Unprecedented dinuclear silver(I)-mediated base pair involving the DNA lesion 1,N(6)-ethenoadenine

Specific binding of Hg2+ to mismatched base pairs involving 5-hydroxyuracil in duplex DNA

Metal ion-nucleic acid interactions contribute significantly to nucleic acid structure and biological activity and have potential applications in nanotechnology. Hg²⁺ specifically binds to the natural T-T mismatched base pair in duplex DNA to form a T-Hg-T base pair. Metal ions may enhance DNA damage induced by DNA-damaging agents, such as oxidative agents. The interactions between metal ions and damaged DNAs, such as mismatched oxidized bases, have not been well characterized. Here, we examined the possibility of Hg²⁺ binding to an asymmetric mismatched base pair involving thymine and 5-hydroxyuracil (OHdU), an oxidized base produced by the oxidative deamination of cytosine. UV melting analyses showed that only the melting temperature of the single T-OHdU mismatched duplex DNA increased upon Hg²⁺ addition. CD spectra indicated no significant change in the higher-order structure of the single T-OHdU mismatched duplex DNA upon Hg²⁺ addition. X-ray crystallographic structure with two consecutive T-OHdU mismatched base pairs and isothermal titration calorimetric analyses with the single T-OHdU mismatched base pair showed that Hg²⁺ specifically binds to the N3 positions of both T and OHdU in T-OHdU at 1:1 molar ratio, with a 5×10⁵ M^-1 binding constant of to form the T-Hg-OHdU base pair. The Hg²⁺-bound structure and the Hg²⁺-binding affinity for T-OHdU was similar to those for T-T. This study on T-Hg-OHdU metal-mediated base pair could aid in studying the molecular mechanism of metal ion-mediated DNA damage and their potential applications in nanotechnology.

CuII-mediated stabilisation of DNA duplexes bearing consecutive ethenoadenine lesions and its application to a metal-responsive DNAzyme

Metal-mediated nucleobase pairing can play a central role in the expression of metal-responsive DNA functions. We report the Cu^II-mediated stabilisation of DNA duplexes bearing damaged nucleobases, 1,N⁶-ethenoadenine (εA), as metal-binding sites, which was utilised to construct a metal-responsive DNAzyme. Consecutive incorporation of three or more εA-εA mismatch pairs allowed for Cu^II-dependent significant duplex stabilisation through metal-mediated εA-Cu^II-εA base pairing. Subsequently, a split DNAzyme with three εA-Cu^II-εA base pairs was strategically designed. The activity of the εA-modified DNAzyme was enhanced by 5.3-fold upon addition of Cu^II ions. This study demonstrates the utility of εA lesions for building metal-responsive DNA architectures.

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.

Modulating aptamer function by copper(II)-mediated base pair formation

Two aptamers, one for ATP and one for arginine, were modified using an artificial 2'-dexoyribonucleoside based on the nucleobase surrogate imidazole-4-carboxylate. This synthetic nucleoside substitute does not engage in hydrogen bonding but is capable of forming Cu(II)-mediated base pairs instead. Hence, the addition of Cu(II) can be used to influence the ability of the aptamer derivatives to adopt the correct fold necessary for binding their respective target molecule. As a result, aptamer function can be modulated via the addition of Cu(II). The extent of modulation ability depends on the identity of the aptamer and on the exact location of the artificial nucleosides within the oligonucleotide sequence.

Organomercury Nucleic Acids: Past, Present and Future

Synthetic efforts towards nucleosides, nucleotides, oligonucleotides and nucleic acids covalently mercurated at one or more of their base moieties are summarized, followed by a discussion of the proposed, realized and abandoned applications of this unique class of compounds. Special emphasis is given to fields in which active research is ongoing, notably the use of HgII -mediated base pairing to improve the hybridization properties of oligonucleotide probes. Finally, this minireview attempts to anticipate potential future applications of organomercury nucleic acids.

