Numerous biological systems in our body continuously carry out their tasks by undergoing specific conformational changes in a particular domain of the functioning object. This conformational change at a specific site is triggered by various stimuli to make the process easily operable in a reversible manner. One well-known example is the spontaneous folding of the human telomeric DNA into a secondary G-quadruplex structure via a conformational change in the presence of K⁺ ions. The reverse process is assisted by the hPOT1 protein, which is essential for the transcription process.^{1, 2} Another instance is the process of transcription that involves the unwinding of the DNA double helix structure with the help of the helicase enzymes.³ As helical structures are widely adopted by natural systems and considering the widespread occurrence of such systems, there is a tremendous interest in the scientific community to mimic the synthetic helical systems that can interchange between multiple forms via a conformational change in response to specific stimuli.

However, artificial systems which can closely mimic the natural congeners, like the unwinding of the DNA double helix into the open conformation, are rare, and only a few examples are reported in the literature. For instance, Yashima and co-workers have reported an oligoresorcinol-based double helix,⁴ which switched to its unwind form in the presence of cyclodextrin. In order to regenerate the double helix structure, adamantane was used, which blocks the beta-cyclodextrin cavity for releasing the monomer. In this work, we have reported a bis(indole) based system, which forms a double helix structure and shows unwinding-rewinding behavior via conformational change. The bis(indole) system forms a stable double helix structure in both solid and solution states. Detailed experimental investigations confirmed that the double helix structure is formed by employing intermolecular hydrogen bonding interactions. In the presence of anions, the double helix structure undergoes conformational change for the unwinding of its helix, which subsequently leads to the formation of ion coordinated supramolecular polymeric structure. The unwinding process was found to be highly selective for Cl^- ions as compared to other halides and nitrate ions. Moreover, the double helix structure was again regenerated upon the addition of Ag⁺ salts.

Anion-coordinated self-assembled supramolecular polymers are of tremendous interest owing to their several biomedical applications. Interestingly, our system forms a supramolecular ion channel structure for efficient and selective transport of chloride ions across the bilayer membranes. We are hopeful that this bis(indole)-based system with excellent bio-functional properties would provide a way to develop smart materials with efficient bio-medical applications. 

1. Burge, S., Parkinson, G. N., Hazel, P., Todd, A. K. & Neidle, S. Quadruplex DNA: sequence, topology and structure. Nucleic Acids Res. 34, 5402-5415 (2006).
2. Zaug, A. J., Podell, E. R. & Cech, T. R. Human POT1 disrupts telomeric G-quadruplexes allowing telomerase extension in vitro. Natl. Acad. Sci. USA 102, 10864-10869 (2005).
3. Burnham, D. R., Kose, H. B., Hoyle, R. B. & Yardimci, H. The mechanism of DNA unwinding by the eukaryotic replicative helicase. Nat. Commun. 10, 2159 (2019).
4. Goto, H., Furusho, Y. & Yashima, E. Supramolecular Control of Unwinding and Rewinding of a Double Helix of Oligoresorcinol Using Cyclodextrin/Adamantane System. J. Am. Chem. Soc. 129, 109-112 (2007).