DNA-induced chirality in water-soluble poly(cobaltoceniumethylene)

Abstract

Poly(cobaltoceniumethylene), a water-soluble cationic metal-containing polyelectrolyte, adopts a chiral structure when bound electrostatically to DNA.

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Metal-containing polymers are emerging as advanced functional materials and have attracted much attention due to the useful physical and catalytic properties that arise from the presence of metal centers.¹ For example, metallopolymers with reversible redox properties are of interest for applications in electrocatalysis² and sensing,³ as responsive surfaces⁴ and capsules,⁵ and as the active components of photonic crystal displays.⁶ Although the formation of helical organic and main group polymers has been observed,⁷ to the best of our knowledge, the introduction of chirality in a metallopolymer has not been achieved to date.

Polyelectrolytes are a class of macromolecules which possess cationic or anionic charges along the polymer chain and are of considerable industrial and commercial importance.⁸ Despite extensive studies on organic polyelectrolytes, examples of inorganic polyelectrolytes are rare,^9,10 especially for transition metal-containing polymers.¹¹ Recently, the development of the ring-opening polymerization (ROP) of sila[1]ferrocenophanes has enabled the synthesis of water-soluble polyferrocenylsilane polyelectrolytes with a variety of anionic or cationic pendant groups attached at the silicon centres.¹² More recently, we have synthesized high-molecular-weight (M_w ∼ 55 000, DP_w ∼ 198) water-soluble poly(cobaltoceniumethylene) nitrate PCE-NO₃ through thermal ROP of dicarba[2]cobaltocenophane 1, followed by oxidation in the presence of NH₄NO₃ (Scheme 1).¹³ One of the unique characteristics of this polymer is the location of the positive charge on the repeating unit of the polymer backbone. The centrally located metal-centered cation may enhance the structural replication of an anionic chiral template (e.g., DNA) compared to polyelectrolytes with pendant charges. Furthermore, the rotational freedom of the cobaltocenium units and the ethylene linkage may enable a facile conformational change to give a helical structure. Herein we describe experiments designed to induce chirality in PCE through complexation to chiral DNA templates.¹⁴

Scheme 1 Synthesis of PCE-NO₃.¹³

The previously synthesized PCE-NO₃^13b and a commercially available native double-stranded DNA (purified from salmon testes, in its sodium salt form) containing ca. 2000 base pairs were used in this work. PCE-NO₃ and DNA were dissolved in deionized water separately and combined at different mass ratios in order to induce complex formation. The light olivine solution became cloudy immediately, indicating a fast complexation of PCE and DNA, presumably as a result of the strong electrostatic interactions between the cobaltocenium cations and the anionic phosphodiester groups of the DNA backbone (Fig. 1). The resultant colloidal solution was found to be stable for several days.

Fig. 1 A speculated helical structure of the DNA/PCE complex and the electrostatic interaction between the cobaltocenium cation and the anionic phosphodiester group of the DNA backbone.

Cryogenic transmission electron microscopy (cryo-TEM) image (Fig. 2a) revealed that the colloidal solution that comprised a 1 : 1 mass ratio of DNA and PCE-NO₃ contained spherical nanoparticles with diameters of 15–40 nm. A large portion of the nanoparticles were fused together and formed small irregular aggregates. Dynamic light scatting (DLS) studies showed a narrow distribution of apparent hydrodynamic radius (R_H,app = 81 nm, Fig. 2b), which is consistent with the formation of aggregates in solution. Atomic force microscopy (AFM) height images confirmed that the complex retained spherical morphology after drying (Fig. 2c). However, owing to drying effects, large particles were observed on the silicon substrate and the size distribution of the particles became much broader (Fig. 2c and d, also see Fig. S2 (ESI†), for drop-cast TEM images).

Fig. 2 (a) Cryo-TEM image, (b) DLS plot, (c) AFM height image and (d) height profiles of the DNA/PCE complex. The mass ratio of DNA to PCE-NO₃ was 1 : 1.

To evaluate the chiral properties of the complexes, circular dichroism (CD) experiments were carried out on the colloidal solutions. The native double-stranded DNA exhibited a positive long wavelength band between 260–280 nm, a negative band around 250 nm, and a positive band around 220 nm (Fig. 3a, red line), implying a right-handed B-form helical conformation.¹⁵ After binding with PCE (DNA/PCE-NO₃ mass ratio >1 : 2), the intensity of the short wavelength bands gradually decreased, probably due to the decreasing proportion of DNA in the complex, while their position and shape remained very similar. These results indicated that DNA retained its helical secondary structure in these complexes in spite of the tight combination with PCE. However, such chirality vanished when the DNA/PCE-NO₃ mass ratio dropped to below 1 : 2 (Fig. 3a, green line), implying the collapse of the helical structure of DNA.

