c-MYC Quadruplex Research Articles

Insights into the mechanism of binding of doxorubicin and a chlorin compound with 22-mer c-Myc G quadruplex

BackgroundThe interaction of small molecules with G quadruplexes is in focus due to its role in molecular recognition and therapeutic drug design. Stabilization of G-quadruplex structures in the promoter regions of oncogenes by small molecule binding has been demonstrated as a potential approach for cancer therapy. MethodsIn this study, electronic spectroscopy (ultraviolet-visible, fluorescence, circular dichroism), differential scanning calorimetry, and molecular modeling were employed to explore the interactions between the chemotherapy drug doxorubicin and a chlorin compound 5,10,15,20-tetraphenyl-[2,3]-[bis(carboxy)-methano]chlorin (H2TPC(DAC)), and the c-Myc 22-mer G quadruplex DNA. ResultsSpectroscopic studies indicated external binding of the compounds with partial stacking at the end quartets. Calorimetric studies and temperature dependent circular dichroism data displayed increased melting temperatures of G quadruplex structure on binding with the compounds. Circular dichroism spectra indicated that the G quadruplex structure is intact upon ligand binding. Both the compounds showed binding affinities of the order of 106 M−1. Fluorescence lifetime studies revealed static quenching as major mechanism for fluorescence quenching. Polymerase chain reaction stop assay hinted that binding of both ligands under study could inhibit the amplification of the DNA sequence. ConclusionResults show that doxorubicin and H2TPC(DAC) bind to the 22-mer c-Myc quadruplex structure with good affinity and induce stability. SignificanceDoxorubicin and H2TPC(DAC) have demonstrated their affinity towards c-Myc G quadruplex DNA, stabilizing it and inhibiting expression and polymerization. The results can be of practical use in designing new analogs for the two compounds, which can become potent anti-cancer agents targeting the c-Myc GQ structure.

Fluorescent copper conjugated curcumin cysteine nanoprobe for selective determination of Fe3+ and G-quadruplex DNA.

Thesynthesis of fluorescent copper-curcumin-cysteine (Cu-CC) as a sensing platformis reported. The synthesized probe has been confirmed by UV-visible spectroscopy, FT-IR, XRD, SEM, and TEM characterization techniques, respectively. The mechanistic aspects of selective sensing of Fe3+ and detection of different G-quadruplex DNA have been illustrated based on the "turn-off-on" concept of a regeneratable fluorescence sensing probe at λex 450nm. Interestingly, we have noticed a high selectivity to Fe3+ ion by the developed Cu-CC sensing probe in comparison withother metal ions. Furthermore, the restoration of fluorescence of the sensing probe in the presence of different DNA sequences is illustrating a cost-effective, convenient, and reliable detection methodology of DNA detection. It is highly sensitive for the determination of Pu27, promoter c-MYC quadruplex DNA in a wide linear range of 100-700nM having a detection limit of 13.1nM (RSD: 0.15%) and sensitivity of 37.2 cps/nM. Whereas, the Pu18 and H-telo telomeric DNA sequences are showing a narrow linear range, i.e., 10nM-200nM and 10nM-180nM, respectively. The real-world sample analysis performance of the regeneratable sensing probe for Pu27 DNA detection in fresh human blood serum samples is showing a satisfactory result.

Sequence driven interaction of amino acids in de-novo designed peptides determines c-Myc G-quadruplex unfolding inducing apoptosis in cancer cells

c-MYC proto-oncogene harbors a putative G-quadruplex structure (Pu27) at the NHEIII1 domain, which can shuffle between transcriptional inhibitor quadruplex and transcriptionally active duplex. In cancer cells this quadruplex destabilization is preferred and NHEIII1 domain assume a duplex topology thereby inducing c-MYC overexpression and tumorigenesis. Hence, the c-MYC quadruplex acts as an excellent target for anti-cancer therapy. Though researcher have tried to develop G-quadruplex targeted small molecules, work with G-quadruplex targeting peptides is very limited. Here we present a peptide that can bind to c-MYC quadruplex, destabilize the tetrad core, and permit the formation of a substantially different structure from the quartet core seen in the canonical G-quadruplexes. Such conformation potentially acted as a roadblock for transcription factors thereby reducing cMYC expression. This event sensitizes the cancer cell to activate apoptotic cascade via the c-MYC-VEGF-A-BCL2 axis. This study provides a detailed insight into the peptide-quadruplex interface that encourages better pharmacophore design to target dynamic quadruplex structure. We believe that our results will contribute to the development, characterization, and optimization of G-quadruplex binding peptides for potential clinical application.

