Nucleic acid duplexes with metal-mediated base pairs and their structures

Controlling the Depth of Hybridization Chain Reaction by Extended Dangling Ends and Its Analytical Applications

Hybridization chain reaction (HCR) is a powerful enzyme-free nucleic acid amplification strategy. Triggered by an initiator strand, it yields nicked double helices analogous to alternating copolymers. However, there is no effective way to regulate the HCR reaction, and the most apparent phenomenon is the uncontrollable polymerization of product after introducing an initiator. Here we explore controlling the depth of the HCR reaction by extended dangling ends on hairpin monomers and report that sequence length, nucleotide composition, and secondary structure can alter HCR polymerization and can be utilized for the desired regulation. Interaction dynamics between initiator and hairpin monomers simulated by oxDNA are in good accordance with experimental results. Such a controlling effect can be utilized for new analytical applications that HCR cannot previously achieve, such as analyzing strand-extension enzymes and identifying short-sequence structures. The finding provides a concise but effective way for controlling the depth of HCR reaction and opens the application scope of HCR to more fields.

Zn2+ ions improve the fidelity of metal-mediated primer extension while suppressing intrinsic and Mn2+-induced mutagenic effects by DNA polymerases

While Mn2+ ions are well-established for reducing the fidelity of DNA polymerases, leading to the misincorporation of nucleotides, our investigation of the effects of metal ions revealed a contrasting role of Zn2+. Here, we demonstrate that Zn2+ ions enhance the fidelity of DNA polymerases (the 3' → 5' exonuclease-deficient Klenow fragment and Taq DNA polymerase) by suppressing misincorporation during primer extension reactions. Remarkably, Zn2+ ions inhibit both intrinsic misincorporation and Mn2+-induced misincorporation of nucleotides. Furthermore, Zn2+ ions also effectively suppressed misincorporation during metal-mediated primer extension reactions, which involved forming Ag+ and Hg2+ ion-mediated base pairs. These findings suggest that Zn2+ ions inhibit both intrinsic and Mn2+-induced mismatched base pair formation. Consequently, the combined use of Mn2+ and Zn2+ ions may offer a strategy for precisely regulating the fidelity of DNA polymerases. Remarkably, Zn2+ ions even suppress misincorporation in primer extension reactions that rely on metal-mediated base pairs, and conversely, this suggests that DNA polymerases recognize metal-mediated base pairs such as T-Hg2+-T, C-Ag+-A, and C-Ag+-T as relatively stable base pairs. These results imply that Zn2+ ions may also enhance the fidelity of DNA polymerases when incorporating non-canonical nucleobases, potentially paving the way for the expansion of the genetic alphabet.

Uridine triphosphate hybrid catalyst for carbon‑carbon bond formation reactions with enhanced enantioselectivity by mercury(II) ions

DNA hybrid catalysts are constructed by embedding active metal species into the chiral scaffolds of DNA, which have been successfully applied to some important aqueous-phase enantioselective transformations. Owing to simple components and inherent chirality, nucleotide hybrid catalysts are emerging in response to soving the unclear locations of catalytic centers and the plausible catalytic mechanisms in DNA-based asymmetric catalysis. However, the tertiary structure of nucleotides lacks tunability, severely impeding further design of nucleotide hybrid catalysts for potential applications. To this end, a design strategy for tunable nucleotide hybrid catalysts is put forward by introducing metal-mediated base pairs. Herein, we found that the formation of uracil‑mercury(II)-uracil (U-Hg2+-U) base pairs could enhance the enantioselectivity in uracil-containing nucleotide-based asymmetric reactions. Compared with uracil triphosphate (UTP) complexing with Cu2+ ions (UTP∙Cu2+), the presence of Hg2+ ions gave rise to an increased enantiomeric excess (ee) of 38 % in Diels-Alder reactions and 22 % ee in Michael reactions. The Hg2+-tuning behaviors of UTP hybrid catalyst have been demonstrated to largely depend on nucleotides, Hg2+ concentrations, metal cofactors, additives and reaction types. Based on ultraviolet-visible, circular dichroism and nuclear magnetic resonance spectroscopic techniques, the chiral enhancement of Hg2+-containing UTP hybrid catalyst is proved to largely depend on the formation of U-Hg2+-U base pairs and the plausible cross-linked structure of UTP-Hg2+-UTP/Cu2+ assembly. This work provides a tunable strategy based on the concept of metal-mediated base pairs, allowing further design of potent oligonucleotide-based catalysts for other enantioselective reactions.

Unveiling the solution structure of a DNA duplex with continuous silver-modified Watson-Crick base pairs

The challenge of transforming organized DNA structures into their metallized counterparts persists in the scientific field. In this context, utilizing DNA molecules modified with 7-deazapurine, provides a transformative solution. In this study, we present the solution structure of a DNA duplex that can be transformed into its metallized equivalent while retaining the natural base pairing arrangement through the creation of silver-modified Watson-Crick base pairs. Unlike previously documented X-ray structures, our research demonstrates the feasibility of preserving the intrinsic DNA self-assembly while incorporating AgI into the double helix, illustrating that the binding of silver does not disrupt the canonical base-pairing organization. Moreover, in our case, the uninterrupted AgI chain deviates from forming conventional straight linear chains; instead, it adheres to a helical arrangement dictated by the underlying DNA structure. This research challenges conventional assumptions and opens the door to precisely design structures based on the organization of highly stable Ag-DNA assemblies.

