Structure of two intramolecular G-quadruplexes formed by natural human telomere sequences in K+ solution

Enhanced Recognition of a Herbal Compound Epiberberine by a DNA Quadruplex-Duplex Structure

The small molecule epiberberine (EPI) is a natural alkaloid with versatile bioactivities against several diseases including cancer and bacterial infection. EPI can induce the formation of a unique binding pocket at the 5' side of a human telomeric G-quadruplex (HTG) sequence with four telomeric repeats (Q4), resulting in a nanomolar binding affinity (KD approximately 26 nM) with significant fluorescence enhancement upon binding. It is important to understand (1) how EPI binding affects HTG structural stability and (2) how enhanced EPI binding may be achieved through the engineering of the DNA binding pocket. In this work, the EPI-binding-induced HTG structure stabilization effect was probed by a peptide nucleic acid (PNA) invasion assay in combination with a series of biophysical techniques. We show that the PNA invasion-based method may be useful for the characterization of compounds binding to DNA (and RNA) structures under physiological conditions without the need to vary the solution temperature or buffer components, which are typically needed for structural stability characterization. Importantly, the combination of theoretical modeling and experimental quantification allows us to successfully engineer Q4 derivative Q4-ds-A by a simple extension of a duplex structure to Q4 at the 5' end. Q4-ds-A is an excellent EPI binder with a KD of 8 nM, with the binding enhancement achieved through the preformation of a binding pocket and a reduced dissociation rate. The tight binding of Q4 and Q4-ds-A with EPI allows us to develop a novel magnetic bead-based affinity purification system to effectively extract EPI from Rhizoma coptidis (Huang Lian) extracts.

Flanking Effect on the Structure and Stability of Human Telomeric G-Quadruplex in Varying Salt Concentrations

The stability of the human telomere G-quadruplex (G4) is directly linked to cancer disease. The human telomere is mostly associated with the flanking nucleobases, which can affect the stability of G4. Hence, in this study, the effect of the flanking nucleobases in the context of their chemical nature, number, and position on the structure and stability of G4 has been investigated in varying concentrations of KCl mimicking the normal and cancer KCl microenvironments. The addition of flanking nucleobases does not alter the G4 topology. However, the presence of merely a single flanking nucleobase destabilizes the telomeric G4. This destabilizing effect is more prominent for thymine than adenine flanking nucleobase, probably due to the formation of the intermolecular G4 topology by thymine. Interestingly, the change in the stability of the telomeric G4 in the presence of thymine flanking nucleobase is sensitive to the concentration of KCl relevant to the normal and cancerous microenvironments, in contrast to adenine. Flanking nucleobases have a greater impact at the 5' end compared to the 3' end, particularly noticeable in KCl concentrations resembling the normal microenvironment rather than the cancerous one. These findings indicate that the effect of the flanking nucleobases on telomeric G4 is different in the KCl salt relevant to normal and cancerous microenvironments. This study may be helpful in attaining molecular-level insight into the role of G4 in telomeric length regulation under normal and cancerous KCl salt conditions.

Exploring the interaction between a fluorescent Ag(I)-biscarbene complex and non-canonical DNA structures: a multi-technique investigation

Silver compounds are mainly studied as antimicrobial agents, but they also have anticancer properties, with the latter, in some cases, being better than their gold counterparts. Herein, we analyse the first example of a new Ag(I)-biscarbene that can bind non-canonical structures of DNA, more precisely G-quadruplexes (G4), with different binding signatures depending on the type of G4. Moreover, we show that this Ag-based carbene binds the i-motif DNA structure. Alternatively, its Au(I) counterpart, which was investigated for comparison, stabilises mitochondrial G4. Theoretical in silico studies elucidated the details of different binding modes depending on the geometry of G4. The two complexes showed increased cytotoxic activity compared to cisplatin, overcoming its resistance in ovarian cancer. The binding of these new drug candidates with other relevant biosubstrates was studied to afford a more complete picture of their possible targets. In particular, the Ag(I) complex preferentially binds DNA structures over RNA structures, with higher binding constants for the non-canonical nucleic acids with respect to natural calf thymus DNA. Regarding possible protein targets, its interaction with the albumin model protein BSA was also tested.

Comprehensive analysis of intramolecular G-quadruplex structures: furthering the understanding of their formalism

G-quadruplexes (G4) are helical structures found in guanine-rich DNA or RNA sequences. Generally, their formalism is based on a few dozen structures, which can produce some inconsistencies or incompleteness. Using the website ASC-G4, we analyzed the structures of 333 intramolecular G4s, of all types, which allowed us to clarify some key concepts and present new information. To each of the eight distinguishable topologies corresponds a groove-width signature and a predominant glycosidic configuration (gc) pattern governed by the directions of the strands. The relative orientations of the stacking guanines within the strands, which we quantified and related to their vertical gc successions, determine the twist and tilt of the helices. The latter impact the minimum groove widths, which represent the space available for lateral ligand binding. The G4 four helices have similar twists, even when these twists are irregular, meaning that they have various angles along the strands. Despite its importance, the vertical gc succession has no strict one-to-one relationship with the topology, which explains the discrepancy between some topologies and their corresponding circular dichroism spectra. This study allowed us to introduce the new concept of platypus G4s, which are structures with properties corresponding to several topologies.

19F Nuclear Magnetic Resonance Fingerprinting Technique for Identifying and Quantifying G-Quadruplex Topology in Human Telomeric Overhangs

G-quadruplexes (G4s) are noncanonical nucleic acid secondary structures with diverse topological features and biological roles. Human telomeric (Htelo) overhangs consisting of TTAGGG repeats can fold into G4s that adopt different topologies under physiological conditions. These G4s are potential targets for anticancer drugs. Despite intensive research, the existence and topology of G4s at Htelo overhangs in vivo are still unclear because there is no method to distinguish and quantify the topology of Htelo overhangs with native lengths that can form more than three tandem G4s in living cells. Herein, we present a novel 19F chemical shift fingerprinting technique to identify and quantify the topology of the Htelo overhangs up to five G-quadruplexes (G4s) and 120 nucleotides long both in vitro and in living cells. Our results show that longer overhang sequences tend to form stable G4s at the 5'- and 3'-ends, while the interior G4s are dynamic and "sliding" along the sequence, with TTA or 1-3 TTAGGG repeats as a linker. Each G4 in the longer overhang is conformationally heterogeneous, but the predominant ones are hybrid-2, two- or three-tetrad antiparallel, and hybrid-1 at the 5'-terminal, interior, and 3'-terminal, respectively. Additionally, we observed a distinct behavior of different lengths of telomeric sequences in living cells, suggesting that the overhang length and protein accessibility are related to its function. This technique provides a powerful tool for quickly identifying the folding topology and relative population of long Htelo overhangs, which may provide valuable insights into telomere functionality and be beneficial for structure-based anticancer drug development targeting G4s.