10-Alkoxy-anthracenyl-isoxazole analogs have sub-micromolar activity against a Glioblastoma multiforme cell line

A series of 10-alkoxy-Anthryl-isoxazole-pyrrole-doubletails (RO-AIMs) were synthesized using a crown ether assisted nucleophilic aromatic substitution followed by a modified Schotten-Baumann reaction. The novel RO-AIMs described here exhibit robust growth inhibition for the human SNB19 CNS glioblastoma cell line, and biphenyl analog 8c had activity in the nanomolar regime, which represents the most efficacious compound in the AIM series to date. Computational modeling for RO-AIMs binding in a ternary complex with c-myc quadruplex DNA and its helicase DHX36 is presented which represents our current working hypothesis.

Transcription-facilitating histone chaperons interact with genomic and synthetic G4 structures

Affinity for G-quadruplex (G4) structures may be a common feature of transcription-facilitating histone chaperons (HCs). This assumption is based on previous unmatched studies of HCs FACT, nucleolin (NCL), BRD3, and ATRX. We verified this assumption and considered its implications for the therapeutic applications of synthetic (exogenous) G4s and the biological significance of genomic G4s. First, we questioned whether exogenous G4s that recognize cell-surface NCL and could trap other HCs in the nucleus are usable as anticancer agents. We performed in vitro binding assays and selected leading multi-targeted G4s. They exhibited minor effects on cell viability. The presumed NCL-regulated intracellular transport of G4s was inefficient or insufficient for tumor-specific G4 delivery. Next, to clarify whether G4s in the human genome could recruit HCs, we compared available HC ChIP-seq data with G4-seq/G4-ChIP-seq data. Several G4s, including the well-known c-Myc quadruplex structure, were found to be colocalized with HC occupancy sites in cancer cell lines. As evidenced by our molecular modeling data, c-Myc G4 might interfere with the HC function of BRD3 but is unlikely to prevent the BRD3-driven assembly of the chromatin remodeling complex. The c-Myc case illustrates the intricate role of genomic G4s in chromatin remodeling, nucleosome remodeling, and transcription.

G-Quadruplex-Specific Cell-Permeable Guanosine-Anthracene Conjugate Inhibits Telomere Elongation and Induces Apoptosis by Repressing the c-MYC Gene.

We herein report a cell-membrane-permeable molecular probe ADG, prepared by conjugating guanosine with anthracene, selectively interacts with c-MYC G-quadruplex over other promoter and telomeric quadruplexes as well as duplex DNA. NMR spectroscopy suggests that ADG interacts with terminal G-quartets as well as with the nearby G-rich tract (G13-G14-G15 and G8-G9-G10) of c-MYC quadruplex. In vitro cellular studies indicate that ADG represses c-MYC expression by stabilizing its promoter G-quadruplex and alters c-MYC-related cellular events. ADG suppresses hTERT and BCL2 gene expressions in a promoter-independent manner, inhibits elongation of telomere length, and activates apoptotic cascades in cancer cells.

A benzo(f)quinolinium fused chromophore-based fluorescent probe for selective detection of c-myc G-Quadruplex DNA with a red emission and a large Stocks shift

Accumulating evidence in recent years has suggested that G-Quadruplex DNAs may play important roles in gene expression and genomic stability in vivo. Specially, c-myc G-Quadruplex that exists in the promoter of oncogene is an attractive target in the pathway of cell proliferation and senescence. The development of small-molecule fluorescent probes for c-myc G-Quadruplex DNA has attracted significant attention. This work presented a benzo(f)quinolinium fused chromophore-based fluorescent probe L-1 for the detection of c-myc G-Quadruplex DNA using N-(2-hydroxyethyl)piperazine moiety as a binding unit. This probe exhibiting a red emission and a large Stocks shift was found to be selective and sensitive for “light-up” detection of c-myc quadruplex DNA over duplex DNA and other G-quadruplexes. It stabilized c-myc G-Quadruplex DNA and effective inhibited in vitro DNA synthesis. Binding studies indicated that the fluorescent response of L-1 mainly arised from end-stacking binding mode. Furthermore, L-1 was applicable to visualize the cellular nucleus in living cells.