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.

Silver(I)-mediated base pairing in DNA involving the artificial nucleobase 7,8-dihydro-8-oxo-1,N6-ethenoadenine

The artificial nucleobase 7,8-dihydro-8-oxo-1,N⁶-ethenoadenine (X) was investigated with respect to its ability to engage in Ag(I)-mediated base pairing in DNA. Spectroscopic data indicate the formation of dinuclear X-Ag(I)₂-X homo base pairs and mononuclear X-Ag(I)-C base pairs (C, cytosine). Density functional theory calculations and molecular dynamics simulations indicate that the nucleobase changes from its lactam tautomeric form prior to the formation of the Ag(I)-mediated base pair to the lactim form after the incorporation of the Ag(I) ions. Fluorescence spectroscopy indicates that the two Ag(I) ions of the homo base pair are incorporated sequentially. Isothermal titration calorimetry confirms that the affinity of one of the Ag(I) ions is about tenfold higher than that of the other Ag(I) ion. The computational analysis by means of density functional theory confirms a much larger reaction energy for the incorporation of the first Ag(I) ion. The thermal stabilization upon the formation of the dinuclear Ag(I)-mediated homo base pair exceeds the one previously observed for the closely related nucleobase 1,N⁶-ethenoadenine by far, despite very similar structures. This additional stabilization may stem from the presence of water molecules engaged in hydrogen bonding with the additional oxygen atom of the artificial nucleobase X. The highly stabilizing Ag(I)-mediated base pair is a valuable addition to established dinuclear metal-mediated base pairs.

Silver(I) Coordination in Silver(I)-Mediated Homo Base Pairs of 6-Pyrazolylpurine in DNA Duplexes Involves the Watson-Crick Edge

DNA duplexes comprising 6‐(1H‐pyrazol‐1‐yl)‐9H‐purine (6PP), 1‐deaza‐6PP (^1D6PP), 7‐deaza‐6PP (^7D6PP) and 1,7‐dideaza‐6PP (^1,7D6PP) 2′‐deoxyribonucleosides, respectively, were investigated towards their ability to form metal‐mediated base pairs in the presence of Ag^I. In 6PP and ^7D6PP, the Ag^I ion can coordinate to the nucleobase via the endocyclic N1 nitrogen atom, that is, via the Watson–Crick edge. In contrast, this nitrogen atom is not available in ^1D6PP and ^1,7D6PP, so that in ^1D6PP an Ag^I coordination is only possible via the Hoogsteen edge (N7). Reference duplexes with either adenine:adenine mispairs or canonical adenine:thymine base pairs were used to investigate the impact of the pyrazolyl moiety on the Ag^I‐binding properties. To determine the thermal and structural duplex stabilities in the absence or presence of Ag^I, all duplexes were examined by UV and circular dichroism spectroscopic studies. These investigations shed light on the question of whether N1‐ or N7‐coordination is preferred in purine‐based metal‐mediated base pairs.

Dynamic Structure and Stability of DNA Duplexes Bearing a Dinuclear Hg(II)-Mediated Base Pair

Quantum mechanical (QM) and hybrid quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations of a recently reported dinuclear mercury(II)-mediated base pair were performed aiming to analyse its intramolecular bonding pattern, its stability, and to obtain clues on the mechanism of the incorporation of mercury(II) into the DNA. The dynamic distance constraint was employed to find initial structures, control the dissociation process in an unbiased fashion and to determine the free energy required. A strong influence of the exocyclic carbonyl or amino groups of neighbouring base pairs on both the bonding pattern and the mechanism of incorporation was observed. During the dissociation simulation, an amino group of an adenine moiety of the adjacent base pair acts as a turnstile to rotate the mercury(II) ion out of the DNA core region. The calculations provide an important insight into the mechanism of formation of this dinuclear metal-mediated base pair and indicate that the exact location of a transition metal ion in a metal-mediated base pair may be more ambiguous than derived from simple model building.