DNA Quadruplex Structure with a Unique Cation Dependency

DNA quadruplex structures provide an additional layer of regulatory control in genome maintenance and gene expression and are widely used in nanotechnology. We report the discovery of an unprecedented tetrastranded structure formed from a native G-rich DNA sequence originating from the telomeric region of Caenorhabditis elegans. The structure is defined by multiple properties that distinguish it from all other known DNA quadruplexes. Most notably, the formation of a stable so-called KNa-quadruplex (KNaQ) requires concurrent coordination of K+ and Na+ ions at two distinct binding sites. This structure provides novel insight into G-rich DNA folding under ionic conditions relevant to eukaryotic cell physiology and the structural evolution of telomeric DNA. It highlights the differences between the structural organization of human and nematode telomeric DNA, which should be considered when using C. elegans as a model in telomere biology, particularly in drug screening applications. Additionally, the absence/presence of KNaQ motifs in the host/parasite introduces an intriguing possibility of exploiting the KNaQ fold as a plausible antiparasitic drug target. The structure's unique shape and ion dependency and the possibility of controlling its folding by using low-molecular-weight ligands can be used for the design or discovery of novel recognition DNA elements and sensors.

Effect of ionic liquids as mobile phase additives on retention behaviors of G-quadruplexes in reversed-phase high performance liquid chromatography

G-quadruplexes (G4s) play an important role in a variety of biological processes and have extensive application prospects. Due to the significance of G4s in physiology and biosensing, studies on G4s have attracted much attention, stimulating the development or improvement of methods for G4 structures and polymorphism analysis. In this work, ionic liquids (ILs) were involved as mobile phase additives in reversed-phase high performance liquid chromatography (RP-HPLC) to analyse G4s with various conformations for the first time. How ILs affected the retention behaviors of G4s was investigated comprehensively. It was found that the addition of ILs markedly enhanced G4 retention, along with obvious amelioration on chromatographic peak shapes and separation. The influence of pH of mobile phase and types of ILs were also included in order to acquire an in-depth understanding. It appeared that the effect of ILs on G4 retention behaviors was the result of a combination of various interactions between G4s with the hydrophobic stationary phase and with the IL-containing mobile phase, where ion pair mechanism and enhanced hydrophobic interaction dominated. The findings of this work revealed that ILs could effectively improve the separation of G4s in RP-HPLC, which was conducive to G4 structural analysis, especially for G4s polymorphism elucidation.

G-Quadruplexes in the Regulation of Viral Gene Expressions and Their Impacts on Controlling Infection

G-quadruplexes (G4s) are noncanonical nucleic acid structures that play significant roles in regulating various biological processes, including replication, transcription, translation, and recombination. Recent studies have identified G4s in the genomes of several viruses, such as herpes viruses, hepatitis viruses, and human coronaviruses. These structures are implicated in regulating viral transcription, replication, and virion production, influencing viral infectivity and pathogenesis. G4-stabilizing ligands, like TMPyP4, PhenDC3, and BRACO19, show potential antiviral properties by targeting and stabilizing G4 structures, inhibiting essential viral life-cycle processes. This review delves into the existing literature on G4's involvement in viral regulation, emphasizing specific G4-stabilizing ligands. While progress has been made in understanding how these ligands regulate viruses, further research is needed to elucidate the mechanisms through which G4s impact viral processes. More research is necessary to develop G4-stabilizing ligands as novel antiviral agents. The increasing body of literature underscores the importance of G4s in viral biology and the development of innovative therapeutic strategies against viral infections. Despite some ligands' known regulatory effects on viruses, a deeper comprehension of the multifaceted impact of G4s on viral processes is essential. This review advocates for intensified research to unravel the intricate relationship between G4s and viral processes, paving the way for novel antiviral treatments.

Protocol for the production and purification of an i-Motif-specific nanobody

Intercalated motifs or i-Motifs (iMs) are nucleic acid structures formed by cytosine-rich sequences, which may regulate cellular processes and have broad applications in nanotechnology due to their pH-dependent nature. We have developed an iM-specific nanobody (iMbody) that can recognize iM DNA structures regardless of their sequences, making it a versatile research tool for studying iMs in various contexts. Here, we provide a protocol for the bacterial expression and His-tag purification of iMbody. We then describe procedures for performing ELISA and immunostaining using iMbody.

G-Quadruplexes in the Viral Genome: Unlocking Targets for Therapeutic Interventions and Antiviral Strategies

G-quadruplexes (G4s) are unique non-canonical four-stranded nucleic acid secondary structures formed by guanine-rich DNA or RNA sequences. Sequences with the potential to form quadruplex motifs (pG4s) are prevalent throughout the genomes of all organisms, spanning from prokaryotes to eukaryotes, and are enriched within regions of biological significance. In the past few years, the identification of pG4s within most of the Baltimore group viruses has attracted increasing attention due to their occurrence in regulatory regions of the genome and the subsequent implications for regulating critical stages of viral life cycles. In this context, the employment of specific G4 ligands has aided in comprehending the intricate G4-mediated regulatory mechanisms in the viral life cycle, showcasing the potential of targeting viral G4s as a novel antiviral strategy. This review offers a thorough update on the literature concerning G4s in viruses, including their identification and functional significance across most of the human-infecting viruses. Furthermore, it delves into potential therapeutic avenues targeting G4s, encompassing various G4-binding ligands, G4-interacting proteins, and oligonucleotide-based strategies. Finally, the article highlights both progress and challenges in the field, providing valuable insights into leveraging this unusual nucleic acid structure for therapeutic purposes.

