centigrades.


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DISCUSSION AND CONCLUSIONS In summary, the present study shows that the formation of a homogeneous liquid crystalline mesophase in aqueous solutions of disodium cromoglycate is impeded by both large concentrations of hydrogen cations c(H+) > ≈10-5 M and large concentrations of magnesium cations c(Mg2+) > 6 × 10-4 M. Comparison of DSCG mixtures in different pH buffer solutions indicate that DSCG precipitates in acidic solutions. This can be explained by the formation of non-ionic cromoglicic acid, which is less soluble in water than the acid residue anions and their columnar aggregates. In contrast, the application of alkaline buffer solutions does not alter the mesophase stability and the transition temperatures significantly, except for a shift of the DSCG concentration, at which the M phase starts to appear. Obviously, the arrangement of the columnar aggregates of the DSCG molecules into the two-dimensional order of the columnar (M) phase appears in a neutral solution at lower DSCG concentrations than in acidic or alkaline solutions. The addition of magnesium chloride to a DSCG mixture can have a profound effect on the solubility of DSCG. For small concentrations [5 × 10-5 M ≤ c(MgCl2) ≤ 6 × 10-4 M], the phase transition temperatures can slightly increase in comparison with DSCG mixtures without added salt. However, DSCG precipitates at c(MgCl2) > ≈ 6 × 10-4 M – 7 × 10-4 M, so that no homogenous liquid crystal mixture with millimolar Mg2+ concentrations can be obtained. This behavior is quite different from the influence of monovalent cations, such as alkali metal ions, which has been studied, previously.33, 34 In agreement with these earlier observations,33, 34 we observed no precipitation at sodium ion concentrations of 1 M and even higher, but found precipitation even at Mg2+ concentrations below 1 mM.


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It is interesting to discuss a counter-intuitive observation. While amounts of magnesium chloride exceeding a fairly low critical concentration prevent the formation of a homogeneous LC mixture, an increase of the transition temperature (N+I) →I with increasing MgCl2 concentration is observed at small concentrations (Fig. 5). The latter finding indicates that the nematic phase is stabilized with respect to the isotropic phase. However, a similar competition between stabilizing and destabilizing effects was also observed in other systems and at different compositions. For example, the stabilizing effect of salt addition was found for alkali and ammonium salts in solutions of DSCG,34 the indanthrone derivative Blue 250,37 and sunset yellow.25 Comparison of different salts added to DSCG solutions, have shown that the shift of transition temperatures can be both positive or negative and is determined by the kind of cations and the ratio between the cation concentration and the DSCG concentration, while the anions play a minor role.34 Small-angle neutron scattering and cryo-transmission electron microscopy revealed an enhancement of the formation of columnar aggregates, but also the formation of bundles of aggregates upon the addition of alkali salts.34 Similar to the results reported here, an increase of the transition temperatures at moderate concentrations (10-3 – 10-2 M in that case), a broadening of the biphasic temperature range and a disappearance of the nematic phase at larger concentrations (10-2 – 10-1 M) were observed upon adding LiI, Na2SO4 or (NH4)2SO4 to solutions of Blue 250.37 This behavior could be attributed to an increase of both the aggregate length and the polydispersity of the aggregates. While the aggregate length promotes the formation of the nematic phase, increasing polydispersity is expected to yield a broadening of the biphasic region.53, 54 The very well-investigated solutions of sunset yellow (SSY) show some similarities, but also a striking difference in comparison with DSCG solutions. Lowering the pH value by adding HCl to a SSY solution was found to yield a decreasing aggregate length and a