Pif1 helicase unfolding of G-quadruplex DNA is highly dependent on sequence and reaction conditions

In addition to unwinding double-stranded nucleic acids, helicase activity can also unfold noncanonical structures such as G-quadruplexes. We previously characterized Pif1 helicase catalyzed unfolding of parallel G-quadruplex DNA. Here we characterized unfolding of the telomeric G-quadruplex, which can fold into antiparallel and mixed hybrid structures and found significant differences. Telomeric DNA sequences are unfolded more readily than the parallel quadruplex formed by the c-MYC promoter in K+ Furthermore, we found that under conditions in which the telomeric quadruplex is less stable, such as in Na+, Pif1 traps thermally melted quadruplexes in the absence of ATP, leading to the appearance of increased product formation under conditions in which the enzyme is preincubated with the substrate. Stable telomeric G-quadruplex structures were unfolded in a stepwise manner at a rate slower than that of duplex DNA unwinding; however, the slower dissociation from G-quadruplexes compared with duplexes allowed the helicase to traverse more nucleotides than on duplexes. Consistent with this, the rate of ATP hydrolysis on the telomeric quadruplex DNA was reduced relative to that on single-stranded DNA (ssDNA), but less quadruplex DNA was needed to saturate ATPase activity. Under single-cycle conditions, telomeric quadruplex was unfolded by Pif1, but for the c-MYC quadruplex, unfolding required multiple helicase molecules loaded onto the adjacent ssDNA. Our findings illustrate that Pif1-catalyzed unfolding of G-quadruplex DNA is highly dependent on the specific sequence and the conditions of the reaction, including both the monovalent cation and the order of addition.

Crystal structure of the major quadruplex formed in the promoter region of the human c-MYC oncogene.

The c-MYC oncogene mediates multiple tumor cell survival pathways and is dysregulated or overexpressed in the majority of human cancers. The NHE III1 region of the c-MYC promoter forms a DNA quadruplex. Stabilization of this structure with small molecules has been shown to reduce expression of c-MYC, and targeting the c-MYC quadruplex has become an emerging strategy for development of antitumor compounds. Previous solution NMR studies of the c-MYC quadruplex have assigned the major conformer and topology of this important target, however, regions outside the G-quartet core were not as well-defined. Here, we report a high-resolution crystal structure (2.35 Å) of the major quadruplex formed in the NHE III1 region of the c-MYC promoter. The crystal structure is in general agreement with the solution NMR structure, however, key differences are observed in the position of nucleotides outside the G-quartet core. The crystal structure provides an alternative model that, along with comparisons to other reported quadruplex crystal structures, will be important to the rational design of selective compounds. This work will aid in development of ligands to target the c-MYC promoter quadruplex with the goal of creating novel anticancer therapies.

Site-specific amino acid substitution in dodecameric peptides determines the stability and unfolding of c-MYC quadruplex promoting apoptosis in cancer cells.

c-MYC proto-oncogene harbours a transcription-inhibitory quadruplex-forming scaffold (Pu27) upstream P1 promoter providing anti-neoplastic therapeutic target. Previous reports showed the binding profile of human Cathelicidin peptide (LL37) and telomeric G-quadruplex. Here, we truncated the quadruplex-binding domain of LL37 to prepare a small library of peptides through site-specific amino acid substitution. We investigated the intracellular selectivity of peptides for Pu27 over other oncogenic quadruplexes and their role in c-MYC promoter repression by dual-luciferase assays. We analysed their thermodynamics of binding reactions with c-MYC quadruplex isomers (Pu27, Myc22, Pu19) by Isothermal Titration Calorimetry. We discussed how amino acid substitutions and peptide helicity enhanced/weakened their affinities for c-MYC quadruplexes and characterized specific non-covalent inter-residual interactions determining their selectivity. Solution NMR structure indicated that KR12C, the best peptide candidate, selectively stabilized the 5′-propeller loop of c-MYC quadruplex by arginine-driven electrostatic-interactions at the sugar-phosphate backbone while KR12A peptide destabilized the quadruplex inducing a single-stranded hairpin-like conformation. Chromatin immunoprecipitations envisaged that KR12C and KR12A depleted and enriched Sp1 and NM23-H2 (Nucleoside diphosphate kinase) occupancy at Pu27 respectively supporting their regulation in stabilizing and unfolding c-MYC quadruplex in MCF-7 cells. We deciphered that selective arresting of c-MYC transcription by KR12C triggered apoptotic-signalling pathway via VEGF-A-BCL-2 axis.