Unveiling the structural mechanism of a G-quadruplex pH-Driven switch

The human telomere oligonucleotide, d[TAGGG(TTAGGG)2TTAGG] (TAGGG), can adopt two distinct 2-G-quartet G-quadruplex structures at pH 7.0 and 5.0, referred to as the TD and KDH+ forms, respectively. By using a combination of NMR and computational techniques, we determined high-resolution structures of both forms, which revealed unique loop architectures, base triples, and base pairs that play a crucial role in the pH-driven structural transformation of TAGGG. Our study demonstrated that TAGGG represents a reversible pH-driven switch system where the stability and pH-induced structural transformation of the G-quadruplexes are influenced by the terminal residues and base triples. Gaining insight into the factors that regulate the formation of G-quadruplexes and their pH-sensitive structural equilibrium holds great potential for the rational design of novel DNA based pH-driven switches. These advancements in understanding create exciting opportunities for applications in the field of nanotechnology, specifically in the development of bio-nano-motors.

Probing telomeric-like G4 structures with full or partial 2'-deoxy-5-hydroxyuridine substitutions

Guanine quadruplexes (G4s) are stable four-stranded secondary DNA structures held together by noncanonical G-G base tetrads. We synthesised the nucleoside analogue 2'-deoxy-5-hydroxyuridine (H) and inserted its phosphoramidite into telomeric repeat-type model oligonucleotides. Full and partial substitutions were made, replacing all guanines in all the three tetrads of a three-tier G4 structure, or only in the putative upper, central, or lower tetrads. We characterised these modified structures using CD, UV absorbance spectroscopy, native gel studies, and a capture oligo-based G4 disruption kinetic assay. The strand separation activity of BLM helicase on these substituted structures was also investigated. Two of the partially H-substituted constructs adopted G4-like structures, but displayed lower thermal stabilities compared to unsubstituted G4. The construct modified in its central tetrad remained mostly denatured, but the possibility of a special structure for the fully replaced variant remained open. H substitutions did not interfere with the G4-resolving activity of BLM helicase, but its efficiency was highly influenced by construct topology and even more by the G4 ligand PhenDC3. Our results suggest that the H modification can be incorporated into G quadruplexes, but only at certain positions to maintain G4 stability. The destabilizing effect observed for 2'-deoxy-5-hydroxyuridine indicates that the cytosine deamination product 5-hydroxyuracil and its nucleoside counterpart in RNA (5-hydroxyuridine), might also be destabilizing in cellular DNA and RNA quadruplexes. The kinetic assay employed in this study can be generally employed for a fast comparison of the stabilities of various G4s either in their free or ligand-bound states.

Why to target G-quadruplexes using peptides: Next-generation G4-interacting ligands

Guanine-rich oligonucleotides existing in both DNA and RNA are able to fold into four-stranded DNA secondary structures via Hoogsteen type hydrogen-bonding, where four guanines self-assemble into a square planar arrangement, which, when stacked upon each other, results in the formation of higher-order structures called G-quadruplexes. Their distribution is not random; they are more frequently present at telomeres, proto-oncogenic promoters, introns, 5'- and 3'-untranslated regions, stem cell markers, ribosome binding sites and so forth and are associated with various biological functions, all of which play a pivotal role in various incurable diseases like cancer and cellular ageing. Several studies have suggested that G-quadruplexes could not regulate biological processes by themselves; instead, various proteins take part in this regulation and can be important therapeutic targets. There are certain limitations in using whole G4-protein for therapeutics purpose because of its high manufacturing cost, laborious structure prediction, dynamic nature, unavailability for oral administration due to its degradation in the gut and inefficient penetration to reach the target site because of the large size. Hence, biologically active peptides can be the potential candidates for therapeutic intervention instead of the whole G4-protein complex. In this review, we aimed to clarify the biological roles of G4s, how we can identify them throughout the genome via bioinformatics, the proteins interacting with G4s and how G4-interacting peptide molecules may be the potential next-generation ligands for targeting the G4 motifs located in biologically important regions.

Genomic Instability of G-Quadruplex Sequences in Escherichia coli: Roles of DinG, RecG, and RecQ Helicases

Guanine-rich DNA can fold into highly stable four-stranded DNA structures called G-quadruplexes (G4). Originally identified in sequences from telomeres and oncogene promoters, they can alter DNA metabolism. Indeed, G4-forming sequences represent obstacles for the DNA polymerase, with important consequences for cell life as they may lead to genomic instability. To understand their role in bacterial genomic instability, different G-quadruplex-forming repeats were cloned into an Escherichia coli genetic system that reports frameshifts and complete or partial deletions of the repeat when the G-tract comprises either the leading or lagging template strand during replication. These repeats formed stable G-quadruplexes in single-stranded DNA but not naturally supercoiled double-stranded DNA. Nevertheless, transcription promoted G-quadruplex formation in the resulting R-loop for (G3T)4 and (G3T)8 repeats. Depending on genetic background and sequence propensity for structure formation, mutation rates varied by five orders of magnitude. Furthermore, while in vitro approaches have shown that bacterial helicases can resolve G4, it is still unclear whether G4 unwinding is important in vivo. Here, we show that a mutation in recG decreased mutation rates, while deficiencies in the structure-specific helicases DinG and RecQ increased mutation rates. These results suggest that G-quadruplex formation promotes genetic instability in bacteria and that helicases play an important role in controlling this process in vivo.

Complexity of Guanine Quadruplex Unfolding Pathways Revealed by Atomistic Pulling Simulations

Guanine quadruplexes (GQs) are non-canonical nucleic acid structures involved in many biological processes. GQs formed in single-stranded regions often need to be unwound by cellular machinery, so their mechanochemical properties are important. Here, we performed steered molecular dynamics simulations of human telomeric GQs to study their unfolding. We examined four pulling regimes, including a very slow setup with pulling velocity and force load accessible to high-speed atomic force microscopy. We identified multiple factors affecting the unfolding mechanism, i.e.,: (i) the more the direction of force was perpendicular to the GQ channel axis (determined by GQ topology), the more the base unzipping mechanism happened, (ii) the more parallel the direction of force was, GQ opening and cross-like GQs were more likely to occur, (iii) strand slippage mechanism was possible for GQs with an all-anti pattern in a strand, and (iv) slower pulling velocity led to richer structural dynamics with sampling of more intermediates and partial refolding events. We also identified that a GQ may eventually unfold after a force drop under forces smaller than those that the GQ withstood before the drop. Finally, we found out that different unfolding intermediates could have very similar chain end-to-end distances, which reveals some limitations of structural interpretations of single-molecule spectroscopic data.