An Atom-Precise Understanding of DNA-Stabilized Silver Nanoclusters

ConspectusDNA-stabilized silver nanoclusters (AgN-DNAs) are sequence-encoded fluorophores. Like other noble metal nanoclusters, the optical properties of AgN-DNAs are dictated by their atomically precise sizes and shapes. What makes AgN-DNAs unique is that nanocluster size and shape are controlled by nucleobase sequence of the templating DNA oligomer. By choice of DNA sequence, it is possible to synthesize a wide range of AgN-DNAs with diverse emission colors and other intriguing photophysical properties. AgN-DNAs hold significant potential as "programmable" emitters for biological imaging due to their combination of small molecular-like sizes, bright and sequence-tuned fluorescence, low toxicities, and cost-effective synthesis. In particular, the potential to extend AgN-DNAs into the second near-infrared region (NIR-II) is promising for deep tissue imaging, which is a major area of interest for advancing biomedical imaging. Achieving this goal requires a deep understanding of the structure-property relationships that govern AgN-DNAs in order to design AgN-DNA emitters with sizes and geometries that support NIR-II emission.In recent years, major advances have been made in understanding the structure and composition of AgN-DNAs, enabling new insights into the correlation of nanocluster structure and photophysical properties. These advances have hinged on combined innovations in mass characterization and crystallography of compositionally pure AgN-DNAs, together with combinatorial experiments and machine learning-guided design. A combined approach is essential due to the major challenge of growing suitable AgN-DNA crystals for diffraction and to the labor-intensive nature of preparing and solving the molecular formulas of atomically precise AgN-DNAs by mass spectrometry. These approaches alone are not feasibly scaled to explore the large sequence space of DNA oligomer templates for AgN-DNAs.This account describes recent fundamental advances in AgN-DNA science that have been enabled by high throughput synthesis and fluorimetry together with detailed analytical studies of purified AgN-DNAs. First, short introductions to nanocluster chemistry and AgN-DNA basics are presented. Then, we review recent large-scale studies that have screened thousands of DNA templates for AgN-DNAs, leading to discovery of distinct classes of these emitters with unique cluster core compositions and ligand chemistries. In particular, the discovery of a new class of chloride-stabilized AgN-DNAs enabled the first ab initio calculations of AgN-DNA electronic structure and present new approaches to stabilize these emitters in biologically relevant conditions. Near-infrared (NIR) emissive AgN-DNAs are also found to exhibit diverse structures and properties. Finally, we conclude by highlighting recent proof-of-principle demonstrations of NIR AgN-DNAs for targeted fluorescence imaging. Continued efforts may future push AgN-DNAs into the tissue transparency window for fluorescence imaging in the NIR-II tissue transparency window.

Formation and Nanomechanical Properties of Silver-Mediated Guanine DNA Duplexes in Aqueous Solution

Silver cations can mediate base pairing of guanine (G) DNA oligomers, yielding linear parallel G-Ag+-G duplexes with enhanced stabilities compared to those of canonical DNA duplexes. To enable their use in programmable DNA nanotechnologies, it is critical to understand solution-state formation and the nanomechanical stiffness of G-Ag+-G duplexes. Using temperature-controlled circular dichroism (CD) spectroscopy, we find that heating mixtures of G oligomers and silver salt above 50 °C fully destabilizes G-quadruplex structures and converts oligomers to G-Ag+-G duplexes. Electrospray ionization mass spectrometry supports that G-Ag+-G duplexes form at stoichiometries of 1 Ag+ per base pair, and CD spectroscopy suggests that as the Ag+/base stoichiometry increases further, G-Ag+-G duplexes undergo additional morphological changes. Using liquid-phase atomic force microscopy, we find that this excess Ag+ enables assembly of long fiberlike structures with ∼2.5 nm heights equivalent to a single DNA duplex but with lengths that far exceed a single duplex. Finally, using the conditions established to form single G-Ag+-G duplexes, we use a surface forces apparatus (SFA) to compare the solution-phase stiffness of single G-Ag+-G duplexes with dG-dC Watson-Crick-Franklin duplexes. SFA shows that G-Ag+-G duplexes are 1.3 times stiffer than dG-dC duplexes, confirming gas-phase ion mobility spectrometry measurements and computational predictions. These findings may guide the development of structural DNA nanotechnologies that rely on silver-mediated base pairing.

Metal-dependent activity control of a compact-sized 8-17 DNAzyme based on metal-mediated unnatural base pairing

A compact 8-17 DNAzyme was modified with a CuII-meditated artificial base pair to develop a metal-responsive allosteric DNAzyme. The base sequence was rationally designed based on the reported three-dimensional structure. The activity of the modified DNAzyme was enhanced 5.1-fold by the addition of one equivalent of CuII ions, showing good metal responsiveness. Since it has been challenging to modify compactly folded DNAzymes without losing their activity, this study demonstrates the utility of the metal-mediated artificial base pairing to create stimuli-responsive functional DNAs.

Synthesis and chemical transformations of glycol nucleic acid (GNA) nucleosides

Xeno nucleic acids (XNA) are an increasingly important class of hypermodified nucleic acids with great potential in bioorganic chemistry and synthetic biology. Glycol nucleic acid (GNA) is constructed from a three-carbon 1,2-propanediol (propylene glycol) backbone attached to a nucleobase entity, representing the simplest known XNA. This review is intended to present GNA nucleosides from a synthetic chemistry perspective-a perspective that serves as a starting point for biological studies. Therefore this account focuses on synthetic methods for GNA nucleoside synthesis, as well as their postsynthetic chemical transformations. The properties and biological activity of GNA constituents are also highlighted. A literature survey shows four major approaches toward GNA nucleoside scaffold synthesis. These approaches pertain to glycidol ring-opening, Mitsunobu, SN2, and dihydroxylation reactions. The general arsenal of reactions used in GNA chemistry is versatile and encompasses the Sonogashira reaction, Michael addition, silyl-Hilbert-Johnson reaction, halogenation, alkylation, cyclization, Rh-catalyzed N-allylation, Sharpless catalytic dihydroxylation, and Yb(OTf)3-catalyzed etherification. Additionally, various phosphorylation reactions have enabled the synthesis of diverse types of GNA nucleotides, dinucleoside phosphates, phosphordiamidites, and oligos. Furthermore, recent advances in GNA chemistry have resulted in the synthesis of previously unknown redox-active (ferrocenyl) and luminescent (pyrenyl and phenanthrenyl) GNA nucleosides, which are also covered in this review.