Fig. 3 (a) CD (top) and UV-vis (bottom) spectra of the DNA/PCE complexes formed with different DNA to PCE-NO₃ mass ratios. (b) CD difference between the DNA/PCE complexes and the native DNA. ΔCD = CD_complex − χCD_DNA, where χ is mass proportion of DNA in the DNA/PCE complex.

The change of the CD spectrum mainly occurred in the long wavelength region. At a lower proportion of PCE (Fig. 3a, dark gray and gray lines), the CD band split into two asymmetric peaks in the region of 250–300 nm, leaving a valley around 280 nm. The valley further grew into a negative band for the 2 : 1 and 1 : 1 complexes (Fig. 3a, light gray and blue lines). The evolution of the CD signal appears to indicate the emergence of a new band corresponding to PCE, which showed an intense UV-vis absorption band around 280 nm (see Fig. S3, ESI†). To efficiently elucidate the subtle changes observed, the “background” CD contribution of DNA was proportionally subtracted from the CD spectrum of the complex (Fig. 3b). The CD difference (ΔCD) patterns revealed a negative band around 280 nm and a positive band around 260 nm with intensity gradually increasing with an increase in PCE content in the complex (until the mass ratio of DNA : PCE-NO₃ = 1 : 1). The former band fits perfectly with the adsorption band of PCE, strongly suggesting that PCE has adopted a chiral structure within the complex as a result of templating by DNA. On the other hand, the second band may suggest a slight structural change of DNA induced by the electrostatic binding with PCE.

Two possibilities would exist for the origin of induced chirality of PCE. One is similar to that of 1,1′-bi-2-naphthol,¹⁶ where the chirality originates from an axially chiral structure of the cobaltoceniumethylene units. As shown in Fig. S4 (ESI†), when binding to DNA, the cobaltoceniumethylene units may orient along the long axis of DNA and adopt a chiral conformation to fit the right-handed double-helices of DNA. However, such orientation would result in a serious mismatch between the positions of positive (cobaltocenium cation) and negative (phosphodiester anion) binding sites, which is thermodynamically unfavourable. Another possibility involves the cobaltocenium units possessing no specific orientation, but rather the main chain of PCE taking on a helical conformation along the DNA helices, most possibly embedded in the major or minor groove of DNA (Fig. 1), to favour electrostatic interactions and to minimize interaction with the solvent. The chromophoric groups (cobaltocenium centres) would then be aligned in a helical fashion and would eventually generate the CD signal. Raman spectra were conducted on freeze-dried complexes (see Fig. S5, ESI†). Unfortunately, structural deformation of DNA occurred during drying and structural analysis was complicated by the absence of a PCE reference due to the strong fluorescent background.

Two shorter samples of DNA (ca. 700 bp and<50 bp, see ESI† for details), showed typical B-type helical structures and were also complexed to PCE. The DNA/PCE complexes had similar CD evolution profiles to those described above (Fig. S6 and S7, ESI†). However, the complexes formed with the short DNA (<50 bp) exhibited relatively weaker induced CD in the region of 250–300 nm, especially for the 1 : 1 complex. In the long DNA-based complexes, a single DNA fiber has ample space for several PCE chains to complex to and replicate its helical structure. In contrast, in the short DNA-based complexes, a single DNA strand is unable to provide sufficient negative binding sites (<100 phosphodiester groups) to neutralize the positive charges of a single PCE chain (DP_w ∼ 198) and thus a single PCE chain may interact simultaneously with two or more DNA strands. This behavior may result in the appearance of template-free randomly oriented PCE in the gap between two DNA strands thus reducing the induced CD intensity.

In conclusion, the template induced chiral structure of a water-soluble metal-containing polymer, PCE, has been achieved for the first time using DNA as an anionic template. The induction of chirality was thought to be facilitated by the location of the positive charges and the structural flexibility of PCE. In principle, the electrostatic templating demonstrated in this study may be applied to other metallopolymers, e.g., polyferrocenylsilanes, to induce chirality. We are currently investigating the use of chiral molecules as templates and the cooperative interaction of chiral templates with self-assembled block copolymers.¹⁷

We thank Dr Paul Verkade and Judith Mantell (Wolfson Bioimaging Facility, University of Bristol) for the cryo-TEM imaging and Professor Shunai Che (Shanghai Jiao Tong University) for her help with the Raman experiments and Prof. Derek N. Woolfson (University of Bristol) for the use of his CD equipment. We thank the EU Marie Curie program (H.Q. and J.B.G.) and the NSERC of Canada (J.B.G.) for postdoctoral fellowships. I.M. also thanks the EU for an ERC Advanced Investigator Grant.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and additional results. See DOI: 10.1039/c2cc37026c

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