Stability of Human Telomeric G-Quadruplexes Complexed with Photosensitive Ligands and Irradiated with Visible Light

Guanine-rich DNA sequences can fold into non-canonical nucleic acid structures called G-quadruplexes (G4s). These nanostructures have strong implications in many fields, from medical science to bottom-up nanotechnologies. As a result, ligands interacting with G4s have attracted great attention as candidates in medical therapies, molecular probe applications, and biosensing. In recent years, the use of G4-ligand complexes as photopharmacological targets has shown significant promise for developing novel therapeutic strategies and nanodevices. Here, we studied the possibility of manipulating the secondary structure of a human telomeric G4 sequence through the interaction with two photosensitive ligands, DTE and TMPyP4, whose response to visible light is different. The effect of these two ligands on G4 thermal unfolding was also considered, revealing the occurrence of peculiar multi-step melting pathways and the different attitudes of the two molecules on the quadruplex stabilization.

Promoters vs. telomeres: AP-endonuclease 1 interactions with abasic sites in G-quadruplex folds depend on topology

The DNA repair endonuclease APE1 is responsible for the cleavage of abasic sites (AP) in DNA as well as binding AP in promoter G-quadruplex (G4) folds in some genes to regulate transcription. The present studies focused on the topological properties of AP-bearing G4 folds and how they impact APE1 interaction. The human telomere sequence with a tetrahydrofuran model (F) of an AP was folded in K+- or Na+-containing buffers to adopt hybrid- or basket-folds, respectively. Endonuclease and binding assays were performed with APE1 and the G4 substrates, and the data were compared to prior work with parallel-stranded VEGF and NEIL3 promoter G4s to identify topological differences. The APE1-catalyzed endonuclease assays led to the conclusion that telomere G4 folds were slightly better substrates than the promoter G4s, but the yields were all low compared to duplex DNA. In the binding assays, G4 topological differences were observed in which APE1 bound telomere G4s with dissociation constants similar to single-stranded DNA, and promoter G4s were bound with nearly ten-fold lower values similar to duplex DNA. An in-cellulo assay with the telomere G4 in a model promoter bearing a lesion failed to regulate transcription. These data support a hypothesis that G4 topology in gene promoters is a critical feature that APE1 recognizes for gene regulation.

ASC-G4, an algorithm to calculate advanced structural characteristics of G-quadruplexes

ASC-G4 is an algorithm for the calculation of the advanced structural characteristics of G-quadruplexes (G4). It allows the unambiguous determination of the intramolecular G4 topology, based on the oriented strand numbering. It also resolves the ambiguity in the determination of the guanine glycosidic configuration. With this algorithm, we showed that the use of the C3' or C5' atoms to calculate the groove width in G4 is more appropriate than the P atoms and that the groove width does not always reflect the space available within the groove. For the latter, the minimum groove width is more appropriate. The application of ASC-G4 to 207 G4 structures guided the choices made for the calculations. A website based on ASC-G4 (http://tiny.cc/ASC-G4) was created, where the user uploads his G4 structure and gets its topology, the types of its loops and their lengths, the presence of snapbacks and bulges, the distribution of guanines in the tetrads and strands, the glycosidic configuration of these guanines, their rise, the groove widths, the minimum groove widths, the tilt and twist angles, the backbone dihedral angles, etc. It also provides a large number of atom-atom and atom-plane distances that are relevant to evaluating the quality of the structure.

Computing the electronic circular dichroism spectrum of DNA quadruple helices of different topology: A critical test for a generalized excitonic model based on a fragment diabatization

In this study, we exploit a recently developed fragment diabatization-based excitonic model, FrDEx, to simulate the electronic circular dichroism (ECD) spectra of three guanine-rich DNA sequences arranged in guanine quadruple helices with different topologies: thrombin binding aptamer (antiparallel), c-Myc promoter (parallel), and human telomeric sequence (3+1 hybrid). Starting from time-dependent density functional theory (TD-DFT) calculations with the M052X functional, we apply our protocol to parameterize the FrDEX Hamiltonian, which accounts for electron density overlap and includes both the coupling with charge transfer transitions and the effect of the surrounding bases on the local excitation of each chromophore. The TD-DFT/M052X spectral shapes are in good agreement with the experimental ones, the main source of discrepancy being related to the intrinsic error on the computed transition energies of guanine monomer. FrDEx spectra are fairly close to the reference TD-DFT ones, allowing a significant advance with respect to a more standard excitonic Hamiltonian. We also show that the ECD spectra are sensitive to the inclusion of the inner K + $$ {}^{+} $$ cation in the calculation.

Synergistic effect of naphthalenediimide and squaraine ligand targeting G-quadruplex DNA in cancer cells

Targeting and stabilizing nonclassical DNA G-quadruplexes (G4s) with a ligand to inhibit cell proliferation is a very promising approach for cancer treatment. Here, we demonstrate that the combination of a naphthalenediimide (NDI) ligand and a squaraine ligand significantly improves the anticancer activity of either ligand alone. The NDI ligand binds the 5'-terminal of hybrid-type G4s and induces the topological conversion from a metastable hybrid to a stable parallel conformation, which allows the end-stacking of the squaraine ligand on the 3'-terminal of the resultant parallel-type G4 structure. Moreover, the NDI ligand promotes the diffusion of the squaraine ligand into the nucleus, and the synergistic effect of the two ligands improves the stability of G4s in cancer cells, blocks the cell cycle in the sub-G1 phase, and induces the DNA damage response. These findings will be helpful in the development of combinational ligands targeting DNA G4s with enhanced bioactivity toward the inhibition of cancer cell proliferation.

Supernatural: Artificial Nucleobases and Backbones to Program Hybridization-Based Assemblies and Circuits

The specificity and predictability of hybridization make oligonucleotides a powerful platform to program assemblies and networks with logic-gated responses, an area of research which has grown into a field of its own. While the field has capitalized on the commercial availability of DNA oligomers with its four canonical nucleobases, there are opportunities to extend the capabilities of the hardware with unnatural nucleobases and other backbones. This Topical Review highlights nucleobases that favor hybridizations that are empowering for assemblies and networks as well as two chiral XNAs than enable orthogonal hybridization networks.