Nucleobase Pair-Metal Dimer/Dinuclear Metal Cation Interaction: A Theoretical Study

Nucleobase pair-metal dimer/dinuclear metal cation interactions play an important role in biological applications because of their highly symmetrical structures and high stabilities. In this work, we have selected five adenine-adenine hydrogen bonding, adenine-thymine (AT), adenine-uracil, adenine-adenine stacking pairs, and Watson-Crick AT stacking pairs and studied their interaction with the coinage metal dimer M2 and M2 2+ metal cations, where M = Ag, Au, and Cu. Quantum chemical calculations have been carried out with density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Electronic structures were analyzed by the partial density of states method. During interactions, we find that M-M distances are shorter than the sum of van der Waals radii of the corresponding two homocoinage metal atoms, which show the existence of significant metallophilic interactions. Results indicated that nucleobase-M2 2+ complexes are stronger as compared to nucleobase-M2 complexes. Also, the replacement of the hydrogen bond by the dinuclear metal cation-coordinated bond forms more stable alternative metallo-DNA sequences in AAST base pairs. TDDFT calculations reveal that nucleobase-Cu2 complexes and nucleobase-Ag2 2+/Au2 2+ complexes can be used for fluorescent markers and logic gate applications. Atom-in-molecules analysis predicted the noncovalent interaction in these complexes.

1,8-Dimercuri-6-Phenyl-1H-Carbazole as a Monofacial Dinuclear Organometallic Nucleobase

A C-nucleoside with 6-phenyl-1H-carbazole as the base moiety has been synthesized and incorporated in the middle of an oligonucleotide. Mercuration of this modified residue at positions 1 and 8 gave the first example of an oligonucleotide featuring a monofacial dinuclear organometallic nucleobase. The dimercurated oligonucleotide formed stable duplexes with unmodified oligonucleotides placing either cytosine, guanine, or thymine opposite to the organometallic nucleobase. A highly stabilizing (ΔT_m =7.3 °C) Hg^II -mediated base pair was formed with thymine. According to DFT calculations performed at the PBE0DH level of theory, this base pair is most likely dinuclear, with the two Hg^II ions coordinated to O2 and O4 of the thymine base.

Light-Induced Formation of Thymine-Containing Mercury(II)-Mediated Base Pairs

By applying caged thymidine residues, DNA duplexes were created in which HgII -mediated base pair formation can be triggered by irradiation with light. When a bidentate ligand was used as the complementary nucleobase, an unprecedented stepwise formation of different metal-mediated base pairs was achieved.

Metal-Modified Nucleic Acids: Metal-Mediated Base Pairs, Triples, and Tetrads

The incorporation of metal ions into nucleic acids by means of metal-mediated base pairs represents a promising and prominent strategy for the site-specific decoration of these self-assembling supramolecules with metal-based functionality. Over the past 20 years, numerous nucleoside surrogates have been introduced in this respect, broadening the metal scope by providing perfectly tailored metal-binding sites. More recently, artificial nucleosides derived from natural purine or pyrimidine bases have moved into the focus of Ag^I -mediated base pairing, due to their expected compatibility with regular Watson-Crick base pairs. This minireview summarizes these advances in metal-mediated base pairing but also includes further recent progress in the field. Moreover, it addresses other aspects of metal-modified nucleic acids, highlighting an expansion of the concept to metal-mediated base triples (in triple helices and three-way junctions) and metal-mediated base tetrads (in quadruplexes). For all types of metal-modified nucleic acids, proposed or accomplished applications are briefly mentioned, too.

