Targeting Human Telomeric Higher-Order DNA: Dimeric G-Quadruplex Units Serve as Preferred Binding Site

Binding and stabilizating effect of RNA triplex poly(U)⋅poly(A)*poly(U) by enantiomers of ruthenium(II) polypyridyl complex [Ru(bpy)2(dppx)]2

Two chiral ruthenium(II) polypyridyl complexes, Λ-[Ru(bpy)₂(dppx)]²⁺ (bpy = 2,2'-bipyridine, dppx = 7,8-dimethyldipyridophenazine; Λ-1) and Δ-[Ru(bpy)₂(dppx)]²⁺ (Δ-1) have been synthesized and characterized in this work. Interactions of Λ-1 and Δ-1 with the RNA triplex poly(U)⋅poly(A)*poly(U) have been investigated by various biophysical techniques. Spectrophotometric titrations and viscosity measurements suggested that enantiomers Λ-1 and Δ-1 bind with the triplex through intercalation, while the binding strengths of the two enantiomers toward the triplex differed only slightly from each other. Fluorescence titrations showed that although enantiomers Λ-1 and Δ-1 exhibited molecular "light switch" effects toward the triplex, the effect of Δ-1 was more marked. Furthermore, Furthermore, thermal denaturation showed that the two enantiomers have significantly different stabilizing effects on the triplex. The obtained results indicate that the racemic complex [Ru(bpy)₂(dppx)]²⁺ is similar to a non-specific metallointercalator for the triplex investigated in this study, and chiralities of Ru(II) polypyridine complexes have an important influence on the binding and stabilizing effects of enantiomers toward the triplex. Two chiral ruthenium(II) polypyridyl complexes, Λ-[Ru(bpy)₂(dppx)]²⁺ (bpy = 2,2'-bipyridine, dppx = 7,8-dimethyldipyridophenazine; Λ-1) and Δ-[Ru(bpy)₂(dppx)]²⁺ (Δ-1) have been synthesized and characterized in this work. Interactions of Λ-1 and Δ-1 with the RNA triplex poly(U)⋅poly(A)*poly(U) have been investigated by various biophysical techniques. The obtained results indicate that the racemic complex [Ru(bpy)₂(dppx)]²⁺ is similar as a non-specific metallointercalator for the triplex investigated in this study, and chiralities of Ru(II) polypyridine complexes have an important influence on the binding and stabilizing effects of enantiomers toward the triplex.

Dimeric Metal-Salphen Complexes Which Target Multimeric G-Quadruplex DNA

G-Quadruplex DNA structures have attracted increasing attention due to their biological roles and potential as targets for the development of new drugs. While most guanine-rich sequences in the genome have the potential to form monomeric G-quadruplexes, certain sequences have enough guanine-tracks to give rise to multimeric quadruplexes. One of these sequences is the human telomere where tandem repeats of TTAGGG can lead to the formation of two or more adjacent G-quadruplexes. Herein we report on the modular synthesis via click chemistry of dimeric metal-salphen complexes (with Ni^II and Pt^II) bridged by either polyether or peptide linkers. We show by circular dichroism (CD) spectroscopy that they generally have higher selectivity for dimeric vs monomeric G-quadruplexes. The emissive properties of the Pt^II-salphen dimeric complexes have been used to study their interactions with monomeric and dimeric G-quadruplexes in vitro as well as to study their cellular uptake and localization.

Effects of Molecular Crowding on the Structure, Stability, and Interaction with Ligands of G-quadruplexes

G-quadruplexes (G4s) are widely found in cells and have significant biological functions, which makes them a target for screening antitumor and antiviral drugs. Most of the previous research on G4s has been conducted mainly in diluted solutions. However, cells are filled with organelles and many biomolecules, resulting in a constant state of a crowded molecular environment. The conformation and stability of some G4s were found to change significantly in the molecularly crowded environment, and interactions with ligands were disturbed to some extent. The structure of the G4s and their biological functions are correlated, and the effect of the molecularly crowded environment on G4 conformational transitions and interactions with ligands should be considered in drug design targeting G4s. This review discusses the changes in the conformation and stability of G4s in a physiological environment. Moreover, the mechanism of action of the molecularly crowded environment affecting the G4 has been further reviewed based on previous studies. Furthermore, current challenges and future research directions are put forward. This review has implications for the design of drugs targeting G4s.

Inhibited complete folding of consecutive human telomeric G-quadruplexes

Noncanonical DNA structures, termed G-quadruplexes, are present in human genomic DNA and are important elements in many DNA metabolic processes. Multiple sites in the human genome have G-rich DNA stretches able to support formation of several consecutive G-quadruplexes. One of those sites is the telomeric overhang region that has multiple repeats of TTAGGG and is tightly associated with both cancer and aging. We investigated the folding of consecutive G-quadruplexes in both potassium- and sodium-containing solutions using single-molecule FRET spectroscopy, circular dichroism, thermal melting and molecular dynamics simulations. Our observations show coexistence of partially and fully folded DNA, the latter consisting of consecutive G-quadruplexes. Following the folding process over hours in sodium-containing buffers revealed fast G-quadruplex folding but slow establishment of thermodynamic equilibrium. We find that full consecutive G-quadruplex formation is inhibited by the many DNA structures randomly nucleating on the DNA, some of which are off-path conformations that need to unfold to allow full folding. Our study allows describing consecutive G-quadruplex formation in both nonequilibrium and equilibrium conditions by a unified picture, where, due to the many possible DNA conformations, full folding with consecutive G-quadruplexes as beads on a string is not necessarily achieved.

Selectively recognizing heptad-interfaced G-quadruplexes by a molecular rotor with an ESIPT emission

Heptad-interfaced G-quadruplexes (G4s), formed with GGA repeats located in the nuclear proto-oncogene c-myb promoter, can be selectively targeted by a prenylated flavonol of sophoflavescenol (Sop) with restriction of molecular rotation to trigger strong excited state intramolecular proton transfer (ESIPT) emission.