Pressure Tuning Studies of Four-Stranded Nucleic Acid Structures

Four-stranded folded structures, such as G-quadruplexes and i-motifs in the genome, have attracted a growing interest nowadays since they have been discovered in the telomere and in several oncogene promoter regions. Their biological relevance is undeniable since their existence in living cells has been observed. In vivo they take part in the regulation of gene expression, in vitro they are used in the analytical biochemistry. They are attractive and promising targets for cancer therapy. Pressure studies can reveal specific aspects of the molecular processes. Pressure tuning experiments allow the determination of the volumetric parameters of the folded structures and of the folding-unfolding processes. Here, we review the thermodynamic parameters with a special focus on the volumetric ones, which were determined using pressure tuning spectroscopic experiments on the G-quadruplex and i-motif nucleic acid forms.

Study on the Interaction of a Peptide Targeting Specific G-Quadruplex Structures Based on Chromatographic Retention Behavior

G-quadruplexes (G4s) are of vital biological significance and G4-specific ligands with conformational selectivity show great application potential in disease treatment and biosensing. RHAU, a RNA helicase associated with AU-rich element, exerts biological functions through the mediation of G4s and has been identified to be a G4 binder. Here, we investigated the interactions between the RHAU peptide and G4s with different secondary structures using size exclusion chromatography (SEC) in association with circular dichroism (CD), ultraviolet-visible (UV-Vis) absorption, and native polyacrylamide gel electrophoresis (Native-PAGE). Spectral results demonstrated that the RHAU peptide did not break the main structure of G4s, making it more reliable for G4 structural analysis. The RHAU peptide was found to display a structural selectivity for a preferential binding to parallel G4s as reflected by the distinct chromatographic retention behaviors. In addition, the RHAU peptide exhibited different interactions with intermolecular parallel G4s and intramolecular parallel G4s, providing a novel recognition approach to G4 structures. The findings of this study enriched the insight into the binding of RHAU to G4s with various conformations. It is noteworthy that SEC technology can be easy and reliable for elucidating G4-peptide interactions, especially for a multiple G4 coexisting system, which supplied an alternative strategy to screen novel specific ligands for G4s.

Selenium-Substituted Monomethine Cyanine Dyes as Selective G-Quadruplex Spectroscopic Probes with Theranostic Potential

The binding interactions of six ligands, neutral and monocationic asymmetric monomethine cyanine dyes comprising benzoselenazolyl moiety with duplex DNA and RNA and G-quadruplex structures were evaluated using fluorescence, UV/Vis (thermal melting) and circular dichroism (CD) spectroscopy. The main objective was to assess the impact of different substituents (methyl vs. sulfopropyl vs. thiopropyl/thioethyl) on the nitrogen atom of the benzothiazolyl chromophore on various nucleic acid structures. The monomethine cyanine dyes with methyl substituents showed a 100-fold selectivity for G-quadruplex versus duplex DNA. Study results indicate that cyanines bind with G-quadruplex via end π-π stacking interactions and possible additional interactions with nucleobases/phosphate backbone of grooves or loop bases. Cyanine with thioethyl substituent distinguishes duplex DNA and RNA and G-quadruplex structures by distinctly varying ICD signals. Furthermore, cell viability assay reveals the submicromolar activity of cyanines with methyl substituents against all tested human cancer cell lines. Confocal microscopy analysis shows preferential accumulation of cyanines with sulfopropyl and thioethyl substituents in mitochondria and indicates localization of cyanines with methyl in nucleus, particularly nucleolus. This confirms the potential of examined cyanines as theranostic agents, possessing both fluorescent properties and cell viability inhibitory effect.

Telomeres expand sphere of influence: emerging molecular impact of telomeres in non-telomeric functions

Although the impact of telomeres on physiology stands well established, a question remains: how do telomeres impact cellular functions at a molecular level? This is because current understanding limits the influence of telomeres to adjacent subtelomeric regions despite the wide-ranging impact of telomeres. Emerging work in two distinct aspects offers opportunities to bridge this gap. First, telomere-binding factors were found with non-telomeric functions. Second, locally induced DNA secondary structures called G-quadruplexes are notably abundant in telomeres, and gene regulatory regions genome wide. Many telomeric factors bind to G-quadruplexes for non-telomeric functions. Here we discuss a more general model of how telomeres impact the non-telomeric genome - through factors that associate at telomeres and genome wide - and influence cell-intrinsic functions, particularly aging, cancer, and pluripotency.

4'-SCF3 -Labeling Constitutes a Sensitive 19 F NMR Probe for Characterization of Interactions in the Minor Groove of DNA

Fluorinated nucleotides are invaluable for ¹⁹ F NMR studies of nucleic acid structure and function. Here, we synthesized 4'-SCF₃ -thymidine (T 4 ' - SCF 3 ${{^{4{^\prime}\hbox{-}{\rm SCF}{_{3}}}}}$ ) and incorporated it into DNA by means of solid-phase DNA synthesis. NMR studies showed that the 4'-SCF₃ group exhibited a flexible orientation in the minor groove of DNA duplexes and was well accommodated by various higher order DNA structures. The three magnetically equivalent fluorine atoms in 4'-SCF₃ -DNA constitute an isolated spin system, offering high ¹⁹ F NMR sensitivity and excellent resolution of the positioning of T 4 ' - SCF 3 ${{^{4{^\prime}\hbox{-}{\rm SCF}{_{3}}}}}$ within various secondary and tertiary DNA structures. The high structural adaptability and high sensitivity of T 4 ' - SCF 3 ${{^{4{^\prime}\hbox{-}{\rm SCF}{_{3}}}}}$ make it a valuable ¹⁹ F NMR probe for quantitatively distinguishing diverse DNA structures with single-nucleotide resolution and for monitoring the dynamics of interactions in the minor groove of double-stranded DNA.

Phen-DC3 Induces Refolding of Human Telomeric DNA into a Chair-Type Antiparallel G-Quadruplex through Ligand Intercalation

Human telomeric G-quadruplex DNA structures are attractive anticancer drug targets, but the target's polymorphism complicates the drug design: different ligands prefer different folds, and very few complexes have been solved at high resolution. Here we report that Phen-DC3 , one of the most prominent G-quadruplex ligands in terms of high binding affinity and selectivity, causes dTAGGG(TTAGGG)3 to completely change its fold in KCl solution from a hybrid-1 to an antiparallel chair-type structure, wherein the ligand intercalates between a two-quartet unit and a pseudo-quartet, thereby ejecting one potassium ion. This unprecedented high-resolution NMR structure shows for the first time a true ligand intercalation into an intramolecular G-quadruplex.