Self-Assembly of DNA Nanostructures in Different Cations

The programmable nature of DNA allows the construction of custom-designed static and dynamic nanostructures, and assembly conditions typically require high concentrations of magnesium ions that restricts their applications. In other solution conditions tested for DNA nanostructure assembly, only a limited set of divalent and monovalent ions are used so far (typically Mg2+ and Na+ ). Here, we investigate the assembly of DNA nanostructures in a wide variety of ions using nanostructures of different sizes: a double-crossover motif (76 bp), a three-point-star motif (~134 bp), a DNA tetrahedron (534 bp) and a DNA origami triangle (7221 bp). We show successful assembly of a majority of these structures in Ca2+ , Ba2+ , Na+ , K+ and Li+ and provide quantified assembly yields using gel electrophoresis and visual confirmation of a DNA origami triangle using atomic force microscopy. We further show that structures assembled in monovalent ions (Na+ , K+ and Li+ ) exhibit up to a 10-fold higher nuclease resistance compared to those assembled in divalent ions (Mg2+ , Ca2+ and Ba2+ ). Our work presents new assembly conditions for a wide range of DNA nanostructures with enhanced biostability.

Heat, pH, and salt: synthesis strategies to favor formation of near-infrared emissive DNA-stabilized silver nanoclusters

We present chemical synthesis strategies for DNA-stabilized silver nanoclusters (AgN-DNAs) with near-infrared (NIR) emission in the biological tissue transparency windows. Elevated temperatures can significantly increase chemical yield of near-infrared nanoclusters. In most cases, basic pH favors near-infrared nanoclusters while micromolar amounts of NaCl inhibit their formation.

Metal-mediated DNA strand displacement and molecular device operations based on base-pair switching of 5-hydroxyuracil nucleobases

Rational design of self-assembled DNA nanostructures has become one of the fastest-growing research areas in molecular science. Particular attention is focused on the development of dynamic DNA nanodevices whose configuration and function are regulated by specific chemical inputs. Herein, we demonstrate the concept of metal-mediated base-pair switching to induce inter- and intramolecular DNA strand displacement in a metal-responsive manner. The 5-hydroxyuracil (UOH) nucleobase is employed as a metal-responsive unit, forming both a hydrogen-bonded UOH-A base pair and a metal-mediated UOH-GdIII-UOH base pair. Metal-mediated strand displacement reactions are demonstrated under isothermal conditions based on the base-pair switching between UOH-A and UOH-GdIII-UOH. Furthermore, metal-responsive DNA tweezers and allosteric DNAzymes are developed as typical models for DNA nanodevices simply by incorporating UOH bases into the sequence. The metal-mediated base-pair switching will become a versatile strategy for constructing stimuli-responsive DNA nanostructures, expanding the scope of dynamic DNA nanotechnology.

An efficient fluorescence reversible regulation strategy with single labelled oligonucleotide HEX-OND successively triggered by Hg(II) and Cysteine: The application and mechanism

An efficient fluorescence reversible regulation system with HEX-OND was developed. Then the application potential was explored in probing Hg(II) & Cysteine (Cys) in real samples and the thermodynamic mechanism was further investigated by precise theory analysis combining multiple spectroscopic methods. The results showed that only mere disturbances were observed among 15 and 11 kinds of other substances for the optimal system in detecting Hg(II) & Cys, respectively; The linear ranges of quantification were identified as 1.0 ∼ 14.0 and 2.0 ∼ 20.0 (×10^-8 mol/L) with LODs of 8.75 and 14.09 (×10^-9 mol/L) for Hg(II) and Cys, respectively; no significant deviations were found in the quantification results of Hg(II) in three traditional Chinese herbs and Cys in two samples between the well-understood methods with ours respectively, showing excellent selectivity, sensitivity, and tremendous application feasibility. The detailed mechanism was further verified as that the introduced Hg(II) forced HEX-OND to transform into the Hairpin structure with the apparent equilibrium association constant of 6.02 ± 0.62 × 10¹⁰ L/mol in the bimolecular ratio, leading to the equimolar quencher, consecutive two guanine bases ((G)₂), approaching and spontaneously static-quenching the reporter HEX (hexachlorofluorescein) (equilibrium constant, 8.75 ± 1.97 × 10⁷ L/mol) in the Photo-induced Electron Transfer (PET) way that was driven by the Electrostatic Interaction. The additional Cys destructed the equimolar Hairpin structure with the apparent equilibrium constant of 8.87 ± 2.47 × 10⁵ L/mol through breaking one of the formed T-Hg(II)-T mismatches by association with the involved Hg(II), occasioning (G)₂ apart from HEX and consequently the fluorescence recovery.