Insight into achirality and chirality effects in interactions of an racemic ruthenium(II) polypyridyl complex and its Δ- and Λ-enantiomers with an RNA triplex

RNA triplexes have a variety of potential applications in molecular biology, diagnostics and therapeutics, while low stabilization of the third strand hinders their practical utilities under physiological conditions. In this regard, achieving the third-strand stabilization by binding small molecules is a promising strategy. Chirality is one of the basic properties of nature. To clarify achirality and chirality effects on the binding and stabilizing effects of RNA triplexes by small molecules, we report for the first time the RNA interactions of an racemic ruthenium(II) polypyridyl complex [Ru(bpy)₂(11-CN-dppz)]²⁺ (rac-Ru1) and its two enantiomers Δ/Λ-[Ru(bpy)₂(11-CN-dppz)]²⁺ (Δ/Λ-Ru1) with an RNA triplex poly(U-A*U) (where "-" represents Watson-Crick base pairing, and "*" denotes Hoogsteen base pairing, respectively) in this work. Research shows that although rac-Ru1 and its two enantiomers Δ/Λ-Ru1 bind to the RNA triplex through the same mode of intercalation, the binding affinity for enantiomer Δ-Ru1 is much higher than that for rac-Ru1 and enantiomer Λ-Ru1. However, compared to enantiomer Λ-Ru1, the binding affinity for rac-Ru1 does not show much of an advantage, which is slightly greater than that for the former. Thermal denaturation measurements reveal both rac-Ru1 and Δ-Ru1 to have a preference for stabilizing the third strand rather than the template duplex of the RNA triplex, while Λ-Ru1 stabilizes the RNA triplex without significant selectivity. Besides, the third-strand stabilizing effects by rac-Ru1 and Δ-Ru1 are not markedly different from each other, but more marked than that by Λ-Ru1. This work shows that the binding properties of the racemic Ru(II) polypyridyl complex with the RNA triplex are not simply an average of its two enantiomers, indicating potentially complicated binding events.

Multimeric G-quadruplexes: A review on their biological roles and targeting

In human cells, nucleic acids adopt several non-canonical structures that regulate key cellular processes. Among them, G-quadruplexes (G4s) are stable structures that form in guanine-rich regions in vitro and in cells. G4 folded/unfolded state shapes numerous cellular processes, including genome replication, transcription, and translation. Moreover, G4 folding is involved in genomic instability. G4s have been described to multimerize, forming high-order structures in both DNA and/or RNA strands. Multimeric G4s can be formed by adjacent intramolecular G4s joined by stacking interactions or connected by short loops. Multimeric G4s can also originate from the assembly of guanines embedded on independent DNA or RNA strands. Notably, crucial regions of the human genome, such as the 3'-terminal overhang of the telomeric DNA as well as the open reading frame of genes involved in the preservation of neuron viability in the human central and peripheral nervous system are prone to form multimeric G4s. The biological importance of such structures has been recently described, with multimeric G4s playing potentially protective or deleterious effects in the pathogenic cascade of various diseases. Here, we portray the multifaceted scenario of multimeric G4s, in terms of structural properties, biological roles, and targeting strategies.

Metallointercalators-DNA Tetrahedron Supramolecular Self-Assemblies with Increased Serum Stability

Self-assembly of metallointercalators into DNA nanocages is a rapid and facile approach to synthesize discrete bioinorganic host/guest structures with a high load of metal complexes. Turberfield's DNA tetrahedron can accommodate one intercalator for every two base pairs, which corresponds to 48 metallointercalators per DNA tetrahedron. The affinity of the metallointercalator for the DNA tetrahedron is a function of both the structure of the intercalating ligand and the overall charge of the complex, with a trend in affinity [Ru(bpy)₂(dppz)]²⁺ > [Tb-DOTAm-Phen]³⁺ ≫ Tb-DOTA-Phen. Intercalation of the metal complex stabilizes the DNA tetrahedron, resulting in an increase of its melting temperature and, importantly, a significant increase in its stability in the presence of serum. [Ru(bpy)₂(dppz)]²⁺, which has a greater affinity for DNA than [Tb-DOTAm-Phen]³⁺, increases the melting point and decreases degradation in serum to a greater extent than the Tb^III complex. In the presence of Lipofectamine, the metallointercalator@DNA nanocage assemblies substantially increase the cell uptake of their respective metal complex. Altogether, the facile incorporation of a large number of metal complexes per assembly, the higher stability in serum, and the increased cell penetration of metallointercalator@DNA make these self-assemblies well-suited as metallodrugs.

Smart Metal-Organic Frameworks for Biotechnological Applications: A Mini-Review

In this pandemic situation it is evident that viruses and bacteria, more specifically, multiple drug resistant (MDR) bacteria, endanger human civilization severely. It is high time to design smart weapons to combat these pathogens for the prevention and cure of allied ailments. Metal-organic frameworks (MOFs) are porous materials designed from metal ions or inorganic clusters and multidentate organic ligands. Due to some unique features like high porosity, tunable pore shape and size, numerous possible metal-ligand combinations, etc., MOFs are ideal candidates to design "smart biotechnological tools". MOFs construct promising fluorescence based biosensing platforms for detection of viruses. MOFs also exhibit excellent antibacterial activity due to their ability for sustained release of active biocidal agents. There are several reviews that summarize the antibacterial applications of MOFs, but the biosensing platforms based on MOFs for detection of viruses have scarcely been summarized. This review carefully covers both the aspects including virus detection (nucleic acid recognition and immunological detection) with underlying mechanisms as well as antibacterial application of MOFs and doped MOFs or composites. This review will deliver valuable information and references for designing new, smarter antimicrobial agents based on MOFs.

Insights into the free energy landscape and salt-controlled mechanism of the conformational conversions between human telomeric G-quadruplex structures

G-quadruplexes have become attractive drug targets in cancer therapy. However, due to the polymorphism of G-quadruplex structures, it is difficult to experimentally verify the relevant structures of multiple intermediates and transition states in dynamic equilibrium. Hence, understanding the mechanism by which structural conversions of G-quadruplexes occur is still challenging. We conducted targeted molecular dynamics simulation with umbrella sampling to investigate how salt affects the conformational conversion of human telomeric G-quadruplex. Our results explore a unique view into the structures and energy barrier of the intermediates and transition states in the interconversion process. The pathway of G-quadruplex conformational interconversion was mapped out by a free energy landscape, consisting of branched parallel pathways with multiple energy basins. We propose a salt-controlled mechanism that as the salt concentration increases, the conformational conversion mechanism switches from multi-pathway folding to sequential folding pathways. The hybrid-I and hybrid-II structures are intermediates in the basket-propeller transformation. In high-salt solutions, the conformational conversion upon K⁺ binding is more feasible than upon Na⁺ binding. The free energy barrier for conformational conversions ranges from 1.6 to 4.6 kcal/mol. Our work will be beneficial in developing anticancer agents.