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destabilization of the mesophases.26 An increase of the transition temperatures upon addition of monovalent salts was attributed to an increasing aggregate density (reduced electrostatic repulsion of the aggregates)27 and collective fluctuations of molecules in the isotropic phase,29 respectively. However, not only NaCl, but also MgCl2 and MgSO4 were found to increase the I → (I + N) transition temperature in SSY solutions at concentrations exceeding 500 mM and 1M, respectively.25 This stabilizing effect on the nematic phase is even larger for Mg2+ ions than for Na+ ions,25 in contrast to the behavior of DSCG solutions. The bundling of DSCG aggregates,34 which was not reported for SSY may account for the quite different behavior of the two mesogens. With respect to the proposed use of LCLCs formed by DSCG as an anisotropic solvent for anisometric DNA origami nanostructures,38 the following can be concluded. (a) The DSCG phase diagram is quite robust against using an alkaline buffer solution instead of de-ionized water. So, there is no impediment to applying the gently alkaline TAE buffer (pH 8.0), which is commonly used in DNA technology. (b) The concentrations of magnesium chloride [c(MgCl2) ≈ 1 × 10-2 M – 2 × 10-2 M], which are typically used to stabilize folded DNA nanostructures, exceed the critical concentration [c(MgCl2) ≈ 6 × 10-4 M – 7 × 10-4 M], at which DSCG precipitates, by more than an order of magnitude. So, the commonly used stabilizer magnesium chloride needs to be replaced by another salt – for example sodium chloride –, when a homogeneous chromonic DSCG nanocomposite containing DNA nanostructures is intended. In experiments, where the strong absorption of the dye sunset yellow is not disturbing, the latter may be preferred to the colorless DSCG, since the folding conditions of DNA origami nanostructures (concentrations of 10 – 20 mM MgCl2) do not prevent mesophase formation in SSY.


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FIGURES

Figure 1. (a) Chemical structure of a DSCG molecule. (b) Geometry of the samples.


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Figure 2. (a) Phase diagram of DSCG solutions in deionized water, which shows isotropic (I), nematic (N) and columnar (M) phases. (b-f) Textures of the mesophases observed in a polarizing microscope. (b) Coexistence of I and N phases, (c) nematic (N) phase, (d) coexistence of nematic (N) and columnar (M) phases, (e) columnar (M) phase, (f) coexistence of I and M phases.


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Figure 3. Influence of acidic buffer solutions on the DSCG/water phase diagram. (a) Transition temperatures versus DSCG concentration for pH = 7 (blue circles) and pH = 5 (red circles). (b) Influence of the pH value on the critical concentrations for the transitions N → (N+M) and (N+M) → M (orange symbols) and onset of the precipitation of DSCG crystals (blue symbols).


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Figure 4. Influence of alkaline buffer solutions on the DSCG/water phase diagram. (a) Transition temperatures versus DSCG concentration for pH = 7 (blue circles), pH = 8 (orange circles), pH = 10 (gray circles), and pH = 12 (yellow circles). (b) Influence of the pH value on


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the critical concentrations for the transitions N → (N+M) (blue symbols) and (N+M) → M (orange symbols).

Figure 5. Transition temperatures versus DSCG concentration in TAE buffer solution for different concentrations of magnesium chloride. Blue circles: DSCG in TAE buffer, no MgCl2; orange circles: 0.050 mM MgCl2; gray circles 0.189 mM MgCl2; yellow circles: 0.389 mM MgCl2; green circles: 0.599 mM MgCl2.


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AUTHOR INFORMATION Corresponding Author *Heinz-S. Kitzerow, email: [email protected], phone: +49 5251 602 156 Funding Sources The President of the University of Paderborn, Germany.

ACKNOWLEDGMENT The authors would like to thank Tim Liedl and his group at the Physics Department of the Ludwig-Maximilians-Universität München for illuminating discussions. Financial support of this work by the President of the University of Paderborn is gratefully acknowledged.

ABBREVIATIONS DSCG, disodium cromoglycate; EDTA, 2-({2-[bis(carboxymethyl)amino]ethyl}(carboxymethyl)amino) acetic acid; LC, liquid crystal; LCLC, lyotropic chromonic liquid crystal; pH, decimal logarithm of the reciprocal of the hydrogen ion activity; TAE, buffer solution (pH 8.0) containing Tris, acetic acid, and EDTA; Tris, 2-amino-2-hydroxymethyl-propane-1,3-diol.


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Table of Contents Graphic and Synopsis


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Fig. 1


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Fig. 2



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Fig. 3


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Fig. 4



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Fig. 5


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table of content 631x234mm (300 x 300 DPI)


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