Human telomeric DNA sequence-specific cleaving by G-quadruplex formation

G-Quadruplexes in Human Telomere: Structures, Properties, and Applications

G-quadruplexes, intricate four-stranded structures composed of G-tetrads formed by four guanine bases, are prevalent in both DNA and RNA. Notably, these structures play pivotal roles in human telomeres, contributing to essential cellular functions. Additionally, the existence of DNA:RNA hybrid G-quadruplexes adds a layer of complexity to their structural diversity. This review provides a comprehensive overview of recent advancements in unraveling the intricacies of DNA and RNA G-quadruplexes within human telomeres. Detailed insights into their structural features are presented, encompassing the latest developments in chemical approaches designed to probe these G-quadruplex structures. Furthermore, this review explores the applications of G-quadruplex structures in targeting human telomeres. Finally, the manuscript outlines the imminent challenges in this evolving field, setting the stage for future investigations.

Organomercury oligonucleotide conjugates as artificial ribonucleases

Two oligonucleotide conjugates sharing the same sequence but incorporating a different 5'-terminal organometallic moiety were synthesized, by either direct mercuration in solution or oximation with an organomercury aldehyde on solid support. The potential of these conjugates to serve as new type of artificial ribonucleases was tested with a complementary 2´-O-methyl-RNA target sequence featuring a single cleavable RNA phosphodiester linkage. Both organomercury oligonucleotides greatly outperformed their metal-free counterparts as well as the previously reported small molecule organomercury RNA cleaving agent in catalytic activity, providing an important proof-of-concept. Compared to state-of-the-art metal-dependent artificial ribonucleases, however, the observed activity was modest.

Multiple hPOT1-TPP1 cooperatively unfold contiguous telomeric G-quadruplexes proceeding from 3' toward 5', a feature due to a 3'-end binding preference and to structuring of telomeric DNA

Telomeres are DNA repeated sequences that associate with shelterin proteins and protect the ends of eukaryotic chromosomes. Human telomeres are composed of 5'TTAGGG repeats and ends with a 3' single-stranded tail, called G-overhang, that can be specifically bound by the shelterin protein hPOT1 (human Protection of Telomeres 1). In vitro studies have shown that the telomeric G-strand can fold into stable contiguous G-quadruplexes (G4). In the present study we investigated how hPOT1, in complex with its shelterin partner TPP1, binds to telomeric sequences structured into contiguous G4 in potassium solutions. We observed that binding of multiple hPOT1-TPP1 preferentially proceeds from 3' toward 5'. We explain this directionality in terms of two factors: (i) the preference of hPOT1-TPP1 for the binding site situated at the 3' end of a telomeric sequence and (ii) the cooperative binding displayed by hPOT1-TPP1 in potassium. By comparing binding in K+ and in Li+, we demonstrate that this cooperative behaviour does not stem from protein-protein interactions, but from structuring of the telomeric DNA substrate into contiguous G4 in potassium. Our study suggests that POT1-TPP1, in physiological conditions, might preferentially cover the telomeric G-overhang starting from the 3'-end and proceeding toward 5'.

Beyond small molecules: targeting G-quadruplex structures with oligonucleotides and their analogues

G-Quadruplexes (G4s) are widely studied secondary DNA/RNA structures, naturally occurring when G-rich sequences are present. The strategic localization of G4s in genome areas of crucial importance, such as proto-oncogenes and telomeres, entails fundamental implications in terms of gene expression regulation and other important biological processes. Although thousands of small molecules capable to induce G4 stabilization have been reported over the past 20 years, approaches based on the hybridization of a synthetic probe, allowing sequence-specific G4-recognition and targeting are still rather limited. In this review, after introducing important general notions about G4s, we aim to list, explain and critically analyse in more detail the principal approaches available to target G4s by using oligonucleotides and synthetic analogues such as Locked Nucleic Acids (LNAs) and Peptide Nucleic Acids (PNAs), reporting on the most relevant examples described in literature to date.