Cell penetrating thiazole peptides inhibit c-MYC expression via site-specific targeting of c-MYC G-quadruplex.

The structural differences among different G-quadruplexes provide an opportunity for site-specific targeting of a particular G-quadruplex structure. However, majority of G-quadruplex ligands described thus far show little selectivity among different G-quadruplexes. In this work, we delineate the design and synthesis of a crescent-shaped thiazole peptide that preferentially stabilizes c-MYC quadruplex over other promoter G-quadruplexes and inhibits c-MYC oncogene expression. Biophysical analysis such as Förster resonance energy transfer (FRET) melting and fluorescence spectroscopy show that the thiazole peptide TH3 can selectively interact with the c-MYC G-quadruplex over other investigated G-quadruplexes and duplex DNA. NMR spectroscopy reveals that peptide TH3 binds to the terminal G-quartets and capping regions present in the 5′- and 3′-ends of c-MYC G-quadruplex with a 2:1 stoichiometry; whereas structurally related distamycin A is reported to interact with quadruplex structures via groove binding and end stacking modes with 4:1 stoichiometry. Importantly, qRT-PCR, western blot and dual luciferase reporter assay show that TH3 downregulates c-MYC expression by stabilizing the c-MYC G-quadruplex in cancer cells. Moreover, TH3 localizes within the nucleus of cancer cells and exhibits antiproliferative activities by inducing S phase cell cycle arrest and apoptosis.

Study of the interaction between indole-based compounds and biologically relevant G-quadruplexes

G-quadruplexes (G4s) are high-order secondary structures that modulate several key cell processes such as telomere function, gene expression and DNA replication. In the past years G4s have emerged as promising targets for drug development due to the discovery of small molecules which bind and stabilize these structures. In this work, we report the synthesis of indole-based compounds and the study of their interaction with the biological relevant G4s c-MYC and human telomeric repeat 22AG using several biophysical techniques. The ligands are G4 specific and they can discriminate different G4 structures namely parallel and hybrid-1 topologies. The NMR study of interaction between the most promising indole-derivative (compound 3) and c-MYC quadruplex suggests that the ligand binds on the external tetrads with additional actions in the loops/grooves in a 2:1 ratio. The molecular docking calculations of compound 3 to c-MYC quadruplex corroborates with the 1H and NOESY NMR studies.Overall, the results suggest that indole-based ligands are promising candidates for future lead optimizations in drug development.

Phenanthroline-bis-oxazole ligands for binding and stabilization of G-quadruplexes

BackgroundG-quadruplexes (G4) are found at important genome regions such as telomere ends and oncogene promoters. One prominent strategy to explore the therapeutic potential of G4 is stabilized it with specific ligands. MethodsWe report the synthesis of new phenanthroline, phenyl and quinoline acyclic bisoxazole compounds in order to explore and evaluate the targeting to c-myc and human telomeric repeat 22AG G4 using FRET-melting, CD-melting, NMR, fluorescence titrations and FID assays. ResultsThe design strategy has led to potent compounds (Phen-1 and Phen-2) that discriminate different G4 structures (human telomeric sequences and c-myc promoter) and selectively stabilize G4 over duplex DNA. CD studies show that Phen-2 binds and induces antiparallel topologies in 22AG quadruplex and also binds c-myc promotor, increasing their Tm in about 12°C and 30°C respectively. In contrast, Phen-1 induces parallel topologies in 22AG and c-myc, with a moderate stabilization of 4°C for both sequences. Consistent with a CD melting study, Phen-2 binds strongly (K=106 to 107M−1) to c-myc and 22AG quadruplexes. ConclusionsPhen-1 and Phen-2 discriminated among various quadruplex topologies and exhibited high selectivity for quadruplexes over duplexes. Phen-2 retains antiparallel topologies for quadruplex 22AG and does not induce conformational changes on the parallel c-myc quadruplex although Phen-1 favors the parallel topology. NMR studies also showed that the Phen-2 binds to the c-myc quadruplex via end stacking. General significanceOverall, the results suggest the importance of Phen-2 as a scaffold for the fine-tuning with substituents in order to enhance binding and stabilization to G4 structures. This article is part of a Special Issue entitled "G-quadruplex" Guest Editor: Dr. Concetta Giancola and Dr. Daniela Montesarchio.