Self-assembly of DNA nanostructures in different cations

The programmable nature of DNA allows the construction of custom-designed static and dynamic nanostructures, and assembly conditions typically require high concentrations of magnesium ions which restricts their applications. In other solution conditions tested for DNA nanostructure assembly, only a limited set of divalent and monovalent ions have been used so far (typically Mg 2+ and Na + ). Here, we investigate the assembly of DNA nanostructures in a wide variety of ions using nanostructures of different sizes: a double-crossover motif (76 bp), a three-point-star motif (∼134 bp), a DNA tetrahedron (534 bp) and a DNA origami triangle (7221 bp). We show successful assembly of a majority of these structures in Ca 2+ , Ba 2+ , Na + , K + and Li + and provide quantified assembly yields using gel electrophoresis and visual confirmation of a DNA origami triangle using atomic force microscopy. We further show that structures assembled in monovalent ions (Na + , K + and Li + ) exhibit up to a 10-fold higher nuclease resistance compared to those assembled in divalent ions (Mg 2+ , Ca 2+ and Ba 2+ ). Our work presents new assembly conditions for a wide range of DNA nanostructures with enhanced biostability.

Platinum-Nucleos(t)ide Compounds as Possible Antimetabolites for Antitumor/Antiviral Therapy: Properties and Perspectives

Nucleoside analogues (NAs) are a family of compounds which include a variety of purine and pyrimidine derivatives, widely used as anticancer and antiviral agents. For their ability to compete with physiological nucleosides, NAs act as antimetabolites exerting their activity by interfering with the synthesis of nucleic acids. Much progress in the comprehension of their molecular mechanisms has been made, including providing new strategies for potentiating anticancer/antiviral activity. Among these strategies, new platinum-NAs showing a good potential to improve the therapeutic indices of NAs have been synthesized and studied. This short review aims to describe the properties and future perspectives of platinum-NAs, proposing these complexes as a new class of antimetabolites.

Replacement of the phosphodiester backbone between canonical nucleosides with a dirhenium carbonyl "click" linker-a new class of luminescent organometallic dinucleoside phosphate mimics

The first-in-class luminescent dinucleoside phosphate analogs with a [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] "click" linker as a replacement for the natural phosphate group are reported together with the synthesis of luminescent adenosine and thymidine derivatives having the [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] entity attached to positions 5' and 3', respectively. These compounds were synthesized by applying inverse-electron-demand Diels-Alder and copper(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition reactions in three or four steps. The obtained compounds exhibited orange emission (λ_PL ≈ 600 nm, Φ_PL ≈ 0.10, and τ = 0.33-0.61 μs) and no toxicity (except for one nucleoside) to human HeLa cervical epithelioid and Ishikawa endometrial adenocarcinoma cancer cells in vitro. Furthermore, the compounds' ability to inhibit the growth of Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacterial strains was moderate and only observed at a high concentration of 100 μM. Confocal microscopy imaging revealed that the "dirhenium carbonyl" dinucleosides and nucleosides localized mainly in the membranous structures of HeLa cells and uniformly inside S. aureus and E. coli bacterial cells. An interesting finding was that some of the tested compounds were also found in the nuclei of HeLa cells.

Metal-dependent base pairing of bifacial iminodiacetic acid-modified uracil bases for switching DNA hybridization partner

Dynamic control of DNA assembly by external stimuli has received increasing attention in recent years. Dynamic ligand exchange in metal complexes can be a central element in the structural and functional transformation of DNA assemblies. In this study, N,N-dicarboxymethyl-5-aminouracil (dcaU) nucleoside with an iminodiacetic acid (IDA) ligand at the 5-position of the uracil base has been developed as a bifacial nucleoside that can form both hydrogen-bonded and metal-mediated base pairs. Metal complexation study of dcaU nucleosides revealed their ability to form a 2:1 complex with a GdIII ion at the monomeric level. The characteristics of base pairing of dcaU nucleosides were then examined inside DNA duplexes. The results revealed that the formation of the metal-mediated dcaU-GdIII-dcaU pair significantly stabilized the DNA duplex containing one dcaU-dcaU mismatch (ΔT m = +16.1 °C). In contrast, a duplex containing a hydrogen-bonded dcaU-A pair was destabilized in the presence of GdIII (ΔT m = -3.5 °C). The GdIII-dependent base pairing of dcaU bases was applied to control the hybridization preference of DNA in response to metal ions. The hybridization partner of a dcaU-containing strand was reversibly exchanged by the addition and removal of GdIII ions. Since the incorporation of a single dcaU base can switch the hybridization behavior of DNA, the bifacial dcaU base would be a versatile building block for imparting metal responsiveness to DNA assemblies, allowing the rational design of dynamic DNA systems.

Mapping H+ in the Nanoscale (A2C4)2-Ag8 Fluorophore

When strands of DNA encapsulate silver clusters, supramolecular optical chromophores develop. However, how a particular structure endows a specific spectrum remains poorly understood. Here, we used neutron diffraction to map protonation in (A₂C₄)₂-Ag₈, a green-emitting fluorophore with a "Big Dipper" arrangement of silvers. The DNA host has two substructures with distinct protonation patterns. Three cytosines from each strand collectively chelate handle-like array of three silvers, and calorimetry studies suggest Ag⁺ cross-links. The twisted cytosines are further joined by hydrogen bonds from fully protonated amines. The adenines and their neighboring cytosine from each strand anchor a dipper-like group of five silvers via their deprotonated endo- and exocyclic nitrogens. Typically, exocyclic amines are strongly basic, so their acidification and deprotonation in (A₂C₄)₂-Ag₈ suggest that silvers perturb the electron distribution in the aromatic nucleobases. The different protonation states in (A₂C₄)₂-Ag₈ suggest that atomic level structures can pinpoint how to control and tune the electronic spectra of these nanoscale chromophores.