G-quadruplex binder pyridostatin as an effective multi-target ZIKV inhibitor

At present, there are still no anti-Zika virus (ZIKV) drugs or vaccines approved by FDA with accurate targets and antiviral mechanisms. Considering the RNA G-quadruplex sequences in ZIKV genome, it is very meaningful to develop G-quadruplex binders as potential anti-ZIKV drugs with novel and accurate targets. In this paper, five classical G-quadruplex binders including Ber, Braco 19, NiL, 360A and PDS have been chosen to discuss their interaction with ZIKV RNA G-quadruplexes. PDS shows higher binding affinity and thermal stability than the other G-quadruplex binders. This property is further evidenced in cells by immunofluorescence microscopy. And PDS shows higher anti-ZIKV activity (EC₅₀ = 4.2 ± 0.4 μM) than the other G-quadruplex binders as well as the positive control ribavirin, with a low cytotoxicity. By time-of-addition assay, PDS exerts antiviral activity at the post-entry process of ZIKV replication cycle, thus inhibiting ZIKV mRNA replication and protein expression. Furthermore, PDS combines with ZIKV NS2B-NS3 protease and reduces its catalytic activity. This study suggests that G-quadruplex binder PDS is an effective multi-target ZIKV inhibitor, which provides more guidance to design some novel anti-ZIKV drugs targeting ZIKV RNA G-quadruplexes.

Structure and Dynamics of RNA Guanine Quadruplexes in SARS-CoV-2 Genome. Original Strategies against Emerging Viruses

Guanine quadruplex (G4) structures in the viral genome have a key role in modulating viruses' biological activity. While several DNA G4 structures have been experimentally resolved, RNA G4s are definitely less explored. We report the first calculated G4 structure of the RG-1 RNA sequence of SARS-CoV-2 genome, obtained by using a multiscale approach combining quantum and classical molecular modeling and corroborated by the excellent agreement between the corresponding calculated and experimental circular dichroism spectra. We prove the stability of the RG-1 G4 arrangement as well as its interaction with G4 ligands potentially inhibiting viral protein translation.

Porphyrin Binding and Irradiation Promote G-Quadruplex DNA Dimeric Structure

Nucleic acid sequences rich in guanines can organize into noncanonical DNA G-quadruplexes (G4s) of variable size. The design of small molecules stabilizing the structure of G4s is a rapidly growing area for the development of novel anticancer therapeutic strategies and bottom-up nanotechnologies. Among a multitude of binders, porphyrins are very attractive due to their light activation that can make them valuable conformational regulators of G4s. Here, a structure-based strategy, integrating complementary probes, is employed to study the interaction between TMPyP4 porphyrin and a 22-base human telomeric sequence (Tel22) before and after irradiation with blue light. Porphyrin binding is discovered to promote Tel22 dimerization, while light irradiation of the Tel22-TMPyP4 complex controls dimer fraction. Such a change in quaternary structure is found to be strictly correlated with modifications at the secondary structure level, thus providing an unprecedented link between the degree of dimerization and the underlying conformational changes in G4s.

The Binding Pocket at the Interface of Multimeric Telomere G-quadruplexes: Myth or Reality?

Human telomeric DNA with hundreds of repeats of the 5'-TTAGGG-3' motif plays a crucial role in several biological processes. It folds into G-quadruplex (G4) structures and features a pocket at the interface of two contiguous G4 blocks. Up to now no structural NMR and crystallographic data are available for ligands interacting with contiguous G4s. Naphthalene diimide monomers and dyads were investigated as ligands of a dimeric G4 of human telomeric DNA comparing the results with those of the model monomeric G4. Time-resolved fluorescence, circular dichroism, isothermal titration calorimetry and molecular modeling were used to elucidate binding features. Ligand fluorescence lifetime and induced circular dichroism unveiled occupancy of the binding site at the interface. Thermodynamic parameters confirmed the hypothesis as they remarkably change for the dyad complexes of the monomeric and dimeric telomeric G4. The bi-functional ligand structure of the dyads is a fundamental requisite for binding at the G4 interface as only the dyads engage in complexes with 1 : 1 stoichiometry, lodging in the pocket at the interface and establishing multiple interactions with the DNA skeleton. In the absence of NMR and crystallographic data, our study affords important proofs of binding at the interface pocket and clues on the role played by the ligand structure.

Cavity-Containing [Fe2L3]4+ Helicates: An Examination of Host-Guest Chemistry and Cytotoxicity

Two new di(2,2′-bipyridine) ligands, 2,6-bis([2,2′-bipyridin]-5-ylethynyl)pyridine (L1) and bis(4-([2,2′-bipyridin]-5-ylethynyl)phenyl)methane (L2) were synthesized and used to generate two metallosupramolecular [Fe₂(L)₃](BF₄)₄ cylinders. The ligands and cylinders were characterized using elemental analysis, electrospray ionization mass spectrometry, UV-vis, ¹H-, ¹³C and DOSY nuclear magnetic resonance (NMR) spectroscopies. The molecular structures of the [Fe₂(L)₃](BF₄)₄ cylinders were confirmed using X-ray crystallography. Both the [Fe₂(L1)₃](BF₄)₄ and [Fe₂(L2)₃](BF₄)₄ complexes crystallized as racemic (rac) mixtures of the ΔΔ (P) and ΛΛ (M) helicates. However, ¹H NMR spectra showed that in solution the larger [Fe₂(L2)₃](BF₄)₄ was a mixture of the rac-ΔΔ/ΛΛ and meso-ΔΛ isomers. The host-guest chemistry of the helicates, which both feature a central cavity, was examined with several small drug molecules. However, none of the potential guests were found to bind within the helicates. In vitro cytotoxicity assays demonstrated that both helicates were active against four cancer cell lines. The smaller [Fe₂(L1)₃](BF₄)₄ system displayed low μM activity against the HCT116 (IC₅₀ = 7.1 ± 0.5 μM) and NCI-H460 (IC₅₀ = 4.9 ± 0.4 μM) cancer cells. While the antiproliferative effects against all the cell lines examined were less than the well-known anticancer drug cisplatin, their modes of action would be expected to be very different.