Modular Design of G-Quadruplex MetalloDNAzymes for Catalytic C-C Bond Formations with Switchable Enantioselectivity

Metal-binding DNA structures with catalytic function are receiving increasing interest. Although a number of metalloDNAzymes have been reported to be highly efficient, the exact coordination/position of their catalytic metal center is often unknown. Here, we present a new approach to rationally develop metalloDNAzymes for Lewis acid-catalyzed reactions such as enantioselective Michael additions. Our strategy relies on the predictable folding patterns of unimolecular DNA G-quadruplexes, combined with the concept of metal-mediated base-pairing. Transition-metal coordination environments were created in G-quadruplex loop regions, accessible by substrates. Therefore, protein-inspired imidazole ligandoside L was covalently incorporated into a series of G-rich DNA strands by solid-phase synthesis. Iterative rounds of DNA sequence design and catalytic assays allowed us to select tailored metalloDNAzymes giving high conversions and excellent enantioselectivities (≥99%). Based on their primary sequence, folding pattern, and metal coordination mode, valuable information on structure-activity relationships could be extracted. Variation of the number and position of ligand L within the sequence allowed us to control the formation of (S) and (R) enantiomeric reaction products, respectively.

Learning to Model G-Quadruplexes: Current Methods and Perspectives

G-quadruplexes have raised considerable interest during the past years for the development of therapies against cancer. These noncanonical structures of DNA may be found in telomeres and/or oncogene promoters, and it has been observed that the stabilization of such G-quadruplexes may disturb tumor cell growth. Nevertheless, the mechanisms leading to folding and stabilization of these G-quadruplexes are still not well established, and they are the focus of much current work in this field. In seminal works, stabilization was observed to be produced by cations. However, subsequent studies showed that different kinds of small molecules, from planar and nonplanar organic molecules to square-planar and octahedral metal complexes, may also lead to the stabilization of G-quadruplexes. Thus, the comprehension and rationalization of the interaction of these small molecules with G-quadruplexes are also important topics of current interest in medical applications. To shed light on the questions arising from the literature on the formation of G-quadruplexes, their stabilization, and their interaction with small molecules, synergies between experimental studies and computational works are needed. In this review, we mainly focus on in silico approaches and provide a broad compilation of different leading studies carried out to date by different computational methods. We divide these methods into twomain categories: (a) classical methods, which allow for long-timescale molecular dynamics simulations and the corresponding analysis of dynamical information, and (b) quantum methods (semiempirical, quantum mechanics/molecular mechanics, and density functional theory methods), which allow for the explicit simulation of the electronic structure of the system but, in general, are not capable of being used in long-timescale molecular dynamics simulations and, therefore, give a more static picture of the relevant processes.

Near-infrared-traceable DNA nano-hydrolase: specific eradication of telomeric G-overhang in vivo

Telomeric DNA, whose length homeostasis is closely correlated with immortality of cancer cells, is regarded as a molecular clock for cellular lifespan. Regarding the capacity in forming G-quadruplex, G-rich 3′-overhang (G-overhang) has been considered as an attractive anticancer target. However, it is still challenging to precisely target telomeric G-overhang with current ligands because of the polymorphism of G-quadruplexes in cells. Herein, we construct a telomeric G-overhang-specific near-infrared-traceable DNA nano-hydrolase, which is composed of four parts: (i) dexamethasone for targeting cell nuclei; (ii) complementary DNA for hybridizing with G-overhang; (iii) multinuclear Ce(IV) complexes for hydrolyzing G-overhang; and (iv) upconversion nanoparticles for real-time tracking. The multivalent targeted DNA nano-hydrolase can be traced to precisely digest telomeric G-overhang, which contributes to telomeric DNA shortening and thereby causes cell aging and apoptosis. The anticancer treatment is further proved by in vivo studies. In this way, this design provides a telomeric G-overhang-specific eradication strategy based on a non-G-quadruplex targeting manner.