Intramolecular Folding of PolyT Oligonucleotides Induced by Cooperative Binding of Silver(I) Ions

Ag⁺-bridged T-Ag⁺-T was recently discovered in a Ag⁺-DNA nanowire crystal, but it was reported that Ag⁺ had little to no affinity to T nucleobases and T-rich oligonucleotides in solution. Therefore, the binding mode for the formation of this type of novel metallo base pair in solution is elusive. Herein, we demonstrate that Ag⁺ can interact with polyT oligonucleotides once the concentration of Ag⁺ in solution exceeds a threshold value. The threshold value is independent of the concentration of the polyT oligonucleotide but is inversely proportional to the length of the polyT oligonucleotide. The polyT oligonucleotides are intramolecularly folded due to their positively cooperative formation and the stack of T-Ag⁺-T base pairs, resulting in the 5'- and 3'-ends being in close proximity to each other. The intramolecular Ag⁺-folded polyT oligonucleotide has a higher thermal stability than the duplex and can be reversibly modulated by cysteine.

Benzothiazole-substituted 1,3-diaza-2-oxophenoxazine as a luminescent nucleobase surrogate for silver(I)-mediated base pairing

A benzothiazole-substituted derivative (X) of 1,3-diaza-2-oxophenoxazine was evaluated with respect to its ability to engage in Ag(I)-mediated homo base pair formation in two different DNA duplexes. The metal binding was determined by a combination of temperature-dependent UV spectroscopy, CD spectroscopy, and fluorescence spectroscopy, indicating the incorporation of two Ag(I) ions to generate a dinuclear X-Ag(I)₂-X base pair. Interestingly, a luminescence increase was observed upon metal binding. Theoretical luminescence spectra were calculated using time-dependent density functional theory (TDDFT) for all possible Ag(I)-mediated X : X base pair geometries to identify the species responsible for the increase in luminescence. The study shows that even bulky non-planar artificial nucleobases can be applied to form stabilizing metal-mediated base pairs.

Metal-mediated DNA base pairing of easily prepared 2-oxo-imidazole-4-carboxylate nucleotides

Metal-mediated DNA base pairs, which consist of two ligand-type artificial nucleobases and a bridging metal ion, have attracted increasing attention in recent years as a different base pairing mode from natural base pairing. Metal-mediated base pairing has been extensively studied, not only for metal-dependent thermal stabilisation of duplexes, but also for metal assembly by DNA templates and construction of functional DNAs that can be controlled by metals. Here, we report the metal-mediated base paring properties of a novel 2-oxo-imidazole-4-carboxylate (Im^OC) nucleobase and a previously reported 2-oxo-imidazole-4-carboxamide (Im^OA) nucleobase, both of which can be easily derived from a commercially available uridine analogue. The Im^OC nucleobases were found to form stable Im^OC–Cu^II–Im^OC and Im^OC–Hg^II–Im^OC base pairs in the presence of the corresponding metal ions, leading to an increase in the duplex melting temperature by +20 °C and +11 °C, respectively. The Im^OC bases did not react with other divalent metal ions and showed superior metal selectivity compared to similar nucleobase design reported so far. The Im^OC–Cu^II–Im^OC base pair was much more stable than mismatch pairs with other natural nucleobases, confirming the base pair specificity in the presence of Cu^II. Furthermore, we demonstrated the quantitative assembly of three Cu^II ions inside a DNA duplex with three consecutive Im^OC–Im^OC pairs, showing great potential of DNA-template based Cu^II nanoarray construction. The study of easily-prepared Im^OC base pairs will provide a new design strategy for metal-responsive DNA materials.

A novel 2-oxo-imidazole-4-carboxylate (Im^OC) nucleobase, which can be easily derived from a commercially available uridine analogue, was found to form stable Cu^II- and Hg^II-mediated base pairs in DNA duplexes.

A review on silver-mediated DNA base pairs: methodology and application

The investigation of the interaction between metal ions and DNA has always attracted much attention in the fields of bioinorganic chemistry, supramolecular coordination chemistry, and DNA nanotechnology. Its mode of action can be simply divided into two aspects. On the one hand, it is non-specific electrostatic adsorption, mainly including Na⁺, K⁺, Mg²⁺, Ca²⁺ and other physiologically regulating ions; on the other hand, it is specific covalent binding, such as Pt²⁺, Hg²⁺, Ag⁺ and other heavy metal ions. This article focuses on the mechanism of action between Ag⁺ and DNA mismatch pair C-C, and summarizes its main characterization methods and various applications. It aims to provide a certain reference for the field of biological devices. With the development of cryo-electron microscopy and liquidcell TEM, the structure of C-Ag⁺-C is expected to be further characterized, which will be more widely used.

Ferrocene-Functionalized Crystalline Biomimetic Catalysts for Efficient CO2 Photoreduction

Photoreducing carbon dioxide (CO2) into highly valued chemicals or energy products has been recognized as one of the most promising proposals to degrade atmospheric CO2 concentration and achieve carbon neutrality. Adenine with a photosensitive amino group and aromatic nitrogen atom can strongly interact with CO2 and has been authenticated for its catalytic activity for the CO2 photoreduction reaction (CO2RR). Herein, two adenine-constructed crystalline biomimetic photocatalysts (Co2-AW and Co2-AF) were designed and synthesized to achieve CO2RR. Between them, Co2-AF displayed higher photocatalytic activity (225.8 μmol g-1 h-1) for CO2-to-HCOOH conversion than that of Co2-AW. It was found that the superior charge transfer capacity of the functional ferrocene group in Co2-AF is the primary reason to facilitate the photocatalytic performance efficiently. Additionally, this work also demonstrated the great potential of the ferrocene group as an electron donor and mediator in improving the photocatalytic activity of crystalline coordination catalysts.