Spectroscopic and molecular docking studies for the binding and interaction aspects of curcumin-cysteine conjugate and rosmarinic acid with human telomeric G-quadruplex DNA

The binding and interaction aspects of potential anticancer ligands like: curcumin-cysteine (CC) and rosmarinic acid (RA) with human telomeric G-quadruplex DNA, a novel anticancer target, have been probed by spectroscopic and molecular docking approach. The circular dichroism study unravels the conformational switching from mixed hybrid to parallel structure for the short sequence of human telomeric G-quadruplex structure in the presence of both the ligands. Further a good correlation for binding affinity has been established from the emission and absorption binding spectrum analysis. Further our spectroscopic and molecular docking studies have suggested that the CC having better binding capability than RA to human telomeric G-quadruplex. The presence of L-cysteine moiety in CC ligand is responsible factor for its binding via both minor as well as major groove of human telomeric G-quadruplex DNA where-as RA binds only via minor groove of telomeric G-DNA.

Quadruplex Ligands in Cancer Therapy

Nucleic acids can adopt alternative secondary conformations including four-stranded structures known as quadruplexes. To date, quadruplexes have been demonstrated to exist both in human chromatin DNA and RNA. In particular, quadruplexes are found in guanine-rich sequences constituting G-quadruplexes, and in cytosine-rich sequences forming i-Motifs as a counterpart. Quadruplexes are associated with key biological processes ranging from transcription and translation of several oncogenes and tumor suppressors to telomeres maintenance and genome instability. In this context, quadruplexes have prompted investigations on their possible role in cancer biology and the evaluation of small-molecule ligands as potential therapeutic agents. This review aims to provide an updated close-up view of the literature on quadruplex ligands in cancer therapy, by grouping together ligands for DNA and RNA G-quadruplexes and DNA i-Motifs.

A highly selective switch-on fluorescence sensor targeting telomeric dimeric G-quadruplex

The fluorescence probes with high selectivity and sensitivity for telomeric multimeric G-quadruplexes have attracted much attention. Nevertheless, few small molecules have exhibited telomeric multimeric G-quadruplexes recognition specificity. Thus, there is an urgent demand to develop specific fluorescence probes for telomeric multimeric G-quadruplexes. We reported herein the specific sensing of telomeric dimeric G-quadruplex TTA45 via a fluorescence light-up response using a commercially available triazine derivative HPTA-1 as a probe. HPTA-1 could discriminate the telomeric dimeric G-quadruplex TTA45 against other types of DNA structures accompanied by a drastic enhancement of the emission intensity without compromising the conformation and stability. Compared with most multimeric G-quadruplex recognition ligands, HPTA-1 had much simpler structure and lower molecular weight. The binding mechanism studies suggested that the distinct fluorescence response was caused by electrostatic and π-π stacking interactions of HPTA-1 with the pocket between two G-quadruplex units of telomeric dimeric G-quadruplex TTA45..

DNA folds threaten genetic stability and can be leveraged for chemotherapy

Damaging DNA is a current and efficient strategy to fight against cancer cell proliferation. Numerous mechanisms exist to counteract DNA damage, collectively referred to as the DNA damage response (DDR) and which are commonly dysregulated in cancer cells. Precise knowledge of these mechanisms is necessary to optimise chemotherapeutic DNA targeting. New research on DDR has uncovered a series of promising therapeutic targets, proteins and nucleic acids, with application notably via an approach referred to as combination therapy or combinatorial synthetic lethality. In this review, we summarise the cornerstone discoveries which gave way to the DNA being considered as an anticancer target, and the manipulation of DDR pathways as a valuable anticancer strategy. We describe in detail the DDR signalling and repair pathways activated in response to DNA damage. We then summarise the current understanding of non-B DNA folds, such as G-quadruplexes and DNA junctions, when they are formed and why they can offer a more specific therapeutic target compared to that of canonical B-DNA. Finally, we merge these subjects to depict the new and highly promising chemotherapeutic strategy which combines enhanced-specificity DNA damaging and DDR targeting agents. This review thus highlights how chemical biology has given rise to significant scientific advances thanks to resolutely multidisciplinary research efforts combining molecular and cell biology, chemistry and biophysics. We aim to provide the non-specialist reader a gateway into this exciting field and the specialist reader with a new perspective on the latest results achieved and strategies devised.

Metallohelices emulate the properties of short cationic α-helical peptides

Naturally occurring peptides in many living systems perform antimicrobial and anticancer host defence roles, but their potential for clinical application is limited by low metabolic stability and relatively high costs of goods. Self-assembled helical metal complexes provide an attractive synthetic platform for non-peptidic architectures that can emulate some of the properties of short cationic α-helical peptides, with tuneable charge, shape, size and amphipathicity. Correspondingly there is a growing body of evidence demonstrating that these supramolecular architectures exhibit bioactivity that emulates that of the natural systems. We review that evidence in the context of synthetic advances in the area, driven by the potential for biomedical applications. We note some design considerations for new biologically-relevant metallohelices, and give our outlook on the future of these compounds as therapeutic peptidomimetics.

BMPQ-1 binds selectively to (3+1) hybrid topologies in human telomeric G-quadruplex multimers

A single G-quadruplex forming sequence from the human telomere can adopt six distinct topologies that are inter-convertible under physiological conditions. This presents challenges to design ligands that show selectivity and specificity towards a particular conformation. Additional complexity is introduced in differentiating multimeric G-quadruplexes over monomeric species, which would be able to form in the single-stranded 3′ ends of telomeres. A few ligands have been reported that bind to dimeric quadruplexes, but their preclinical pharmacological evaluation is limited. Using multidisciplinary approaches, we identified a novel quinoline core ligand, BMPQ-1, which bound to human telomeric G-quadruplex multimers over monomeric G-quadruplexes with high selectivity, and induced the formation of G-quadruplex DNA along with the related DNA damage response at the telomere. BMPQ-1 reduced tumor cell proliferation with an IC₅₀ of ∼1.0 μM and decreased tumor growth rate in mouse by half. Biophysical analysis using smFRET identified a mixture of multiple conformations coexisting for dimeric G-quadruplexes in solution. Here, we showed that the titration of BMPQ-1 shifted the conformational ensemble of multimeric G-quadruplexes towards (3+1) hybrid-2 topology, which became more pronounced as further G-quadruplex units are added.

