A Cationic Tetraphenylethene as a Light-Up Supramolecular Probe for DNA G-Quadruplexes

Abstract

Guanine-quadruplexes (G4s) are targets for anticancer therapeutics. In this context, human telomeric DNA (HT-DNA) that can fold into G4s sequences are of particular interest, and their stabilization with small molecules through a visualizable process has become a challenge. As a new type of ligand for HT-G4, we designed a tetraimidazolium tetraphenylethene ( TPE-Im ) as a water-soluble light-up G4 probe. We study its G4-binding properties with HT-DNA by UV-Visible absorption, circular dichroism and fluorescence spectroscopies, which provide insights into the interactions between TPE-Im and G4-DNA. Remarkably, TPE-Im shows a strong fluorescence enhancement and large shifts upon binding to G4, which is valuable for detecting G4s. The association constants for the TPE-Im /G4 complex were evaluated in different solution conditions via isothermal titration calorimetry (ITC), and its binding modes were explored by molecular modeling showing a groove-binding mechanism. The stabilization of G4 by TPE-Im has been assessed by Fluorescence Resonance Energy Transfer (FRET) melting assays, which show a strong stabilization (Δ T 1/2 around +20°C), together with a specificity toward G4 with respect to double-stranded DNA.

Synthesis, Self‐Assembly, and Nucleic Acid Recognition of an Acylhydrazone‐Conjugated Cationic Tetraphenylethene Ligand

“…The results show a significant increase in the melting temperature (ΔT 1/2 =16 °C) that indicates a strong stabilization of this G4 structure (Figure 6D). This value is close to that reported for tetraimidazolium tetraphenylethene (TPE−Im) (ΔT 1/2 =19.5 °C) [26a] and slightly lower than ligands with larger aromatic cores such as tetrakis(N‐methylpyridinium‐4‐yl)porphyrin (ΔT 1/2 =23 °C) or tetraimidazolium‐fused porphyrin (ΔT 1/2 =25 °C) [45c] . Nevertheless, no decrease of this melting temperature is noted after addition of 10 equivalents of a random sequence dsDNA (with 43 base pairs), which points toward a great selectivity of TPE‐Gir for G4 compared to dsDNA, possibly due to more important π‐π interactions (see below).…”

“…Interestingly, we can also notice, from the CD spectra, changes in DNA structure (positive band shifted from 280 to 260 nm; negative band shifted from 245 to 230 nm) that could indicate a B‐to‐A helix conversion in the presence of TPE‐Gir (Figure 5C) [42] . The marked AIE effect was further evidenced in gel electrophoresis experiments where luminescent bands for complexed plasmids are observed when irradiated at 320 nm (Figure S3, ESI) – an essential but not sufficient prerequisite for “light‐up” probes [25b,26a] …”

“…Examples of ligand families are acridines, bisquinoliniums, porphyrin, naphthalene‐ and perylene‐diimides which interact with G4 targets by electrostatic interactions with the loops and by π‐type interactions with G‐quartets [44] . In this line, we have recently evidenced the selective binding of different ligands for G4s, [45] including some based on TPE that have also been explored by us [26a] and others, [26d] finding important effects related to the nature of the spacers connecting the central aromatic core with the peripheral cationic groups [26b,c] . Due to the strong propensity of TPE‐Gir to engage in π stacking interactions with nucleobases and its original acylhydrazone spacer, we were interested in its binding properties to G4s.…”

“…Finally, we assessed the stabilization of G‐quadruplex and the binding selectivity of TPE‐Gir by thermal denaturation through a FRET assay [48] . For this purpose, we used a modified Tel22, named F21T, appended with a FAM dye at position 5’ and a TAMRA dye at position 3’ [26a] . The results show a significant increase in the melting temperature (ΔT 1/2 =16 °C) that indicates a strong stabilization of this G4 structure (Figure 6D).…”

From Interaction to Function in DNA‐Templated Supramolecular Self‐Assemblies

“…In a collaboration between the group of Clément and ours, we have recently reported a water‐soluble tetraimidazolium tetraphenylethene (Figure D) acting as a light‐up probe for sensing DNA guanine‐quadruplexes (G4s) by fluorescence spectroscopy . The compound displays fluorescence turn‐on and large emission wavelengths shifts triggered by G4 binding, which are attractive features for future sensing applications.…”