“Metal-modified base pairs” vs. “metal-mediated pairs of bases”: not just a semantic issue!

A “nucleobase pair” is not identical with a “pair of basic ligands”, as only in the first case, the existence of inter-base hydrogen bonds is implied. The cross-linking of two nucleobases or two basic ligands by a metal ion of suitable geometry produces either “metal-modified” or “metal-mediated” species, but in the author’s opinion, this difference is not always properly made. This commentary is an attempt to provide a clearer distinction between the two scenarios.

Towards polymerase-mediated synthesis of artificial RNA–DNA metal base pairs

Polymerase-mediated synthesis of RNA-DNA metal base pairs.

New chemistries and enzymes for synthetic genetics

Beyond the natural nucleic acids DNA and RNA, nucleic acid chemistry has unlocked a whole universe of modifications to their canonical chemical structure, which can in various ways modify and enhance nucleic acid function and utility for applications in biotechnology and medicine. Unlike the natural modifications of tRNA and rRNA or the epigenetic modifications in mRNA and genomic DNA, these altered chemistries are not found in nature and therefore these molecules are referred to as xeno-nucleic acids (XNAs). In this review we aim to focus specifically on recent progress in a subsection of this vast field-synthetic genetics-concerned with encoded synthesis, reverse transcription, and evolution of XNAs.

Triggering Copper(II)-Mediated Base Pair Formation by Light

Metal-mediated base pairs enable a site-specific incorporation of transition metal ions into nucleic acid structures. The resulting nucleic acid-metal complex conjugates are of interest in the context of functionalized nucleic acids, as they bear metal-based functionality. It is desirable to devise nucleic acids with an externally triggered metal-binding affinity, as this may allow regulating this functionality. Toward this end, a caged deoxyribonucleoside analog H^NPP was devised for the site-specific binding of copper(II) ions upon irradiation by light, based on the ligand 3-hydroxy-2-methylpyridin-4(1H)-one (H) and the photocleavable 2-(2-nitrophenyl)propoxy protecting group (NPP). The formation of both H-Cu(II)-H homo base pairs and H-Cu(II)-X hetero base pairs (involving a second artificial deoxyribonucleoside X, based on imidazole-4-carboxylate) was achieved upon irradiation of DNA duplexes bearing the respective H^NPP:H^NPP or H^NPP:X mispairs in the presence of copper(II) ions. The H-Cu(II)-X pair shows an exceptional DNA duplex stabilization of up to 43 °C upon its formation, exceeding that of the H-Cu(II)-H pair. It therefore represents one of the most stabilizing Cu(II)-mediated base pairs reported so far. Our findings expand the scope of light-triggered metal-mediated base pair formation by introducing a copper(II)-binding ligand.

Mass Spectrometry of Nucleic Acid Noncovalent Complexes

Nucleic acids have been among the first targets for antitumor drugs and antibiotics. With the unveiling of new biological roles in regulation of gene expression, specific DNA and RNA structures have become very attractive targets, especially when the corresponding proteins are undruggable. Biophysical assays to assess target structure as well as ligand binding stoichiometry, affinity, specificity, and binding modes are part of the drug development process. Mass spectrometry offers unique advantages as a biophysical method owing to its ability to distinguish each stoichiometry present in a mixture. In addition, advanced mass spectrometry approaches (reactive probing, fragmentation techniques, ion mobility spectrometry, ion spectroscopy) provide more detailed information on the complexes. Here, we review the fundamentals of mass spectrometry and all its particularities when studying noncovalent nucleic acid structures, and then review what has been learned thanks to mass spectrometry on nucleic acid structures, self-assemblies (e.g., duplexes or G-quadruplexes), and their complexes with ligands.

Chemical reduction of Ag+ to Ag employing organic electron donors: evaluation of the effect of Ag+-mediated cytosine-cytosine base pairing on the aggregation of Ag nanoparticles

Ag⁺-mediated base pairing is valuable for synthesising DNA-based silver nanoparticles (AgNPs) and nanoclusters (AgNCs). Recently, we reported the formation of a [Ag(cytidine)₂]⁺ complex in dimethyl sulfoxide (DMSO), which facilitated the evaluation of the effect of cytosine-Ag⁺-cytosine (C-Ag⁺-C) base pairing on the degree of AgNP aggregation in solution. As an aprotic solvent, DMSO was expected to dissolve the [Ag(cytidine)₂]⁺ complex, and powerful reducing agents, such as organic electron donors. In this study, the chemical reduction of a cytidine/Ag⁺ system using a powerful reducing agent tetrakis(dimethylamino)ethylene (TDAE) was investigated. ¹H/¹³C/¹⁵N NMR spectroscopic evidence was obtained to identify the iminium dication (TDAE²⁺), which is an oxidised form of TDAE. The results were compared with those obtained using another organic electron donor, tetrathiafulvalene (TTF), which exhibits a relatively lower reduction activity than TDAE. AgNPs prepared via redox reaction between [Ag(cytidine)₂]⁺ and organic electron donors (TDAE and TTF) were characterised using UV-Vis spectroscopy and nanoparticle tracking analysis. It was found that the formation of C-Ag⁺-C base pairing inhibited the aggregation of AgNPs in solution. In addition, in the presence of cytidine, the total concentration of the AgNP solution was affected by the reduction activity of the reducing agent.