Dimeric G-Quadruplex: An Effective Nucleic Acid Scaffold for Lighting Up Thioflavin T

As a recently identified higher-order quadruplex (G4) structure, the G4 dimer possesses unique structure and biological functions. In this work, we found accidentally that two tandem PW17 (one known G4-forming DNA) sequences can fold into a stable G4 dimer, and the G4 dimer can enhance dramatically the fluorescence intensity of thioflavin T (ThT). The G4 dimer/ThT fluorescence intensity is about ninefold that of the corresponding G4 monomer/ThT. Meanwhile, compared with the common G4/ThT system, G4 dimer/ThT exhibited more stable fluorescence emission in the media with various concentrations of Na⁺ and K⁺. On the basis of these findings, G4 dimer/ThT was used as a fluorescence indicator to construct one arched DNA probe for label-free detection of DNA. By incorporating a G4 dimer sequence in amplified products, we further designed one rolling circle amplification-based biosensing strategy to show the utility of this G4 dimer/ThT fluorescence indicator. This study demonstrates that dimeric G4 is an effective nucleic acid scaffold for lighting up ThT, showing promising applications in a label-free bioassay.

On the binding of naphthalene diimides to a human telomeric G-quadruplex multimer model

To selectively target telomeric G-quadruplex (G4) DNA, monomeric and dimeric naphthalene diimides (NDIs) were investigated as binders of multimeric G4 structures able to discriminate duplex DNA. These NDIs were analysed by the affinity chromatography-based screening G4-CPG (G-quadruplex on Controlled Pore Glass), using the sequence d[AGGG(TTAGGG)₇] (tel46), folding into two consecutive G4s, as model of the human telomeric G4 multimer. In parallel, a telomeric G4 monomer (tel26) and a duplex structure (ds27) were used as controls. According to G4-CPG screening, NDI-5 proved to be the best ligand in terms of dimeric G4 vs. duplex DNA selectivity and was analysed by circular dichroism (CD), gel electrophoresis, isothermal titration calorimetry (ITC) and fluorescence spectroscopy in its interactions with tel46. NDI-5 strongly binds and stabilizes tel46 G4, favouring a hybrid folding in K⁺-containing buffer. Under these conditions, the binding process comprises a first event involving three molecules of NDI-5 and a second one in which other six molecules bind to the DNA. In a metal cation-free system, NDI-5 induces tel46 G4 folding, as indicated by CD and PAGE, favouring an antiparallel structuring. Docking simulations showed that NDI-5 can effectively bind to the pocket between two G4 units, representing a promising ligand for multimeric G4s.

Comparative studies on the binding interaction of two chiral Ru(II) polypyridyl complexes with triple- and double-helical forms of RNA

Two chiral Ru(II) polypyridyl complexes, Δ-[Ru(bpy)₂(6-F-dppz)]²⁺ (Δ-1; bpy = 2,2'-bipyridine, 6-F-dppz = 6-fluorodipyrido[3,2-a:2',3'-c]phenazine) and Λ-[Ru(bpy)₂(6-F-dppz)]²⁺ (Λ-1), have been synthesized and characterized as binders for the RNA poly(U)•poly(A)*poly(U) triplex and poly(A)•poly(U) duplex in this work. Analysis of the UV-Vis absorption spectra and fluorescence emission spectra indicates that the binding of intercalating Δ-1 with the triplex and duplex RNA is greater than that of Λ-1, while the binding affinities of the two enantiomers to triplex structure is stronger than that of duplex structure. Fluorescence titrations show that the two enantiomers can act as molecular "light switches" for triple- and double-helical RNA. Thermal denaturation studies revealed that that the two enantiomers are more stable to Watson-Crick base-paired double strand of the triplex than the Hoogsteen base-paired third strand, but their stability and selectivity are different. For Δ-enantiomer, the increase of the thermal stability of the Watson-Crick base-paired duplex (13 °C) is slightly stronger than of the Hoogsteen base-paired strand (10 °C), displaying no obvious selectivity. However, compared to the Hoogsteen base-paired strand (5 °C), the stability of the Λ-enantiomer to the Watson-Crick base-paired duplex (13 °C) is more significant, which has obvious selectivity. The overall increase in viscosity of the RNA-(Λ-1) system and its curve shape are similar to that of the RNA-(Δ-1) system, suggesting that the binding modes of two enantiomers with RNA are intercalation. The obtained results in this work may be useful for understanding the binding differences in chiral Ru(II) polypyridyl complexes toward RNA triplex and duplex.

Specific recognition of telomeric multimeric G-quadruplexes by a simple-structure quinoline derivative

The development of highly sensitive fluorescence probes for telomeric multimeric G-quadruplexes has attracted extensive attention. However, few probes reported have exhibited selectivity for telomeric multimeric G-quadruplexes. Thus, it is challenging to design fluorescence probes with high specificity and selectivity for telomeric multimeric G-quadruplexes. This study employed a commercially available quinoline derivative BEPQ-1 as an effective switch-on sensor for telomeric multimeric G-quadruplexes. The fluorescence intensity enhanced more than 20 folds upon the addition of telomeric multimeric G-quadruplexes. This probe exhibited good selectivity and sensitivity for telomeric multimeric G-quadruplexes. And the detection limit of BEPQ-1 for the telomeric multimeric G-quadruplex TTA45 was calculated to be 0.11 μM. The distinctive feature of BEPQ-1 is the simple structure and small size. In the light of binding mode, BEPQ-1 could even simultaneously bind to the end two G-quartets of the two adjacent G-quadruplex units in telomeric multimeric G-quadruplex by π-π stacking. To our knowledge, this is the first simple-structure fluorescence probe for telomeric multimeric G-quadruplex. This finding might provide a strategy to design specific probes for telomeric multimeric G-quadruplexes and contribute to understand the structures and functions of G-quadruplexes in the telomere region.