Binding of metal ions and water molecules to nucleic acid bases: the influence of water molecule coordination to a metal ion on water-nucleic acid base hydrogen bonds

The interactions of nucleic acid bases with non-coordinated and coordinated water molecules were studied by analyzing data in the Protein Data Bank (PDB) and by quantum chemical calculations. The analysis of the data in the crystal structures from the PDB indicates that hydrogen bonds involving oxygen or nitrogen atoms of nucleic acid bases and water molecules are shorter when water is bonded to a metal ion. These results are in agreement with the quantum chemical calculations on geometries and interaction energies of hydrogen bonds; the calculations on model systems show that hydrogen bonds of nucleic acid bases with water bonded to a metal ion are stronger than hydrogen bonds with non-coordinated water. These calculated values are similar to the strength of hydrogen bonds between nucleic acid bases. The results presented in this paper may be relevant to understand the role of water molecules and metal ions in the process of replication and stabilization of nucleic acids and also to understand the possible toxicity of metal ion interactions with nucleic acids.

1,3,9-Triaza-2-oxophenoxazine: An Artificial Nucleobase Forming Highly Stable Self-Base Pairs with Three AgI Ions in a Duplex

Metal-mediated base pairs (MMBPs) formed by natural or artificial nucleobases have recently been developed. The metal ions can be aligned linearly in a duplex by MMBP formation. The development of a three- or more-metal-coordinated MMBPs has the potential to improve the conductivity and enable the design of metal ion architectures in a duplex. This study aimed to develop artificial self-bases coordinated by three linearly aligned Ag^I ions within an MMBP. Thus, artificial nucleic acids with a 1,3,9-triaza-2-oxophenoxazine (9-TAP) nucleobase were designed and synthesized. In a DNA/DNA duplex, self-base pairs of 9-TAP could form highly stable MMBPs with three Ag^I ions. Nine equivalents of Ag^I led to the formation of three consecutive 9-TAP self-base pairs with extremely high stability. The complex structures of 9-TAP MMBPs were determined by using electrospray ionization mass spectrometry and UV titration experiments. Highly stable self-9-TAP MMBPs with three Ag^I ions are expected to be applicable to new DNA nanotechnologies.

Copper(ii)-mediated base pairing involving the artificial nucleobase 3H-imidazo[4,5-f]quinolin-5-ol

A highly stabilizing Cu(ii)-mediated base pair is introduced into DNA using a large artificial nucleobase.

2,6-Dimercuriphenol as a Bifacial Dinuclear Organometallic Nucleobase

A C-nucleoside having 2,6-dimercuriphenol as the base moiety has been synthesized and incorporated into an oligonucleotide. NMR and UV melting experiments revealed the ability of this bifacial organometallic nucleobase surrogate to form stable dinuclear Hg^II -mediated base triples with adenine, cytosine, and thymine (or uracil) in solution as well as within a triple-helical oligonucleotide. A single Hg^II -mediated base triple between 2,6-dimercuriphenol and two thymines increased both Hoogsteen and Watson-Crick melting temperatures of a 15-mer pyrimidine⋅purine*pyrimidine triple helix by more than 10 °C relative to an unmodified triple helix of the same length. This novel binding mode could be exploited in targeting certain pathogenic nucleic acids as well as in DNA nanotechnology.

A dinuclear Cu(i)-mediated complex: Theoretical studies of the G2Cu2 4+ cluster ion

Recently, the T-Hg(ii)₂-A base pair containing two equivalents of Hg(ii) has been prepared and characterized experimentally, which implies that there might exist considerable stable metal-mediated base pairs holding two neighbouring metal centers. Here we report a quantum chemical study on geometries, electronic structures, and bonding of various G₂Cu₂ ⁴⁺ (G = guanine) isomers including one di-copper(i) unit. Different density functional methods [Becke 3-parameter-Lee-Yang-Parr, Perdew-Becke-Ernzerhof, Becke-Perdew, Density Functional Theory with Dispersion Corrections (DFT-D)] assign ambiguous relative energies to these isomers with the singlet and triplet states. High-level ab initio [domain-based local pair natural orbital (DLPNO) coupled-cluster with single and double excitations and DLPNO-coupled-cluster with single, double, and perturbative triple excitations] calculations confirm that the lowest-lying isomer of the G₂Cu₂ ⁴⁺ ion has C _2h symmetry with the singlet state and is comparable to the singly and doubly charged homologues (G₂Cu₂ ⁺ and G₂Cu₂ ²⁺). The extended transition state (ETS)-natural orbitals for the chemical valence (ETS-NOCV) calculations point out that it has larger instantaneous interaction energy and bond dissociation energy than the corresponding singly and doubly charged complexes due to its relatively stronger attractive energies and weaker Pauli repulsion. The orbital interactions in the quadruply charged cluster chiefly come from Cu₂ ⁴⁺ ← G⋯G π donations. The results may help the understanding of the bonding properties of other potential metal-base pair complexes with the electron transfer.