Metalloenzyme-mimic innate G-quadruplex DNAzymes using directly coordinated metal ions as active centers

G-quadruplex DNAs (G4s) have been reported to exhibit the DNAzyme activities by binding with some metal complexes and functional organic ligands. However, there is a challenge to develop metalloenzyme-mimic G4-based innate DNAzymes using the complexed metal ions directly serving as the active centers. This will diversify DNAzymes for developing novel devices since G4 structures are more polymorphic than the other DNA foldings. In this work, we found that the lanthanide trivalent cerium ion of Ce3+ can bind to the human telomere G4 (htG4) according to a 1 : 2 binding mode favorable for creating metalloenzymes-mimic G4 DNAzymes. This Ce3+-G4 entity exhibits a peroxidase activity towards the oxidation of the substrate of 3,3,5,5-tetramethylbenzidine (TMB) by hydrogen peroxide. The 5' G4 tetrads with the orderly arranged carbonyl oxygen atoms are believed to be the coordination sites for Ce3+ and favor the conversion between Ce3+ and Ce4+. Our work provides an alternative feasibility in developing the G4-based innate DNAzymes for variant applications.

Heteroleptic Coordination Environments in Metal-Mediated DNA G-Quadruplexes

The presence of metal centers with often highly conserved coordination environments is crucial for roughly half of all proteins, having structural, regulatory, or enzymatic function. To understand and mimic the function of metallo-enzymes, bioinorganic chemists pursue the challenge of synthesizing model compounds with well-defined, often heteroleptic metal sites. Recently, we reported the design of tailored homoleptic coordination environments for various transition metal cations based on unimolecular DNA G-quadruplex structures, templating the regioselective positioning of imidazole ligandosides L^I. Here, we expand this modular system to more complex, heteroleptic coordination environments by combining L^I with a new benzoate ligandoside L^B within the same oligonucleotide. The modifications still allow the correct folding of parallel tetramolecular and antiparallel unimolecular G-quadruplexes. Interestingly, the incorporation of L^B results in strong destabilization expressed in lower thermal denaturation temperatures T_m. While no transition metal cations could be bound by G-quadruplexes containing only L^B, heteroleptic derivatives containing both L^I and L^B were found to complex Cu^II, Ni^II, and Zn^II. Especially in case of Cu^II we found strong stabilizations of up to ΔT_m = +34°C. The here shown system represents an important step toward the design of more complex coordination environments inside DNA scaffolds, promising to culminate in the preparation of functional metallo-DNAzymes.

Tailored Transition-Metal Coordination Environments in Imidazole-Modified DNA G-Quadruplexes

Two types of imidazole ligands were introduced both at the end of tetramolecular and into the loop region of unimolecular DNA G‐quadruplexes. The modified oligonucleotides were shown to complex a range of different transition‐metal cations including Ni^II, Cu^II, Zn^II and Co^II, as indicated by UV/Vis absorption spectroscopy and ion mobility mass spectrometry. Molecular dynamics simulations were performed to obtain structural insight into the investigated systems. Variation of ligand number and position in the loop region of unimolecular sequences derived from the human telomer region (htel) allows for a controlled design of distinct coordination environments with fine‐tuned metal affinities. It is shown that Cu^II, which is typically square‐planar coordinated, has a higher affinity for systems offering four ligands, whereas Ni^II prefers G‐quadruplexes with six ligands. Likewise, the positioning of ligands in a square‐planar versus tetrahedral fashion affects binding affinities of Cu^II and Zn^II cations, respectively. Gaining control over ligand arrangement patterns will spur the rational development of transition‐metal‐modified DNAzymes. Furthermore, this method is suited to combine different types of ligands, for example, those typically found in metalloenzymes, inside a single DNA architecture.

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.

A multi-functional guanine derivative for studying the DNA G-quadruplex structure

In the present study, we developed a multi-functional guanine derivative, ^8FG, as a G-quadruplex stabilizer, a fluorescent probe for the detection of G-quadruplex formation, and a ¹⁹F sensor for the observation of the G-quadruplex. We demonstrate that the functional nucleoside bearing a 3,5-bis(trifluoromethyl)benzene group at the 8-position of guanine stabilizes the DNA G-quadruplex structure and fluoresces following the G-quadruplex formation. Furthermore, we show that the functional sensor can be used to directly observe DNA G-quadruplexes by ¹⁹F-NMR in living cells. To our knowledge, this is the first study showing that the nucleoside derivative simultaneously allows for three kinds of functions at a single G-quadruplex DNA. Our results suggest that the multi-functional nucleoside derivative can be broadly used for studying the G-quadruplex structure and serves as a powerful tool for examining the molecular basis of G-quadruplex formation in vitro and in living cells.