Lighting up metallohelices: from DNA binders to chemotherapy and photodynamic therapy

The design of novel agents that specifically target DNA and interrupt its normal biological processes is an attractive goal in drug design. Among the promising metallodrugs, metal-directed self-assembled metallohelices with defined three-dimensional stereochemical structures display unique structure-inherent and unprecedented noncovalent targeting abilities towards DNA, resulting in excellent anticancer or antibiotic activities. A newly burgeoning hotspot is focusing on lighting them up by embedding luminescent metal ions as the vertices. The photoactive metallohelices that combine strong interactions toward DNA targets and efficient ¹O₂ quantum yield may provide new motivation in diagnostic and photodynamic therapy (PDT) areas. This perspective focuses on research progress on metallohelices as DNA binders and chemotherapeutic agents, and highlights recent advances in fabricating luminescent examples for PDT. The relative assembly strategies are also discussed and compared. Finally, perspectives on the future development of the lit-up metallohelices are presented.

A new approach to construct and modulate G-quadruplex by cationic surfactant

HYPOTHESIS

G-quadruplex structure has raised increasing attention in supramolecular chemistry as an effective template for ordered functional materials. Thus, it is of practical significance to advance our understanding regarding G-quadruplex structures. Typically, G-quadruplex structures are formed in the presence of suitable metal ions. New methods to construct such structures need to be explored.

EXPERIMENTS

The supramolecular assembly between CTAB and a guanosine derivative at different molar ratios was systematically studied, including assembly mechanisms, morphology, and macroscopic properties. Cationic surfactants with different alkyl chains were studied as control experiments.

FINDINGS

A novel strategy to construct G-quadruplex with the promotion of the cationic surfactant CTAB is presented in this work. The structure-property relationships of G-quadruplex gels are characterized by rheology and shrinkage ratio experiments. MacKintosh's theory was used to rationalize the relationship between gel elasticity and water content. The transition of G-quadruplex structures could be easily enabled by modulating CTAB concentration, which promotes the phase transition from gel/sol biphase to homogeneous sol phase. This work will provide a new viewpoint for the construction and modulation of G-quadruplex structures.

Human telomere double G-quadruplex recognition by berberine-bisquinolinium imaging conjugates in vitro and cells

Molecular tools of double or multimeric G-quadruplexes have been given higher requirements on detection sensitivity, thermal stabilization and cell imaging to establish functions of these G-quadruplex aggregates and biological mechanisms as anticancer reagents. Here, two smart berberine-bisquinolinium conjugates (Ber-360A and Ber-PDS) by linking the berberine fluorophore ligand and an established G-quadruplex binder (i.e. bisquinolinium scaffold), have been designed and evaluated their activities and mechanisms for G-quadruplex aggregation. Two conjugates, especially Ber-PDS, are two highly selective, sensitive and fluorescent sensors which can distinguish human telomere double G-quadruplexes from other type G-quadruplexes and ds DNA. These two ligands could be the first example to stack two adjacent G-quadruplex units and fluorescently recognize human telomere double G-quadruplexes. Furthermore, conjugate Ber-PDS could enter the nucleoli and target G-quadruplex DNA through microscopy experiments, and also display strong telomerase inhibition and antitumor activities.

Chiral Ru(ii) complexes act as a potential non-viral gene carrier for directional transportation to the nucleus and cytoplasm

Guanine-rich DNA sequences can spontaneously fold into four-stranded structures called G-quadruplexes (G4s). G4s have been identified extensively in the promoter regions of several proto-oncogenes, including c-myc, as well as telomeres. G4s have attracted an increasing amount of attention in the field of nanotechnology because of their use as versatile building blocks of DNA-based nanostructures. In this study, we report the self-assembly of c-myc G-quadruplex DNA controlled by a pair of chiral ruthenium(ii) complexes coordinated by 2-(4-phenyacetylenephenyl)-1H-imidazo[4,5f][1,10]phenanthroline (PBEPIP), Λ-[Ru(bpy)₂(PBEPIP)](ClO₄)₂ (Λ-RM0627, bpy = bipyridine) and Δ-[Ru(bpy)₂(PBEPIP)](ClO₄)₂ (Δ-RM0627). Λ-RM0627 could promote the high-order self-assembly of c-myc G-quadruplex DNA into a nanowire structure, whereas Δ-RM0627 could induce DNA condensation into G-quadruplex aggregates. Moreover, in vitro studies on human liver carcinoma HepG2 cells showed that the nanowire of c-myc G-quadruplex DNA promoted by Λ-RM0627 could be localized in the nuclei of cells, whereas the nanoparticle of c-myc G-quadruplex DNA generated by Δ-RM0627 was taken up and localized in the cytoplasm. This study provides examples of the enantioselective self-assembly of G4 DNA molecules controlled by chiral ruthenium(ii) complexes and suggests the potential applications of assembled nanostructures as non-viral DNA vectors for gene therapy.

Multicolorfully probing intramolecular G-Quadruplex tandem interface

A long guanine-rich oliogonucleotide sequence can form multiple G-quadruplex (G4) tandem individuals in a single molecule with internal G4-G4 (inG4-G4) interfaces. The interface can exist at the stacked (s-inG4-G4) or unstacked (us-inG4-G4) state, dependent of the G4 conformation and environment. Because of the vital bioactivity of the G4 interface state, there is a great demand for developing a reliable multicolor fluorescence method to identify the interface state using a fluorophore that can emit at the individual wavelength for a specific interface. Herein, we found that a porphyrin with four dihydroxyphenyl substituents (OH₂PP) can multicolorfully recognize the s-inG4-G4 dimer interface against the us-inG4-G4 dimer one. The s-inG4-G4 dimer cause significant red shifts in the excitation and emission bands of OH₂PP in contrast to the us-inG4-G4 dimer and G4 monomers. OH₂PP adopts a 1:1 binding mode with the s-inG4-G4 dimer, whereas a 2:1 binding mode occurs to the us-inG4-G4 dimer. The limit of detection (LOD) for the s-inG4-G4 structure is about tens of nM level. The observed binding dependence of OH₂PP on the linker length between the G4 individuals suggests the interface binding with the s-inG4-G4 dimer. Deformation of the porphyrin macrocycle within the s-inG4-G4 interface confinement most likely contributes to the multicolorful response with the hyperporphyrin effect. Our work demonstrates that OH₂PP is a promising fluorophore to fluorescently recognize the G4 multimer with an ideal interface-sensitive multicolor response.

Dimers formed with the mixed-type G-quadruplex binder pyridostatin specifically recognize human telomere G-quadruplex dimers

By adjusting the length of the polyether linkers, pyridostatin (PDS) dimers displayed higher binding selectivities and thermal stabilization towards human telomere antiparallel and mixed-type G-quadruplex dimers (G2T1).