Specific Stabilization of c-MYC and c-KIT G-Quadruplex DNA Structures by Indolylmethyleneindanone Scaffolds.

Stabilization of G-quadruplex DNA structures by small molecules has emerged as a promising strategy for the development of anticancer drugs. Since G-quadruplex structures can adopt various topologies, attaining specific stabilization of a G-quadruplex topology to halt a particular biological process is daunting. To achieve this, we have designed and synthesized simple structural scaffolds based on an indolylmethyleneindanone pharmacophore, which can specifically stabilize the parallel topology of promoter quadruplex DNAs (c-MYC, c-KIT1, and c-KIT2), when compared to various topologies of telomeric and duplex DNAs. The lead ligands (InEt2 and InPr2) are water-soluble and meet a number of desirable criteria for a small molecule drug. Highly specific induction and stabilization of the c-MYC and c-KIT quadruplex DNAs (ΔT1/2 up to 24 °C) over telomeric and duplex DNAs (ΔT1/2 ∼ 3.2 °C) by these ligands were further validated by isothermal titration calorimetry and electrospray ionization mass spectrometry experiments (Ka ∼ 10(5) to 10(6) M(-1)). Low IC50 (∼2 μM) values were emerged for these ligands from a Taq DNA polymerase stop assay with the c-MYC quadruplex forming template, whereas the telomeric DNA template showed IC50 values >120 μM. Molecular modeling and dynamics studies demonstrated the 5'- and 3'-end stacking modes for these ligands. Overall, these results demonstrate that among the >1000 quadruplex stabilizing ligands reported so far, the indolylmethyleneindanone scaffolds stand out in terms of target specificity and structural simplicity and therefore offer a new paradigm in topology specific G-quadruplex targeting for potential therapeutic and diagnostic applications.

Small molecule regulated dynamic structural changes of human G-quadruplexes.

The changes in structure and dynamics of oncogenic (c-MYC) and telomeric (h-TELO) G-rich DNA sequences due to the binding of a novel carbazole derivative (BTC) are elucidated using single-molecule Förster resonance energy transfer (sm-FRET), fluorescence correlation spectroscopy (FCS) and NMR spectroscopy. In contrast to the previous reports on the binding of ligands to pre-folded G-quadruplexes, this work illustrates how ligand binding changes the conformational equilibria of both unstructured G-rich DNA sequences and K+-folded G-quadruplexes. The results demonstrate that K+ free c-MYC and h-TELO exist as unfolded and partially folded conformations. The binding of BTC shifts the equilibria of both investigated DNA sequences towards the folded G-quadruplex structure, increases the diffusion coefficients and induces faster end-to-end contact formation. BTC recognizes a minor conformation of the c-MYC quadruplex and the two-tetrad basket conformations of the h-TELO quadruplex.

Preparation of 4-([2,2′:6′,2″-terpyridin]-4′-yl)-N,N-diethylaniline NiII and PtII complexes and exploration of their in vitro cytotoxic activities

Two metal complexes of NiLCl2 (1) and [PtLCl]Cl (2) with 4-([2,2′:6′,2″-terpyridin]-4′-yl)-N,N-diethylaniline (L) were synthesized and characterized. 1 and 2 exhibited selective cytotoxicity to T-24 cells more than L, compared with the normal liver cell line (HL-7702). Various experiments showed that L, 1 and 2 caused T-24 cell cycle arrest at S phase, as shown by the down-regulation of cdc25 A, cyclin A, cyclin B and CDK2 and the up-regulation of p21, p27 and p53. Furthermore, complexes 1 and 2, especially complex 2, acted as telomerase inhibitors targeting c-myc G-quadruplex DNA and triggered cell apoptosis. In addition, 1 and 2 also caused mitochondrial dysfunction. Taken together, we found that 1 and 2 exerted their cytotoxic activity mainly via inhibiting telomerase by interaction with c-myc quadruplex and disruption of mitochondrial function.