A phosphate-targeted dinuclear Cu(II) complex combining major groove binding and oxidative DNA cleavage

Free radical generation is an inevitable consequence of aerobic existence and is implicated in a wide variety of pathological conditions including cancer, cardiovascular disease, ageing and neurodegenerative disorder. Free radicals can, however, be used to our advantage since their production is catalysed by synthetic inorganic molecules-termed artificial metallonucleases-that cut DNA strands by oxidative cleavage reactions. Here, we report the rational design and DNA binding interactions of a novel di-Cu2+ artificial metallonuclease [Cu2(tetra-(2-pyridyl)-NMe-naphthalene)Cl4] (Cu2TPNap). Cu2TPNap is a high-affinity binder of duplex DNA with an apparent binding constant (Kapp) of 107 M(bp)-1. The agent binds non-intercalatively in the major groove causing condensation and G-C specific destabilization. Artificial metallonuclease activity occurs in the absence of exogenous reductant, is dependent on superoxide and hydrogen peroxide, and gives rise to single strand DNA breaks. Pre-associative molecular docking studies with the 8-mer d(GGGGCCCC)2, a model for poly[d(G-C)2], identified selective major groove incorporation of the complex with ancillary Cu2+-phosphate backbone binding. Molecular mechanics methods then showed the d(GGGGCCCC)2 adduct to relax about the complex and this interaction is supported by UV melting experiments where poly[d(G-C)2] is selectively destabilized.

Structure-Property Relationships in CuII -Binding Tetramolecular G-Quadruplex DNA

A series of artificial metal-base tetrads composed of a Cu^II cation coordinating to four pyridines, covalently attached to the ends of tetramolecular G-quadruplex DNA strands [L^A-D d(G₄ )]₄ (L^A-D =ligand derivatives), was systematically studied. Structurally, the square-planar [Cu(pyridine)₄ ] complex behaves analogously to the canonical guanine quartet. Copper coordination to all studied ligand derivatives was found to increase G-quadruplex thermodynamic stability, tolerating a great variety of ligand linker lengths (1-5 atoms) and thus demonstrating the robustness of the chosen ligand design. Only at long linker lengths, the stabilizing effect of copper binding is compensated by the loss of conformational freedom. A previously reported ligand L^E with chiral backbone enables incorporation at any oligonucleotide position. We show that ligand chirality distinctly steers Cu^II -induced G-quadruplex stabilization. 5'-End formation of two metal-base tetrads by tetramolecular G-quadruplex [L^E₂ d(G)₄ ]₄ shows that stabilization in the presence of Cu^II is not additive. All results are based on UV/Vis thermal denaturation, thermal difference, circular dichroism experiments and molecular dynamics simulations.

In What Ways Do Synthetic Nucleotides and Natural Base Lesions Alter the Structural Stability of G-Quadruplex Nucleic Acids?

Synthetic analogs of natural nucleotides have long been utilized for structural studies of canonical and noncanonical nucleic acids, including the extensively investigated polymorphic G-quadruplexes (GQs). Dependence on the sequence and nucleotide modifications of the folding landscape of GQs has been reviewed by several recent studies. Here, an overview is compiled on the thermodynamic stability of the modified GQ folds and on how the stereochemical preferences of more than 70 synthetic and natural derivatives of nucleotides substituting for natural ones determine the stability as well as the conformation. Groups of nucleotide analogs only stabilize or only destabilize the GQ, while the majority of analogs alter the GQ stability in both ways. This depends on the preferred syn or anti N-glycosidic linkage of the modified building blocks, the position of substitution, and the folding architecture of the native GQ. Natural base lesions and epigenetic modifications of GQs explored so far also stabilize or destabilize the GQ assemblies. Learning the effect of synthetic nucleotide analogs on the stability of GQs can assist in engineering a required stable GQ topology, and exploring the in vitro action of the single and clustered natural base damage on GQ architectures may provide indications for the cellular events.