Label-free single-molecule identification of telomere G-quadruplexes with a solid-state nanopore sensor

Telomere sequences can spontaneously form G-quadruplexes (G4) in the presence of some cations. In view of their relevance to genetic processes and potential as therapeutic-targets, hitherto, a wealth of conventional techniques have been reported for interrogation of G4 conformation diversity and corresponding folding kinetics, most of which are limited in precision and sensitivity. This work introduces a label-free solid-state nanopore (SSN) approach for the determination of inter-, intra- and tandem molecular G4 with distinct base permutation in various cation buffers with a tailored aperture and meanwhile captures the single-molecule G4 folding process. SSN translocation properties elucidated that both inter- and intramolecular G4 generated higher current blockage with longer duration than flexible homopolymer nucleotide, and intramolecular G4 are structurally more stable with higher event frequency and longer blockage time than intermolecular ones; base arrangement played weak role in translocation behaviors; the same sequences with one, two and three G4 skeletons displayed an increase in current blockage and a gradual extension in dwell time with the increase of molecule size recorded in the same nanopore. We observed the conformation change of single-molecule G4 which indicated the existence of folding/unfolding equilibration in nanopore, and real-time test suggested a gradual formation of G4 with time. Our results could provide a continued and improved understanding of the underlying relevance of structural stability and G4 folding process by utilizing SSN platform which exhibits strategic potential advances over the other methods with high spatial and temporal resolution.

Nanopore detection of single-molecule G-quadruplexes.

Dimeric aryl-substituted imidazoles may inhibit ALT cancer by targeting the multimeric G-quadruplex in telomere

In 10-15% of cancers, telomere maintenance is provided by a telomerase-independent mechanism known as alternative lengthening of telomere (ALT), making telomerase inhibitors ineffective on these cancers. Ligands that stabilize telomeric G-quadruplex (G4) are considered to be able to inhibit either the ALT process or disrupt the T-loop structure, which would be promising therapeutic agents for ALT cancers. Notably, the 3'-terminal overhang of telomeric DNA might fold into multimeric G4 containing consecutive G4 subunits, which offers an attractive target for selective ligands considering large numbers of G4s widespread in the genome. In this study, a dimeric aryl-substituted imidazole (DIZ-3) was developed as a selective multimeric G4 ligand based on a G4-ligand-dimerizing strategy. Biophysical experiments revealed that DIZ-3 intercalated into the G4-G4 interface, stabilizing the higher-order structure. Furthermore, this ligand was demonstrated to induce cell cycle arrest and apoptosis, and thus inhibited cell proliferation in an ALT cancer cell line. Cancer cells were more sensitive to DIZ-3, relative to normal cells. Notably, DIZ-3 had little effect on the transcription of several G4-dependent oncogenes. This study provides a nice example for discovering dimeric agents to potentially treat ALT cancers via targeting telomeric multimeric G4.

Structure and Function of Multimeric G-Quadruplexes

G-quadruplexes are noncanonical nucleic acid structures formed from stacked guanine tetrads. They are frequently used as building blocks and functional elements in fields such as synthetic biology and also thought to play widespread biological roles. G-quadruplexes are often studied as monomers, but can also form a variety of higher-order structures. This increases the structural and functional diversity of G-quadruplexes, and recent evidence suggests that it could also be biologically important. In this review, we describe the types of multimeric topologies adopted by G-quadruplexes and highlight what is known about their sequence requirements. We also summarize the limited information available about potential biological roles of multimeric G-quadruplexes and suggest new approaches that could facilitate future studies of these structures.

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

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 (ΔT _1/2 around +20°C), together with a specificity toward G4 with respect to double-stranded DNA.

Cyclic Naphthalene Diimide Dimer with a Strengthened Ability to Stabilize Dimeric G-Quadruplex

A new type of dimeric cyclic naphthalene diimide derivatives (cNDI-dimers) carrying varied linker length were designed and synthesized to recognize dimeric G-quadruplex structures. All of the cNDI-dimers exhibited a high preference for recognizing G-quadruplex structures, and significantly enhanced the thermal stability of the dimeric G-quadruplex structure over the cNDI monomer by increasing the melting temperature by more than 23 °C, which indicated the strengthened ability of cNDI dimers for stabilizing dimeric G-quadruplex. cNDI dimers also showed a stronger ability to inhibit telomerase activity and stop telomere DNA elongation than cNDI monomer, which showed an improved anticancer potentiality for further therapeutic application.

Cationic porphyrins with large side arm substituents as resonance light scattering ratiometric probes for specific recognition of nucleic acid G-quadruplexes

Specific G-quadruplex-probing is crucial for both biological sciences and biosensing applications. Most reported probes are focused on fluorescent or colorimetric recognition of G-quadruplexes. Herein, for the first time, we reported a new specific G-quadruplex-probing technique-resonance light scattering (RLS)-based ratiometric recognition. To achieve the RLS probing of G-quadruplexes in the important physiological pH range of 7.4-6.0, four water soluble cationic porphyrin derivatives, including an unreported octa-cationic porphyrin, with large side arm substituents were synthesized and developed as RLS probes. These RLS probes were demonstrated to work well for ratiometric recognition of G-quadruplexes with high specificity against single- and double-stranded DNAs, including long double-stranded ones. The working mechanism was speculated to be based on the RLS signal changes caused by porphyrin protonation that was promoted by the end-stacking of porphyrins on G-quadruplexes. This work adds an important member in G-quadruplex probe family, thus providing a useful tool for studies on G-quadruplex-related events concerning G-quadruplex formation, destruction and changes in size, shape and aggregation. As a proof-of-concept example of applications, the RLS probes were demonstrated to work well for label-free and sequence-specific sensing of microRNA. This work also provides a simple and useful way for the preparation of cationic porphyrins with high charges.

Recent Progress of Targeted G-Quadruplex-Preferred Ligands Toward Cancer Therapy

A G-quadruplex (G4) is a well-known nucleic acid secondary structure comprising guanine-rich sequences, and has profound implications for various pharmacological and biological events, including cancers. Therefore, ligands interacting with G4s have attracted great attention as potential anticancer therapies or in molecular probe applications. To date, a large variety of DNA/RNA G4 ligands have been developed by a number of laboratories. As protein-targeting drugs face similar situations, G-quadruplex-interacting drugs displayed low selectivity to the targeted G-quadruplex structure. This low selectivity could cause unexpected effects that are usually reasons to halt the drug development process. In this review, we address the recent research on synthetic G4 DNA-interacting ligands that allow targeting of selected G4s as an approach toward the discovery of highly effective anticancer drugs.