Binding-Induced Diversity of a Human Telomeric G-Quadruplex Stability Phase Space

The structural polymorphism of G-quadruplex nucleic acids is an important factor in their recognition by proteins and small-molecule ligands. However, it is not clear why the binding of several ligands alters G-quadruplex topology. We addressed this question by following the (un)folding and binding of the human telomeric fragment 5′-(GGGTTA)₃GGGT-3′ (22GT) by calorimetry (DSC, ITC) and spectroscopy (CD). A thermodynamic analysis of the obtained data led to a detailed description of the topological phase space of stability (phase diagram) of 22GT and shows how it changes in the presence of a specific bisquinolinium ligand (360A). Various 1:1 and 2:1 ligand–quadruplex complexes were observed. With increasing temperature, the 1:1 complexes transformed into 2:1 complexes, which is attributed to the preferential binding of the ligand to the folding intermediates. Overall, the dissection of the thermodynamic parameters in combination with molecular modelling clarified the driving forces of the topological quadruplex transformations in a wide range of ligand concentrations and temperatures.

G-quadruplex structural transition driven by a platinum compound

G-quadruplex (G4) transitions play integral roles in regulating biological functions and can be modified by ligands. However, little is known about G4 transitions. Herein, we reveal distinct pathways of a platinum(II) compound Pt-phen converting parallel-stranded MYC G4 to a hybrid-type structure. Three NMR structures, 1:1 5'-end binding, 1:1 3'-end binding and 2:1 Pt-phen-MYC G4 complexes, were determined by NMR. We find that Pt-phen drives G4 transition at a low ratio. Under physiological 100 mM K+ conditions, a significant stable hydrogen-bonded T:T:A triad is formed at 3'-end of hybrid-type Myc1234, and consequently, Pt-phen first binds the 5'-end to form a 1:1 5'-end binding complex and then disrupts the 3' T:T:A triad and binds 3'-end to form a 2:1 complex with more Pt-phen. Remarkably, the G4 transition pathway is different in 5 mM K+ with Pt-phen first binding the 3'-end and then the 5'-end. 'Edgewise-loop and flanking/ligand/G-tetrad' sandwich structure formation and terminal T:T:A triad stabilization play decisive roles in advancing and altering transition pathways. Our work is the first to elucidate the molecular structures of G4 transitions driven by a small molecule. The ligand-driven G4 transition is a dynamic process that includes a quick G4 transition and multiple complexes formation.

G-quadruplex structure of the C. elegans telomeric repeat: a two tetrads basket type conformation stabilized by a non-canonical C-T base-pair

The Caenorhabditis elegans model has greatly contributed to the understanding of the role of G-quadruplexes in genomic instability. The GGCTTA repeats of the C. elegans telomeres resemble the GGGTTA repeats of the human telomeres. However, the comparison of telomeric sequences (Homo sapiens, Tetrahymena, Oxytricha, Bombyx mori and Giardia) revealed that small changes in these repeats can drastically change the topology of the folded G-quadruplex. In the present work we determined the structure adopted by the C. elegans telomeric sequence d[GG(CTTAGG)3]. The investigated C. elegans telomeric sequence is shown to fold into an intramolecular two G-tetrads basket type G-quadruplex structure that includes a C-T base pair in the diagonal loop. This work sheds light on the telomeric structure of the widely used C. elegans animal model.

Spectroscopic and Molecular Dynamics Aspect of Antimalarial Drug Hydroxychloroquine Binding with Human Telomeric G-Quadruplex

Hydroxychloroquine (HCQ) is an important drug that is in the trial stage for different types of cancer diseases; however, insight about the mechanism of its action is almost unknown. G-quadruplex (Gq) has been considered one of the potential targets for the cure of cancer; hence, it is essential to understand the possibility of the binding of HCQ with Gq to get a better understanding of its action. In this study, the molecular insight into the possibility of the binding of HCQ with different topological forms of Gq of the human telomere (htel) has been investigated using spectroscopic, thermochemical, and molecular dynamics simulation techniques. The spectroscopic and thermochemical studies clearly suggest that HCQ has a topological preference in the binding with htel in the form of a hybrid structure rather than the antiparallel form and the binding of HCQ stabilizes preferably to the hybrid form. The molecular dynamics simulation study suggests that the interaction of HCQ in the groove and loop regions of the hybrid structure is more stable compared to the antiparallel form, which is the probable reason for the topological preference of HCQ. This study depicts that HCQ has a topological preference in the binding and stabilization of the Gq of htel, which makes it potentially an important drug for targeting the telomere region associated with cancer disease.

Guiding the folding of G-quadruplexes through loop residue interactions

A G-rich sequence was designed to allow folding into either a stable parallel or hybrid-type topology. With the parent sequence featuring coexisting species, various related sequences with single and double mutations and with a shortened central propeller loop affected the topological equilibrium. Two simple modifications, likewise introduced separately to all sequences, were employed to lock folds into one of the topologies without noticeable structural alterations. The unique combination of sequence mutations, high-resolution NMR structural information, and the thermodynamic stability for both topological competitors identified critical loop residue interactions. In contrast to first loop residues, which are mostly disordered and exposed to solvent in both propeller and lateral loops bridging a narrow groove, the last loop residue in a lateral three-nucleotide loop is engaged in stabilizing stacking interactions. The propensity of single-nucleotide loops to favor all-parallel topologies by enforcing a propeller-like conformation of an additional longer loop is shown to result from their preference in linking two outer tetrads of the same tetrad polarity. Taken together, the present studies contribute to a better structural and thermodynamic understanding of delicate loop interactions in genomic and artificially designed quadruplexes, e.g. when employed as therapeutics or in other biotechnological applications.

Photoactivatable V-Shaped Bifunctional Quinone Methide Precursors as a New Class of Selective G-quadruplex Alkylating Agents

Combining the selectivity of G-quadruplex (G4) ligands with the spatial and temporal control of photochemistry is an emerging strategy to elucidate the biological relevance of these structures. In this work, we developed six novel V-shaped G4 ligands that can, upon irradiation, form stable covalent adducts with G4 structures via the reactive intermediate, quinone methide (QM). We thoroughly investigated the photochemical properties of the ligands and their ability to generate QMs. Subsequently, we analyzed their specificity for various topologies of G4 and discovered a preferential binding towards the human telomeric sequence. Finally, we tested the ligand ability to act as photochemical alkylating agents, identifying the covalent adducts with G4 structures. This work introduces a novel molecular tool in the chemical biology toolkit for G4s.