DFT Functionals for Modeling of Polyethylene Chains Cross-Linked by Metal Atoms. DLPNO-CCSD(T) Benchmark Calculations

Density functional theory (DFT) functionals for calculations of binding energies (BEs) of the polyethylene (PE) chains cross-linked by selected metal atoms (M) are benchmarked against DLPNO-CCSD(T) and DLPNO-CCSD(T1) data. PEX-M-PEX complexes as compared with plain parallel PEX···PEX chains with X = 3-9 carbon atoms are model species characterized by a cooperative effect of covalent C-M-C bonds and interchain dispersion interactions. The accuracy of DLPNO-CC methods was assessed by a comparison of BEs with the canonical CCSD(T) results for small PE3-M-PE3 complexes. Functionals for PEX···PEX and closed-shell PEX-M-PEX complexes (M = Be, Mg, Zn) were benchmarked against DLPNO-CCSD(T) BEs; open-shell complexes (M = Li, Ag, Au) were benchmarked against the DLPNO-CCSD(T1) method with iterative triples. Three dispersion corrections were combined with 25 DFT functionals for calculations of BEs with respect to PEX-M and PEX fragments employing def2-TZVPP and def2-QZVPP basis sets. Accuracy to within 5% for the closed-shell PEX-M-PEX complexes was achieved with five functionals. Less accurate are functionals for the open-shell PEX-M-PEX complexes; only two functionals deviate by less than 15% from DLPNO-CCSD(T1). Particularly problematic were PEX-Li-PEX complexes. A reasonable overall performance across all complexes in terms of the mean absolute percentage error is found for the range-separated hybrid functionals ωB97X-D3 and CAM-B3LYP/D3(BJ)-ABC.

Loading Linear Arrays of CuII Inside Aromatic Amide Helices

The very stable helices of 8-amino-2-quinolinecarboxylic acid oligoamides are shown to uptake CuII ions in their cavity through deprotonation of their amide functions with minimal alteration of their shape, unlike most metallo-organic structures which generally differ from their organic precursors. The outcome is the formation of intramolecular linear arrays of a defined number of CuII centers (up to sixteen in this study) at a 3 Å distance, forming a molecular mimic of a metal wire completely surrounded by an organic sheath. The helices pack in the solid state so that the arrays of CuII extend intermolecularly. Conductive-AFM and cyclic voltammetry suggest that electrons are transported throughout the metal-loaded helices in contrast with hole transport observed for analogous foldamers devoid of metal ions.

Controllable DNA strand displacement by independent metal-ligand complexation

Introduction of artificial metal-ligand base pairs can enrich the structural diversity and functional controllability of nucleic acids. In this work, we revealed a novel approach by placing a ligand-type nucleoside as an independent toehold to control DNA strand-displacement reactions based on metal-ligand complexation. This metal-mediated artificial base pair could initiate strand invasion similar to the natural toehold DNA, but exhibited flexible controllability to manipulate the dynamics of strand displacement that was only governed by its intrinsic coordination properties. External factors that influence the intrinsic properties of metal-ligand complexation, including metal species, metal concentrations and pH conditions, could be utilized to regulate the strand dynamics. Reversible control of DNA strand-displacement reactions was also achieved through combination of the metal-mediated artificial base pair with the conventional toehold-mediated strand exchange by cyclical treatments of the metal ion and the chelating reagent. Unlike previous studies of embedded metal-mediated base pairs within natural base pairs, this metal-ligand complexation is not integrated into the nucleic acid structure, but functions as an independent toehold to regulate strand displacement, which would open a new door for the development of versatile dynamic DNA nanotechnologies.

Effect of cytosine-Ag+-cytosine base pairing on the redox potential of the Ag+/Ag couple and the chemical reduction of Ag+ to Ag by tetrathiafulvalene

The redox properties of metallo-base pairs remain to be elucidated. Herein, we report the detailed 1H/13C/109Ag NMR spectroscopic and cyclic voltammetric characterisation of the [Ag(cytidine)2]+ complex as isolated cytosine-Ag+-cytosine (C-Ag+-C) base pairs. We also performed comparative studies between cytidine/Ag+ and other nucleoside/Ag+ systems by using cyclic voltammetry measurements. In addition, to evaluate the effect of [Ag(cytidine)2]+ formation on the chemical reduction of Ag+ to Ag, we utilised the redox reaction between Ag+ and tetrathiafulvalene (TTF). We found that Ag+-mediated base pairing lowers the redox potential of the Ag+/Ag couple. In addition, C-Ag+-C base pairing makes it more difficult to reduce captured Ag+ ions than in other nucleoside/Ag+ systems. Remarkably, the cytidine/Ag+ system can be utilised to control the redox potential of the Ag+/Ag couple in DMSO. This feature of the cytidine/Ag+ system may be exploited for Ag nanoparticle synthesis by using the redox reaction between Ag+ and TTF.