G-quadruplex DNA targeted metal complexes acting as potential anticancer drugs

This review summarizes the recent development of G4 DNA targeted metal complexes and discusses their potential as anticancer drugs.

A Multinuclear Metal Complex Based DNase-Mimetic Artificial Enzyme: Matrix Cleavage for Combating Bacterial Biofilms

Extracellular DNA (eDNA) is an essential structural component during biofilm formation, including initial bacterial adhesion, subsequent development, and final maturation. Herein, the construction of a DNase-mimetic artificial enzyme (DMAE) for anti-biofilm applications is described. By confining passivated gold nanoparticles with multiple cerium(IV) complexes on the surface of colloidal magnetic Fe3 O4  /SiO2 core/shell particles, a robust and recoverable artificial enzyme with DNase-like activity was obtained, which exhibited high cleavage ability towards both model substrates and eDNA. Compared to the high environmental sensitivity of natural DNase in anti-biofilm applications, DMAE exhibited a much better operational stability and easier recoverability. When DMAE was coated on substratum surfaces, biofilm formation was inhibited for prolonged periods of time, and the DMAE excelled in the dispersion of established biofilms of various ages. Finally, the presence of DMAE remarkably potentiated the efficiency of traditional antibiotics to kill biofilm-encased bacteria and eradiate biofilms.

Oxidative DNA cleavage promoted by two phenolate-bridged binuclear copper(ii) complexes

Both the binuclear copper complexes 1 and 2 could efficiently cleave the supercoiled pUC19 plasmid DNA into its nicked and linear forms in the presence of excessive ascorbic acid (Vc) under nearly physiological conditions. The DNA binding ability and the DNA cleavage efficacy of 1 are superior to 2.

Chemical modifications of artificial restriction DNA cutter (ARCUT) to promote its in vivo and in vitro applications

Recently, completely chemistry-based tools for site-selective scission of DNA (ARCUT) have been prepared by combining 2 strands of pseudo-complementary PNA (pcPNA: site-selective activator) and a Ce(IV)-EDTA complex (molecular scissors). Its site-specificity is sufficient to cut the whole human genome at one predetermined site. In this first-generation ARCUT, however, there still remain several problems to be solved for wider applications. This review presents recent approaches to solve these problems. They are divided into (i) covalent modification of pcPNA with other functional groups and (ii) new strategies using conventional PNA, in place of pcPNA, as site-selective activator. Among various chemical modifications, conjugation with positively-charged nuclear localization signal peptide is especially effective. Furthermore, unimolecular activators, a single strand of which successfully activates the target site in DNA for site-selective scission, have been also developed. As the result of these modifications, the site-selective scission by Ce(IV)-EDTA was achieved promptly even under high salt conditions which are otherwise unfavourable for double-duplex invasion. Furthermore, it has been shown that "molecular crowding effect," which characterizes the inside of living cells, enormously promotes the invasion, and thus the invasion seems to proceed effectively and spontaneously in the cells. Strong potential of pcPNA for further applications in vivo and in vitro has been confirmed.

Toward the design of a catalytic metallodrug: selective cleavage of G-quadruplex telomeric DNA by an anticancer copper-acridine-ATCUN complex

Telomeric DNA represents a novel target for the development of anticancer drugs. By application of a catalytic metallodrug strategy, a copper-acridine-ATCUN complex (CuGGHK-Acr) has been designed that targets G-quadruplex telomeric DNA. Both fluorescence solution assays and gel sequencing demonstrate the CuGGHK-Acr catalyst to selectively bind and cleave the G-quadruplex telomere sequence. The cleavage pathway has been mapped by matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) experiments. CuGGHK-Acr promotes significant inhibition of cancer cell proliferation and shortening of telomere length. Both senescence and apoptosis are induced in the breast cancer cell line MCF7.