Metal-mediated base pairs in parallel-stranded DNA

In nucleic acid chemistry, metal-mediated base pairs represent a versatile method for the site-specific introduction of metal-based functionality. In metal-mediated base pairs, the hydrogen bonds between complementary nucleobases are replaced by coordinate bonds to one or two transition metal ions located in the helical core. In recent years, the concept of metal-mediated base pairing has found a significant extension by applying it to parallel-stranded DNA duplexes. The antiparallel-stranded orientation of the complementary strands as found in natural B-DNA double helices enforces a cisoid orientation of the glycosidic bonds. To enable the formation of metal-mediated base pairs preferring a transoid orientation of the glycosidic bonds, parallel-stranded duplexes have been investigated. In many cases, such as the well-established cytosine-Ag(I)-cytosine base pair, metal complex formation is more stabilizing in parallel-stranded DNA than in antiparallel-stranded DNA. This review presents an overview of all metal-mediated base pairs reported as yet in parallel-stranded DNA, compares them with their counterparts in regular DNA (where available), and explains the experimental conditions used to stabilize the respective parallel-stranded duplexes.

Dinuclear Metal-Mediated Homo Base Pairs with Metallophilic Interactions: Theoretical Studies of G2M22+ (M = Cu, Ag, and Au) Ions

Dinuclear metal-mediated homo base pairs are interesting clusters with highly symmetric structures and significant stabilities. The geometric and electronic structures of G₂M₂²⁺ (G = Guanine, M = Cu, Ag or Au) cluster ions were studied with quantum chemical calculations. The lowest-energy isomers of G₂M₂²⁺ cluster ions have C_2h symmetries with an approximately antiparallel alignment of two sets of N-M∙∙∙O groups being formed in the planar structures. The M-M distances are shorter than the sum of van der Waals radii of corresponding two homo coinage metal atoms, showing that metallophilic interactions significantly exist in these complexes. They have the large HOMO−LUMO gaps of about 5.80 eV at the DFT level and the bond dissociation energies of more than 5.60 eV at the DFT/B3LYP level, indicating that these cluster dications are highly stable. The second lowest-energy isomers stabilized by an approximately parallel alignment of one set of O-M-O group and one set of N-M-N group are found to be close to the lowest-energy isomers in energy. The barrier between the two isomers of G₂M₂²⁺ cluster ions is significantly large, also showing that these lowest-energy isomers are very stable.

Silver-Mediated Double Helix: Structural Parameters for a Robust DNA Building Block

The DNA double helix is a versatile building block used in DNA nanotechnology. To potentiate the discovery of new DNA nanoscale assemblies, recently, silver cations have been introduced to pair DNA strands by base-Ag⁺-base bonding rather than by Watson-Crick pairing. In this work, we study the classical dynamics of a parallel silver-mediated homobase double helix and compare it to the dynamics of the antiparallel double helix. Our classical simulations show that only the parallel double helix is highly stable through the 100 ns simulation time. A new type of H-bond previously proposed by our collaboration and recently observed in crystal-determined helices drives the physicochemical stabilization. Compared to the natural B-DNA form, the metal-mediated helix has a contracted axial base pair rise and smaller numbers of base pairs per turn. These results open the path for the inclusion of this robust metal-mediated building block into new nanoscale DNA assemblies.