Ultrafast excited state dynamics of silver ion-mediated cytosine-cytosine base pairs in metallo-DNA

To better understand the nexus between structure and photophysics in metallo-DNA assemblies, the parallel-stranded duplex formed by the all-cytosine oligonucleotide, dC₂₀, and silver nitrate was studied by circular dichroism (CD), femtosecond transient absorption spectroscopy, and time-dependent-density functional theory calculations. Silver(I) ions mediate Cytosine-Cytosine (CC) base pairs by coordinating to the N3 atoms of two cytosines. Although these silver(I) mediated CC base pairs resemble the proton-mediated CC base pairs found in i-motif DNA at first glance, a comparison of experimental and calculated CD spectra reveals that silver ion-mediated i-motif structures do not form. Instead, the parallel-stranded duplex formed between dC₂₀ and silver ions is proposed to contain consecutive silver-mediated base pairs with high propeller twist-like ones seen in a recent crystal structure of an emissive, DNA-templated silver cluster. Femtosecond transient absorption measurements with broadband probing from the near UV to the near IR reveal an unusually long-lived (>10 ns) excited state in the dC₂₀ silver ion complex that is not seen in dC₂₀ in single-stranded or i-motif forms. This state is also absent in a concentrated solution of cytosine-silver ion complexes that are thought to assemble into planar ribbons or sheets that lack stacked silver(I) mediated CC base pairs. The large propeller twist angle present in metal-mediated base pairs may promote the formation of long-lived charged separated or triplet states in this metallo-DNA.

Enzymatic construction of metal-mediated nucleic acid base pairs

Artificial metal base pairs have become increasingly important in nucleic acids chemistry due to their high thermal stability, water solubility, orthogonality to natural base pairs, and low cost of production. These interesting properties combined with ease of chemical and enzymatic synthesis have prompted their use in several practical applications, including the construction of nanomolecular devices, ions sensors, and metal nanowires. Chemical synthesis of metal base pairs is highly efficient and enables the rapid screening of novel metal base pair candidates. However, chemical synthesis is limited to rather short oligonucleotides and requires rather important synthetic efforts. Herein, we discuss recent progress made for the enzymatic construction of metal base pairs that can alleviate some of these limitations. First, we highlight the possibility of generating metal base pairs using canonical nucleotides and then describe how modified nucleotides can be used in this context. We also provide a description of the main analytical techniques used for the analysis of the nature and the formation of metal base pairs together with relevant examples of their applications.

Differentiating a Least-Stable Single Nucleotide Mismatch in DNA Via Metal Ion-Mediated Base Pairing and Using Thioflavin T as an Extrinsic Fluorophore

Monitoring the DNA dynamics in solution has great potential to develop new nucleic acid-based sensors and devices. With spectroscopic approaches, both at the ensemble average and single-molecule resolution, this study is directed to differentiate a single nucleotide mismatch (SNM) via a metal ion-stabilized mismatched base-pairing (C-Ag⁺-C/C-Cu²⁺-T) (C = cytosine, T = thymine) and site-selective extrinsic fluorophore, specifically, Thioflavin T (ThT). This is the first approach of its kind where dynamic quantities like molecular diffusion coefficients and diffusion times have been utilized to distinguish the least-stable SNM (CC & CT) formed by the most discriminating nucleobase, specifically, cytosine in a 20-mer duplex DNA. Additionally, this work also quantifies metal ions (Ag⁺ and Cu²⁺) at lower concentrations using fluorescence correlation spectroscopy. Our results can provide greater molecular-level insights into the mismatch-dependent metal-DNA interactions and also illuminate ThT as a new fluorophore to monitor the dynamics involved in DNA-metal composites.

Structure and luminescence of DNA-templated silver clusters

DNA serves as a versatile template for few-atom silver clusters and their organized self-assembly. These clusters possess unique structural and photophysical properties that are programmed into the DNA template sequence, resulting in a rich palette of fluorophores which hold promise as chemical and biomolecular sensors, biolabels, and nanophotonic elements. Here, we review recent advances in the fundamental understanding of DNA-templated silver clusters (Ag N -DNAs), including the role played by the silver-mediated DNA complexes which are synthetic precursors to Ag N -DNAs, structure-property relations of Ag N -DNAs, and the excited state dynamics leading to fluorescence in these clusters. We also summarize the current understanding of how DNA sequence selects the properties of Ag N -DNAs and how sequence can be harnessed for informed design and for ordered multi-cluster assembly. To catalyze future research, we end with a discussion of several opportunities and challenges, both fundamental and applied, for the Ag N -DNA research community. A comprehensive fundamental understanding of this class of metal cluster fluorophores can provide the basis for rational design and for advancement of their applications in fluorescence-based sensing, biosciences, nanophotonics, and catalysis.

Crystallographic Structure of Novel Types of AgI -Mediated Base Pairs in Non-canonical DNA Duplex Containing 2'-O,4'-C-Methylene Bridged Nucleic Acids

Metal-mediated base pairs have widespread applications, such as in DNA-metal nanodevices and sensors. Here, we focused on their sugar conformation in duplexes and observed the crystallographic structure of the non-canonical DNA/DNA duplex containing 2'-O,4'-C-methylene bridged nucleic acid in the presence of Ag^I ions. The X-ray crystallographic structure was successfully obtained at a resolution of 1.5 Å. A novel type of Ag^I -mediated base pair between the N1 positions of anti-conformation of adenines in the duplex was observed. In the central non-canonical region, a hexad nucleobase structure containing Ag^I -mediated base pairs between the N7 positions of guanines was formed. A highly bent non-canonical structure was formed at the origin of Ag^I -mediated base pairs in the central region. The bent duplex structure induced by the addition of Ag^I ions might become a powerful tool for dynamic structural changes in DNA nanotechnology applications.

Construction of hydrogen bonding and coordination networks based on ethynylpyridine-appended nucleobases

A series of ethynylpyridine-appended nucleobases have been designed, synthesized, characterized and employed for the formation of crystalline molecular networks by hydrogen/coordination bonding.

Recent progress in non-native nucleic acid modifications

While Nature harnesses RNA and DNA to store, read and write genetic information, the inherent programmability, synthetic accessibility and wide functionality of these nucleic acids make them attractive tools for use in a vast array of applications.