Investigating actinomycin D binding to G-quadruplex, i-motif and double-stranded DNA in 27-nt segment of c-MYC gene promoter

c-MYC DNA is an attractive target for drug design, especially for cancer chemotherapy. Around 90% of c-MYC transcription is controlled by NHE III1, whose 27-nt purine-rich strand has the ability to form G-quadruplex structure. In this investigation, interaction of ActD with 27-nt G-rich strand (G/c-MYC) and its equimolar mixture with the complementary sequence, (GC/c-MYC) as well as related C-rich oligonucleotide (C/c-MYC) was evaluated. Molecular dynamic simulations showed that phenoxazine and lactone rings of ActD come close to the outer G-tetrad nucleotides indicating that ActD binds through end-stacking to the quadruplex DNA. RMSD and RMSF revealed that fluctuation of the quadruplex DNA increases upon interaction with the drug. The results of spectrophotometry and spectrofluorometry indicated that ActD most probably binds to the c-MYC quadruplex and duplex DNA via end-stacking and intercalation, respectively and polarity of ActD environment decreases due to the interaction. It was also found that binding of ActD to the GC-rich DNA is stronger than the two other forms of DNA. Circular dichroism results showed that the type of the three forms of DNA structures doesn't change, but their compactness alters due to their interaction with ActD. Finally, it can be concluded that ActD binds differently to double stranded DNA, quadruplex DNA and i-motif.

A Nucleus-Imaging Probe That Selectively Stabilizes a Minor Conformation of c-MYC G-quadruplex and Down-regulates c-MYC Transcription in Human Cancer Cells.

The c-MYC proto-oncogene is a regulator of fundamental cellular processes such as cell cycle progression and apoptosis. The development of novel c-MYC inhibitors that can act by targeting the c-MYC DNA G-quadruplex at the level of transcription would provide potential insight into structure-based design of small molecules and lead to a promising arena for cancer therapy. Herein we report our finding that two simple bis-triazolylcarbazole derivatives can inhibit c-MYC transcription, possibly by stabilizing the c-MYC G-quadruplex. These compounds are prepared using a facile and modular approach based on Cu(I) catalysed azide and alkyne cycloaddition. A carbazole ligand with carboxamide side chains is found to be microenvironment-sensitive and highly selective for “turn-on” detection of c-MYC quadruplex over duplex DNA. This fluorescent probe is applicable to visualize the cellular nucleus in living cells. Interestingly, the ligand binds to c-MYC in an asymmetric fashion and selects the minor-populated conformer via conformational selection.

Synthesis of a platinum(II) complex with 2-(4-methoxy-phenyl) imidazo [4,5-f]-[1,10] phenanthrolin and study of its antitumor activity.

A new platinum(II) complex of [Pt(II)(L) (pn)]Cl·2H2O (1) (pn=1,3-propanediamine) with 2-(4-methoxy-phenyl)imidazo [4,5-f]-[1,10]phenanthrolin (H-L) was synthesized and characterized. In complex 1, the platinum adopts a four-coordinated square planar geometry. Complex 1 exhibited selective cytotoxicity against NCI-H460, BEL-7402, SK-OV-3, SK-OV-3/DDP and HeLa cell lines with IC50 values in the micromolar range (9.7-35.8μM), but low cytotoxicity toward normal human liver HL-7702 cells. Complex 1 caused HeLa cell cycle arrest at S phase and it induced HeLa apoptosis by the activation of caspase-3/9. Various experiments showed that complex 1 preferred to bind with G-quadruplex in c-myc. Taken together, we found that complex 1 exerted its antitumor activity mainly via inhibiting telomerase by interaction with c-myc quadruplex and activation of caspase-3/9.

Photo‐Cross‐Linking Probes for Trapping G‐Quadruplex DNA

We have developed a straightforward synthetic pathway to a set of six photoactivatable G-quadruplex ligands with a validated G4-binding motif (the bisquinolinium pyridodicarboxamide PDC-360A) tethered through various spacers to two different photo-cross-linking groups: benzophenone and an aryl azide. The high quadruplex-versus-duplex selectivity of the PDC core was retained in the new derivatives and resulted in selective alkylation of two well-known G-quadruplexes (human telomeric G4 and oncogene promoter c-myc G4) under conditions of harsh competition. The presence of two structurally different photoactivatable functions allowed the selective alkylation of G-quadruplex structures at specific nucleobases and irreversible G4 binding. The topology and sequence of the quadruplex matrix appear to influence strongly the alkylation profile, which differs for the telomeric and c-myc quadruplexes. The new compounds are photoactive in cells and thus provide new tools for studying G4 biology.