Metal-Mediated Base Pairs: From Characterization to Application

The investigation of metal-mediated base pairs and the development of their applications represent a prominent area of research at the border of bioinorganic chemistry and supramolecular coordination chemistry. In metal-mediated base pairs, the complementary nucleobases in a nucleic acid duplex are connected by coordinate bonds to an embedded metal ion rather than by hydrogen bonds. Because metal-mediated base pairs facilitate a site-specific introduction of metal-based functionality into nucleic acids, they are ideally suited for use in DNA nanotechnology. This minireview gives an overview of the general requirements that need to be considered when devising a new metal-mediated base pair, both from a conceptual and from an experimental point of view. In addition, it presents selected recent applications of metal-modified nucleic acids to indicate the scope of metal-mediated base pairing.

Directed adenine functionalization for creating complex architectures for material and biological applications

In this feature article, targeted design strategies are outlined for modified adenine nucleobase derivatives in order to construct metal-mediated discrete complexes, ring-expanded purine skeletons, linear and catenated coordination polymers, shape-selective MOFs, and purine-capped nanoparticles, with a wide range of applications from gas and solvent adsorption to bioimaging agents and anticancer metallodrugs. The success of such design strategies could be ascribed to the rich chemistry of purine and pyrimidine derivatives, versatile coordination behavior, ability to bind a host of metal ions, which could be further tuned by the introduction of additional functionalities, and their inherent propensity to hydrogen bond and exhibit π-π interactions. These noncovalent interactions produce stable frameworks and network solids that are useful as advanced materials, and the biocompatibility of these ligand complexes provides an impetus for assessing novel biological applications.

Metal-mediated DNA assembly with ligand-based nucleosides

Nucleic acids such as DNA are increasingly being applied in nanotechnology, as a result of their capability to self-assemble reversibly. The formal replacement of canonical base pairs by metal-mediated ones enables a site-specific introduction of metal-based functionality into these biomolecules, leading to the formation of predesigned metal arrays. This article offers an overview of structural aspects of metal-mediated base pairs, reviews recent advances in the field of metal-mediated base pairing and presents potential applications of the resulting metal-modified nucleic acids. It particularly focuses on recently developed metal-mediated base pairs with purine-derived nucleosides, gives an overview of metal-responsive systems relying on metal-mediated base pairs and summarizes various applications beyond metal-ion sensors.

Crystal structure of a DNA duplex containing four Ag(i) ions in consecutive dinuclear Ag(i)-mediated base pairs: 4-thiothymine–2Ag(i)–4-thiothymine

The crystal structures of 4-thiothymine–2Ag^I–4-thiothymine base pairs in B-form DNA duplexes have been solved.

Relative Strand Orientation in a DNA Duplex Controls the Nuclearity of a Metal-Mediated Base Pair

1,N⁶ -Ethenoadenine (ϵA) and cytosine (C) are able to form two different metal-mediated base pairs. When the glycosidic bonds are arranged in a cisoid manner (i.e., in antiparallel-stranded DNA), the ϵA:C mispair binds one Ag^I ion, leading to a mononuclear ϵA-Ag^I -C base pair that contains a synergistic hydrogen bond. In contrast, a transoid orientation of the glycosidic bonds (as found in parallel-stranded DNA) results in the formation of a dinuclear metal-mediated base pair ϵA-Ag^I₂ -C.