Biomolecules under Pressure: Phase Diagrams, Volume Changes, and High Pressure Spectroscopic Techniques

Pressure is an equally important thermodynamical parameter as temperature. However, its importance is often overlooked in the biophysical and biochemical investigations of biomolecules and biological systems. This review focuses on the application of high pressure (>100 MPa = 1 kbar) in biology. Studies of high pressure can give insight into the volumetric aspects of various biological systems; this information cannot be obtained otherwise. High-pressure treatment is a potentially useful alternative method to heat-treatment in food science. Elevated pressure (up to 120 MPa) is present in the deep sea, which is a considerable part of the biosphere. From a basic scientific point of view, the application of the gamut of modern spectroscopic techniques provides information about the conformational changes of biomolecules, fluctuations, and flexibility. This paper reviews first the thermodynamic aspects of pressure science, the important parameters affecting the volume of a molecule. The technical aspects of high pressure production are briefly mentioned, and the most common high-pressure-compatible spectroscopic techniques are also discussed. The last part of this paper deals with the main biomolecules, lipids, proteins, and nucleic acids: how they are affected by pressure and what information can be gained about them using pressure. I I also briefly mention a few supramolecular structures such as viruses and bacteria. Finally, a subjective view of the most promising directions of high pressure bioscience is outlined.

Chromene Derivatives as Selective TERRA G-Quadruplex RNA Binders with Antiproliferative Properties

In mammalian cells, telomerase transcribes telomeres in large G-rich non-coding RNA, known as telomeric repeat-containing RNA (TERRA), which folds into noncanonical nucleic acid secondary structures called G-quadruplexes (G4s). Since TERRA G4 has been shown to be involved in telomere length and translation regulation, it could provide valuable insight into fundamental biological processes, such as cancer growth, and TERRA G4 binders could represent an innovative strategy for cancer treatment. In this work, the three best candidates identified in our previous virtual screening campaign on bimolecular DNA/RNA G4s were investigated on the monomolecular Tel DNA and TERRA G4s by means of molecular modelling simulations and in vitro and in cell analysis. The results obtained in this work highlighted the stabilizing power of all the three candidates on TERRA G4. In particular, the two compounds characterized by a chromene scaffold were selective TERRA G4 binders, while the compound with a naphthyridine core acted as a dual Tel/TERRA G4-binder. A biophysical investigation by circular dichroism confirmed the relative stabilization efficiency of the compounds towards TERRA and Tel G4s. The TERRA G4 stabilizing hits showed good antiproliferative activity against colorectal and lung adenocarcinoma cell lines. Lead optimization to increase TERRA G4 stabilization may provide new powerful tools against cancer.

Targeting G-Quadruplex Dna For Cancer Chemotherapy

The self-association of DNA formed by Hoogsteen hydrogen bonding comprises several layers of four guanine or G-tetrads or G4s. The distinct feature of G4s, such as the G-tetrads and loops, qualify structure-selective recognition by small molecules and various ligands and can act as potential anticancer therapeutic molecules. The G4 selective-ligands, can influence gene expression by targeting a nucleic acid structure rather than sequence. Telomere G4 can be targeted for cancer treatment by small molecules inhibiting the telomerase activity whereas c-MYC is capable of controlling transcription, can be targeted to influence transcription. The k-RAS is one of the most frequently encountered oncogenic driver mutations in pancreatic, colorectal, and lung cancers. The k-RAS oncogene plays important role in acquiring and increasing the drug resistance and can also be directly targeted by small molecules to combat k-RAS mutant tumors. Modular G4 ligands with different functional groups, side chains and rotatable bonds as well as conformation affect the binding affinity/selectivity in cancer chemotherapeutic interventions. These modular G4 ligands act by targeting the diversity of G4 loops and groves and assists to develop more drug-like compounds with selectivity. In this review, we present the recent research on synthetic G4 DNA-interacting ligands as an approach toward the discovery of target specific anticancer chemotherapeutic agents.

A Chimeric DNA/RNA Antiparallel Quadruplex with Improved Stability

Nucleic acid quadruplexes are proposed to play a role in the regulation of gene expression, are often present in aptamers selected for specific binding functions and have potential applications in medicine and biotechnology. Therefore, understanding their structure and thermodynamic properties and designing highly stable quadruplexes is desirable for a variety of applications. Here, we evaluate DNA→RNA substitutions in the context of a monomolecular, antiparallel quadruplex, the thrombin-binding aptamer (TBA, GGTTGGTGTGGTTGG) in the presence of either K+ or Sr2+ . TBA predominantly folds into a chair-type configuration containing two G-tetrads, with G residues in both syn and anti conformation. All chimeras with DNA→RNA substitutions (G→g) at G residues requiring the syn conformation demonstrated strong destabilization. In contrast, G→g substitutions at Gs with anti conformation increased stability without affecting the monomolecular chair-type topology. None of the DNA→RNA substitutions in loop positions affected the quadruplex topology; however, these substitutions varied widely in their stabilizing or destabilizing effects in an unpredictable manner. This analysis allowed us to design a chimeric DNA/RNA TBA construct that demonstrated substantially improved stability relative to the all-DNA construct. These results have implications for a variety of quadruplex-based applications including for the design of dynamic nanomachines.

Fluorescent AgNCs Formed on Bifunctional DNA Template for Potassium Ion Detection

In this study, we examined properties of silver nanoclusters, which are AgNCs stabilized by DNA oligonucleotide scaffold containing G-quadruplex-forming sequences: human telomeric (Tel22) or thrombin-binding aptamer (TBA). Thus, we obtained two fluorescent probes abbreviated as Tel22C12-AgNCs and TBAC12-AgNCs, which were characterized using absorption, circular dichroism and fluorescence spectroscopy. Both probes emit green and red fluorescence. The presence of silver nanoclusters did not destabilize the formed G-quadruplexes. The structural changes of probes upon binding K+ or Na+ ions cause quenching in their red emission. Green emission was slightly quenched only in the case of Tel22C12-AgNCs; on the contrary, for TBAC12-AgNC’s green emission, we observed an increasing fluorescence signal. Moreover, the Tel22C12-AgNCs system shows not only a higher binding preference for K+ over Na+, but it was able to monitor small changes in K+ concentrations in the buffer mimicking extracellular conditions (high content of Na+ ions). These results suggest that Tel22C12-AgNCs exhibit the potential to monitor transmembrane potassium transport.