Synthesis, hydrolytic DNA-cleaving activities and cytotoxicities of EDTA analogue-tethered pyrrole-polyamide dimer-based Ce(IV) complexes

Two EDTA analogue-tethered C2-symmetrical dimeric monopyrrole-polyamide 5 and dipyrrole-polyamide 6, and their corresponding Ce(IV) complexes Ce-5 and Ce-6 were synthesized and fully characterized. Agarose gel electrophoresis studies on pBR322 DNA cleavage indicate that complexes Ce-5 and Ce-6 exhibited potent DNA-cleaving activities under physiological conditions. The maximal first-order rate constants (kmax's) were (0.42 ± 0.02) h(-1) for Ce-5 and (0.52 ± 0.02) h(-1) for Ce-6, respectively, suggesting that both complexes catalyzed the cleavage of supercoiled DNA by up to approximately 10(8)-fold. Complex Ce-6 exhibited ca 10-fold higher overall catalytic activity (kmax/KM) than Ce-5, which may be ascribed to its higher DNA-binding affinity. Inhibition experiments and a model study convincingly suggest that both complexes Ce-5 and Ce-6 functioned as hydrolytic DNA-cleavers. In addition, both complexes were found to display moderate inhibitory activity toward A549 and HepG-2 cells.

G-quadruplexes incorporating modified constituents: a review

This review summarizes the results of structural studies carried out with analogs of G-quadruplexes built from natural nucleotides. Several dozens of base-, sugar-, and phosphate derivatives of the biological building blocks have been incorporated into more than 50 potentially quadruplex forming DNA and RNA oligonucleotides and the stability and folding topology of the resultant intramolecular, bimolecular and tetramolecular architectures characterized. The TG4T, TG5T, the 15 nucleotide-long thrombin binding aptamer, and the human telomere repeat AG3(TTAG3)3 sequences were modified in most cases, and four guanine analogs can be noted as being particularly useful in structural studies. These are the fluorescent 2-aminopurine, the 8-bromo-, and 8-methylguanines, and the hypoxanthine. The latter three analogs stabilize a given fold in a mixture of structures making possible accurate structural determinations by circular dichroism and nuclear magnetic resonance measurements.

Fluorescence quenchers for hydrazone and oxime orthogonal bioconjugation

We describe the synthesis and properties of new fluorescence quenchers containing aldehyde, hydrazine, and aminooxy groups, allowing convenient bioconjugation as oximes or hydrazones. Conjugation to oligonucleotides proceeded in high yield with aniline as catalyst. Kinetics studies of conjugation show that, under optimal conditions, a hydrazine or aminooxy quencher can react with aldehyde-modified DNA to form a stable hydrazone or oxime adduct in as little as five minutes. The resulting quencher-containing DNAs were assessed for their ability to quench the emission of fluorescein in labeled complements and compared to the commercially available dabcyl and Black Hole Quencher 2 (BHQ2), which were conjugated as phosphoramidites. Results show that the new quenchers possess slightly different absorbance properties compared to dabcyl and are as efficient as the commercial quenchers in quenching fluorescein emission. Hydrazone-based quenchers were further successfully incorporated into molecular beacons and shown to give high signal to background ratios in single nucleotide polymorphism detection in vitro. Finally, aminooxy and hydrazine quenchers were applied to quenching of an aldehyde-containing fluorophore associated with living cells, demonstrating cellular quenching within one hour.

G-rich sequence-specific recognition and scission of human genome by PNA/DNA hybrid G-quadruplex formation

Hole in one: A single peptide nucleic acid (PNA) effectively targets the G-rich region in double-stranded DNA through formation of a PNA/DNA hybrid G-quadruplex. Only one target site in the whole human genome was selectively cleaved by the hybrid G-quadruplex. Such site-selective scission of DNA is central to gene manipulation for molecular biology, biotechnology, and therapy.

Introduction of multiphosphonate ligand to peptide nucleic acid for metal ion conjugation

Peptide nucleic acid (PNA) is one of the most widely used synthetic DNA analogs. Conjugation of functional molecules to PNA is very effective to further widen its potential applications. For this purpose, here we report the synthesis of several ligand monomers and introduced them to PNA. These ligand-modified PNAs attract cerium ion and are useful for site-selective DNA hydrolysis. It should be noted that these ligands on PNA are also effective even under the conditions of invasion complex.