A Dinuclear Mercury(II)-Mediated Base Pair in DNA

The first dinuclear metal-mediated base pair containing divalent metal ions has been prepared. A combination of the neutral bis(monodentate) purine derivative 1,N⁶ -ethenoadenine (ϵA), which preferentially binds two metal ions with a parallel alignment of the N-M bonds, and the canonical nucleobase thymine (T), which readily deprotonates in the presence of Hg^II and thereby partially compensates the charge accumulation due to the two closely spaced divalent metal ions, yields the dinuclear T-Hg^II₂ -ϵA base pair. This metal-mediated base pair stabilizes the DNA oligonucleotide duplex as shown by an increase of 8 °C in its melting temperature. Formation of the base pair was demonstrated by temperature-dependent UV spectroscopy as well as by titration experiments monitored by UV and CD spectroscopy.

6-Pyrazolylpurine as an Artificial Nucleobase for Metal-Mediated Base Pairing in DNA Duplexes

The artificial nucleobase 6-pyrazol-1-yl-purine (6PP) has been investigated with respect to its usability in metal-mediated base pairing. As was shown by temperature-dependent UV spectroscopy, 6PP may form weakly stabilizing 6PP–Ag(I)–6PP homo base pairs. Interestingly, 6PP can be used to selectively recognize a complementary pyrimidine nucleobase. The addition of Ag(I) to a DNA duplex comprising a central 6PP:C mispair (C = cytosine) leads to a slight destabilization of the duplex. In contrast, a stabilizing 6PP–Ag(I)–T base pair is formed with a complementary thymine (T) residue. It is interesting to note that 6PP is capable of differentiating between the pyrimidine moieties despite the fact that it is not as sterically crowded as 6-(3,5-dimethylpyrazol-1-yl)purine, an artificial nucleobase that had previously been suggested for the recognition of nucleic acid sequences via the formation of a metal-mediated base pair. Hence, the additional methyl groups of 6-(3,5-dimethylpyrazol-1-yl)purine may not be required for the specific recognition of the complementary nucleobase.

Formation of Silver Nanoclusters from a DNA Template Containing Ag(I)-Mediated Base Pairs

A series of DNA double helices containing different numbers of silver(I)-mediated base pairs involving the artificial nucleobases imidazole or 2-methylimidazole has been applied for the generation of DNA-templated silver nanoclusters. The original Ag(I)-containing nucleic acids as well as the resulting nanoclusters and nanoparticles have been characterized by means of UV/Vis spectroscopy, circular dichroism (CD) spectroscopy, fluorescence spectroscopy, and transmission electron microscopy (TEM). The results show for the first time that metal-mediated base pairs can be used for the templated growth of metal nanoclusters.

A comparative antimicrobial and toxicological study of gold(iii) and silver(i) complexes with aromatic nitrogen-containing heterocycles: synergistic activity and improved selectivity index of Au(iii)/Ag(i) complexes mixture

Combination therapy may be applied in the case of gold(iii) and silver(i) complexes with aromatic nitrogen-containing heterocycles to improve their antimicrobial activity and reduce toxic-side effects.

Self-pairing of 1-methylthymine mediated by two and three Ag(i) ions: a gas phase study using infrared dissociation spectroscopy and density functional theory

Metal base pairs comprised of silver dimer or trimer and two thymines were studied by IR dissociation spectroscopy and density functional theory.

Silver(I) complexes with phthalazine and quinazoline as effective agents against pathogenic Pseudomonas aeruginosa strains