19F NMR Spectroscopy for the Analysis of DNA G-Quadruplex Structures Using 19F-Labeled Nucleobase

G-quadruplex structures have been suggested to be biologically important in processes such as transcription and translation, gene expression and regulation in human cancer cells, and regulation of telomere length. Investigation of G-quadruplex structures associated with biological events is therefore essential to understanding the functions of these molecules. We developed the ¹⁹F-labeled nucleobases and introduced them into DNA sequences for the ¹⁹F NMR spectroscopy analysis. We present the ¹⁹F NMR methodology used in our research group for the study of G-quadruplex structures in vitro and in living cells.

Random Formation of G-Quadruplexes in the Full-Length Human Telomere Overhangs Leads to a Kinetic Folding Pattern with Targetable Vacant G-Tracts

G-Quadruplexes formed in the 3' telomere overhang (∼200 nucleotides) have been shown to regulate biological functions of human telomeres. The mechanism governing the population pattern of multiple telomeric G-quadruplexes is yet to be elucidated inside the telomeric overhang in a time window shorter than thermodynamic equilibrium. Using a single-molecule force ramping assay, we quantified G-quadruplex populations in telomere overhangs over a full physiological range of 99-291 nucleotides. We found that G-quadruplexes randomly form in these overhangs within seconds, which leads to a population governed by a kinetic, rather than a thermodynamic, folding pattern. The kinetic folding gives rise to vacant G-tracts between G-quadruplexes. By targeting these vacant G-tracts using complementary DNA fragments, we demonstrated that binding to the telomeric G-quadruplexes becomes more efficient and specific for telomestatin derivatives.

Ligand Design to Acquire Specificity to Intended G-Quadruplex Structures

A G-quadruplex is a nucleic acid secondary structure that is adopted by guanine-rich sequences, and is considered to be relevant in various pharmacological and biological contexts. G-Quadruplexes have also attracted great attention in the field of DNA nanotechnology because of their extremely high thermal stability and the availability of many defined structures. To date, a large repertory of DNA/RNA G-quadruplex-interactive ligands has been developed by numerous laboratories. Several relevant reviews have also been published that have helped researchers to grasp the full scope of G-quadruplex research from its outset to the present. This review focuses on the G-quadruplex ligands that allow targeting of specific G-quadruplexes. Moreover, unique ligands, successful methodologies, and future perspectives in relation to specific G-quadruplex recognition are also addressed.

Fluorescence Spectroscopic Insight into the Supramolecular Interactions in DNA-Based Enantioselective Sulfoxidation

Interactions of copper(II)-bipyridine cofactors and thioanisole substrate with human telomeric G-quadruplex DNA were studied by UV/Vis absorption, circular dichroism, and fluorescence quenching titration. Three copper(II)-bipyridine complexes are equivalently anchored to the G-quadruplex scaffold at all five fluorescently labeled sites. Thioanisole interacts with the DNA architecture at both the second loop and 3' terminus in the absence or presence of copper(II)-bipyridine complexes. These nonspecificities in the weak interactions of Cu^II complexes and thioanisole with G-quadruplex might explain why DNA only affords a modest enantioselectivity in the oxidation of thioanisole. These findings provide insights toward the construction of highly enantioselective DNA-based catalysts.

Solution structures of multiple G-quadruplex complexes induced by a platinum(II)-based tripod reveal dynamic binding

DNA G-quadruplexes are not only attractive drug targets for cancer therapeutics, but also have important applications in supramolecular assembly. Here, we report a platinum(II)-based tripod (Pt-tripod) specifically binds the biological relevant hybrid-1 human telomeric G-quadruplex (Tel26), and strongly inhibits telomerase activity. Further investigations illustrate Pt-tripod induces the formation of monomeric and multimeric Pt-tripod‒Tel26 complex structures in solution. We solve the 1:1 and the unique dimeric 4:2 Pt-tripod–Tel26 complex structures by NMR. The structures indicate preferential binding of Pt-tripod to the 5ʹ-end of Tel26 at a low Pt-tripod/Tel26 ratio of 0–1.0. After adding more Pt-tripod, the Pt-tripod binds the 3ʹ-end of Tel26, unexpectedly inducing a unique dimeric 4:2 structure interlocked by an A:A non-canonical pair at the 3ʹ-end. Our structures provide a structural basis for understanding the dynamic binding of small molecules with G-quadruplex and DNA damage mechanisms, and insights into the recognition and assembly of higher-order G-quadruplexes.

DNA G-quadruplexes occur in oncologically relevant regions, thus are interesting targets for cancer research and treatment. Here, the authors solved the 1:1 and 4:2 (ligand/DNA) NMR structures of human telomeric DNA in complex with platinum(II)-tripod ligand and show that the binding is dynamic.

Fluorescence-based tools to probe G-quadruplexes in cell-free and cellular environments

Biophysical and biochemical investigations provide compelling evidence connecting the four-stranded G-quadruplex (GQ) structure with its role in regulating multiple cellular processes. Hence, modulating the function of GQs by using small molecule binders is being actively pursued as a strategy to develop new chemotherapeutic agents. However, sequence diversity and structural polymorphism of GQs have posed immense challenges in terms of understanding what conformation a G-rich sequence adopts inside the cell and how to specifically target a GQ motif amidst several other GQ-forming sequences. In this context, here we review recent developments in the applications of biophysical tools that use fluorescence readout to probe the GQ structure and recognition in cell-free and cellular environments. First, we provide a detailed discussion on the utility of covalently labeled environment-sensitive fluorescent nucleoside analogs in assessing the subtle difference in GQ structures and their ligand binding abilities. Furthermore, a detailed discussion on structure-specific antibodies and small molecule probes used to visualize and confirm the existence of DNA and RNA GQs in cells is provided. We also highlight the open challenges in the study of tetraplexes (GQ and i-motif structures) and how addressing these challenges by developing new tools and techniques will have a profound impact on tetraplex-directed therapeutic strategies.