Pilot imaging study of o-BMVC foci for discrimination of indeterminate cytology in diagnosing fine-needle aspiration of thyroid nodules

Fluorescence lifetime imaging microscopy of a fluorescence probe, 3,6-bis(1-methyl-2-vinylpyridinium) carbazole diiodide (o-BMVC), provides an objective method for preoperative diagnosis of fine-needle aspiration (FNA) of thyroid nodules. The key of this o-BMVC test of FNA smears is the measurement of the digital number of o-BMVC foci in the nucleus. Thus, there are three categories classified in the o-BMVC test, which are nondiagnostic for unsatisfactory samples, benign for less numbers of o-BMVC foci, and malignant for more numbers of o-BMVC foci. The discrimination of indeterminate (including atypia, follicular neoplasm, suspicious) cytology into benign or malignant cases can reduce diagnostic uncertainty and benefit clinical decision making. This pilot study strongly suggests that the o-BMVC test is an invaluable method for diagnosing FNA samples. Particularly, the combination of FNA cytology and the o-BMVC test holds great promise to improve the efficacy of diagnosis and reduce the healthcare costs.

G4-PROTAC: targeted degradation of a G-quadruplex binding protein

G-quadruplex (G4) binding proteins regulate important biological processes, but their interaction networks are poorly understood. We report the first use of G4 as a warhead of a proteolysis-targeting chimera (G4-PROTAC) for targeted degradation of a G4-binding protein (RHAU/DHX36). G4-PROTAC provides a new way to explore G4-protein networks and to develop potential therapeutics.

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.

Molecular insights into the selective binding mechanism targeting parallel human telomeric G-quadruplex

Stabilizing human telomere DNA G-quadruplex (G4) proves a promising anti-cancer strategy. Though plenty of G4 stabilizing molecules have been reported, little is known about their selective binding mechanism among various G4s. Recently, a designed monohydrazone derivative (compound 15) was reported to display specific preference in binding and stabilizing parallel human telomeric G4. To reveal the selective binding mechanism, a comparative theoretical investigation was performed on two monohydrazone derivatives (compounds 1 and 15) and three telomeric G4s showing parallel, hybrid-I, and hybrid-II conformations. Two probable binding modes, i.e. the end-stacking binding and the groove binding, were predicted by molecular dockings for each monohydrazone in its binding with the telomeric G4s. Further long-timescale molecular dynamics simulations reveal the conversion from the groove binding to the end-stacking binding for both compounds, indicating the preference of the end-stacking binding mode. Structural analysis together with binding free energy calculations show that the van der Waals interaction plays a leading role in ranking the binding affinity. By forming extensive van der Waals interactions, the parallel G4-15 binding complex shows the highest binding affinity, and the corresponding compound 15 exhibits the strongest stabilizing effect to the telomeric G4. These findings agree well with the experimental observations. Through characterizing the selective binding between monohydrazones and telomeric G4s at the atomic level, the current study provides support to the design of novel selective stabilizers targeting telomeric G4s.

The combined action of cations and anions of ionic liquids modulates the formation and stability of G-quadruplex DNA

G-Quadruplex (Gq) formation and stabilization by any molecule is an essential requirement for its application in therapy, especially in oncology. Metal cations have shown higher propensity of the formation of the Gq structure and its stabilization. In this study, the role of both cations and anions of ionic liquids (ILs) on the Gq formation of human telomere (hTeloG) and its stability was investigated using spectroscopic and molecular dynamics simulation techniques. Irrespective of the nature of anions of ILs, tetramethylguanidinium (TMG) cations associated with different anions can form an antiparallel Gq structure in hTeloG. However, the propensity of the formation of an antiparallel Gq structure and its stability depend on the chain length of anions of ILs. Gq is significantly less stable in ILs having longer hydrocarbon chain anions compared to the short chain anions suggesting that the hydrophobicity of the anion plays a critical role in the stability and formation of the Gq structure by ILs. The data indicate that longer hydrocarbon chain anions of ILs preferably interact in the loop region of Gq through hydrophobic interaction which enhances the overall binding of the cation of ILs with Gq causing a decrease in the stacking energy between the G-quartets as well as Hoogsteen hydrogen bonds between the guanine bases leading to the destabilization of the antiparallel Gq structure.

Two coexisting pseudo-mirror heteromolecular telomeric G-quadruplexes in opposite loop progressions differentially recognized by a low equivalent of Thioflavin T

The final 3′-terminal residue of the telomeric DNA G-overhang is inherently less precise. Here, we describe how alteration of the last 3′-terminal base affects the mutual recognition between two different G-rich oligomers of human telomeric DNA in the formation of heteromolecular G-quadruplexes (hetero-GQs). Associations between three- and single-repeat fragments of human telomeric DNA, target d(GGGTTAGGGTTAGGG) and probe d(TAGGGT), in Na⁺ solution yield two coexisting forms of (3 + 1) hybrid hetero-GQs: the kinetically favourable LLP-form (left loop progression) and the thermodynamically controlled RLP-form (right loop progression). However, only the adoption of a single LLP-form has been previously reported between the same probe d(TAGGGT) and a target variant d(GGGTTAGGGTTAGGGT) having one extra 3′-end thymine. Moreover, the flanking base alterations of short G-rich probe variants also significantly affect the loop progressions of hetero-GQs. Although seemingly two pseudo-mirror counter partners, the RLP-form exhibits a preference over the LLP-form to be recognized by a low equivalent of fluorescence dye thioflavin T (ThT). To a greater extent, ThT preferentially binds to RLP hetero-GQ than with the corresponding telomeric DNA duplex context or several other representative unimolecular GQs.

Thermodynamic Stability of G-Quadruplexes: Impact of Sequence and Environment

G-quadruplexes have attracted growing interest in recent years due to their occurrence in vivo and their possible biological functions. In addition to being promising targets for drug design, these four-stranded nucleic acid structures have also been recognized as versatile tools for various technological applications. Whereas a large number of studies have yielded insight into their remarkable structural diversity, our current knowledge on G-quadruplex stabilities as a function of sequence and environmental factors only gradually emerges with an expanding collection of thermodynamic data. This minireview provides an overview of general rules that may be used to better evaluate quadruplex thermodynamic stabilities but also discusses present challenges in predicting most stable folds for a given sequence and environment.