Five silver(I) complexes with aromatic nitrogen-containing heterocycles, phthalazine (phtz) and quinazoline (qz), were synthesized, characterized and analyzed by single-crystal X-ray diffraction analysis. Although different AgX salts reacted with phtz, only dinuclear silver(I) complexes of the general formula {[Ag(X-O)(phtz-N)]2(μ-phtz-N,N')2} were formed, X=NO3(-) (1), CF3SO3(-) (2) and ClO4(-) (3). However, reactions of qz with an equimolar amount of AgCF3SO3 and AgBF4 resulted in the formation of polynuclear complexes, {[Ag(CF3SO3-O)(qz-N)]2}n (4) and {[Ag(qz-N)][BF4]}n (5). Complexes 1-5 were evaluated by in vitro antimicrobial studies against a panel of microbial strains that lead to many skin and soft tissue, respiratory, wound and nosocomial infections. The obtained results indicate that all tested silver(I) complexes have good antibacterial activity with MIC (minimum inhibitory concentration) values in the range from 2.9 to 48.0μM against the investigated strains. Among the investigated strains, these complexes were particularly efficient against pathogenic Pseudomonas aeruginosa (MIC=2.9-29μM) and had a marked ability to disrupt clinically relevant biofilms of strains with high inherent resistance to antibiotics. On the other hand, their activity against the fungus Candida albicans was moderate. In order to determine the therapeutic potential of silver(I) complexes 1-5, their antiproliferative effect on the human lung fibroblastic cell line MRC5, has been also evaluated. The binding of complexes 1-5 to the genomic DNA of P. aeruginosa was demonstrated by gel electrophoresis techniques and well supported by molecular docking into the DNA minor groove. All investigated complexes showed an improved cytotoxicity profile in comparison to the clinically used AgNO3.

Sequence-Dependent Duplex Stabilization upon Formation of a Metal-Mediated Base Pair

An artificial nucleoside surrogate with 1H-imidazo[4,5-f][1,10]phenanthroline (P) acting as an aglycone has been introduced into DNA oligonucleotide duplexes. This nucleoside surrogate can act as a bidentate ligand, and so is useful in the context of metal-mediated base pairs. Several duplexes involving a hetero base pair with an imidazole nucleoside have been investigated. The stability of DNA duplexes incorporating the respective Ag(I) -mediated base pairs strongly depends on the sequence context. Quantum mechanical/molecular mechanical (QM/MM) calculations have been performed in order to gain insight into the factors determining this sequence dependence. The results indicated that, in addition to the stabilizing effect that results from the formation of coordinative bonds, destabilizing effects may occur when the artificial base pair does not fit optimally into the surrounding B-DNA duplex.

Single-Molecule Force Spectroscopy of an Artificial DNA Duplex Comprising a Silver(I)-Mediated Base Pair

Single-molecule force spectroscopy measurements of a DNA duplex comprising an artificial metal-mediated base pair are reported. The measurements reveal that DNA duplexes comprising one central imidazole:imidazole mispair rupture at lower forces than a related duplex with canonical base pairs only. In contrast, DNA duplexes with one central imidazole-Ag(+)-imidazole base pair (formed by the addition of Ag(+) to the aforementioned duplex with the mispair) rupture at higher forces. These measurements indicate for the first time that the increase in thermal stability of a nucleic acid duplex that is observed upon the formation of a metal-mediated base pair is accompanied by a concomitant mechanical stabilization. In fact, the mechanical stabilization even exceeds the thermal one. This result indicates that nucleic acids with metal-mediated base pairs should be ideal building blocks for rigid functionalized DNA nano-objects.

The Role of Hydrogen Bonds in the Stabilization of Silver-Mediated Cytosine Tetramers

DNA oligomers can form silver-mediated duplexes, stable in gas phase and solution, with potential for novel biomedical and technological applications. The nucleobase-metal bond primarily drives duplex formation, but hydrogen (H-) bonds may also be important for structure selection and stability. To elucidate the role of H-bonding, we conducted theoretical and experimental studies of a duplex formed by silver-mediated cytosine homopobase DNA strands, two bases long. This silver-mediated cytosine tetramer is small enough to permit accurate, realistic modeling by DFT-based quantum mechanics/molecular mechanics methods. In gas phase, our calculations found two energetically favorable configurations distinguished by H-bonding, one with a novel interplane H-bond, and the other with planar H-bonding of silver-bridged bases. Adding solvent favored silver-mediated tetramers with interplane H-bonding. Overall agreement of electronic circular dichroism spectra for the final calculated structure and experiment validates these findings. Our results can guide use of these stabilization mechanisms for devising novel metal-mediated DNA structures.