Synthesis of a dibromoperylene phosphoramidite building block and its incorporation at the 5' end of a G-quadruplex forming oligonucleotide: spectroscopic properties and structural studies of the resulting dibromoperylene conjugate

Previous studies indicate that some perylene bisimide derivatives can drive the assembly of DNA G-quadruplexes, thus suggesting the possible advantage in the adoption of perylene-conjugated G-rich oligonucleotides in biological and biotechnological applications. Nevertheless, the typical poor solubility of perylene bisimides strongly limits the number of suitable chemical strategies to prepare perylene-conjugated oligonucleotides. In order to overcome these difficulties, we employed the earlier described core twisted perylene derivatives possessing unique optical and electronic properties, besides good solubility in common solvents. As a first result, the large-scale synthesis of a new dibromoperylene derivative (PEOEBr) phosphoramidite building block is herein reported. Furthermore, the structural behavior of the conjugated PEOEBr-GGGTTAGGG (HTRp2) human telomeric repeat was investigated by using CD, UV, fluorescence, and gel electrophoresis techniques in desalted water and in K(+)- and Na(+)-containing buffers. We observed that the peculiar property of PEOEBr moieties to form dimers instead of extended aggregates drives the HTRp2 strands toward dimerization and mainly promotes the formation of quadruplex species having both the 5'-ends located at the same side of the structures. However, the counterions present in solutions (K(+) or Na(+)) as well as the strand concentration, also contribute to influence the topology and the stoichiometry of formed structures. Furthermore, unlike the unmodified sequence GGGTTAGGG (HTR2), HTRp2 strands quickly associate into G-quadruplexes even in desalted water, as assessed by CD experiments.

Interaction of metal complexes with G-quadruplex DNA

Guanine-rich sequences of DNA can assemble into tetrastranded structures known as G-quadruplexes. It has been suggested that these secondary DNA structures could be involved in the regulation of several key biological processes. In the human genome, guanine-rich sequences with the potential to form G-quadruplexes exist in the telomere as well as in promoter regions of certain oncogenes. The identification of these sequences as novel targets for the development of anticancer drugs has sparked great interest in the design of molecules that can interact with quadruplex DNA. While most reported quadruplex DNA binders are based on purely organic templates, numerous metal complexes have more recently been shown to interact effectively with this DNA secondary structure. This Review provides an overview of the important roles that metal complexes can play as quadruplex DNA binding molecules, highlighting the unique properties metals can confer to these molecules.

A 6-mer photocontrolled oligonucleotide as an effective telomerase inhibitor

Telomerase is a relevant target for cancer therapy. A small 6-mer psoralen-containing oligonucleotide is shown to cause efficient photo-cross-linking to human telomere DNA by G-quadruplex formation. The small, photocontrolled oligonucleotide shows effective telomerase inhibition, both in vitro and in vivo, only when light irradiated. Data suggest that it is a promising agent for the treatment of cancer.

Quinone methides tethered to naphthalene diimides as selective G-quadruplex alkylating agents

We have developed novel G-quadruplex (G-4) ligand/alkylating hybrid structures, tethering the naphthalene diimide moiety to quaternary ammonium salts of Mannich bases, as quinone-methide precursors, activatable by mild thermal digestion (40 degrees C). The bis-substituted naphthalene diimides were efficiently synthesized, and their reactivity as activatable bis-alkylating agents was investigated in the presence of thiols and amines in aqueous buffered solutions. The electrophilic intermediate, quinone-methide, involved in the alkylation process was trapped, in the presence of ethyl vinyl ether, in a hetero Diels-Alder [4 + 2] cycloaddition reaction, yielding a substituted 2-ethoxychroman. The DNA recognition and alkylation properties of these new derivatives were investigated by gel electrophoresis, circular dichroism, and enzymatic assays. The alkylation process occurred preferentially on the G-4 structure in comparison to other DNA conformations. By dissecting reversible recognition and alkylation events, we found that the reversible process is a prerequisite to DNA alkylation, which in turn reinforces the G-quadruplex structural rearrangement.