Excess Electron Transfer through DNA Duplexes Comprising a Metal‐Mediated Base Pair

Organometallic modification confers oligonucleotides new functionalities

To improve their properties or to introduce entirely new functionalities, the intriguing scaffolds of nucleic acids have been decorated with various modifications, most recently also organometallic ones. While challenging to introduce, organometallic modifications offer the potential of expanding the field of application of metal-dependent functionalities to metal-deficient conditions, notably those of biological media. So far, organometallic moieties have been utilized as probes, labels and catalysts. This Feature Article summarizes recent efforts and predicts likely future developments in each of these lines of research.

6-Pyrazolylpurine and its deaza derivatives as nucleobases for silver(I)-mediated base pairing with pyrimidines

The artificial nucleobase 6-pyrazolylpurine (6PP) and its deaza derivatives 1-deaza-6-pyrazolylpurine (1D6PP), 7-deaza-6-pyrazolylpurine (7D6PP), and 1,7-dideaza-6-pyrazolylpurine (1,7D6PP) were investigated with respect to their ability to differentiate between the canonical nucleobases cytosine and thymine by means of silver(I)-mediated base pairing. As shown by temperature-dependent UV spectroscopy and by circular dichroism spectroscopy, 6PP and (to a lesser extent) 7D6PP form stable silver(I)-mediated base pairs with cytosine, but not with thymine. 1D6PP and 1,7D6PP do not engage in the formation of stabilizing silver(I)-mediated base pairs with cytosine or thymine. The different behavior of 1D6PP, 7D6PP, and 1,7D6PP indicates that silver(I) binding occurs via the N1 position of the purine derivative, i.e. via the Watson-Crick face. The data show that 6PP is capable of differentiating between cytosine and thymine, which is potentially relevant in the context of detecting single-nucleotide polymorphisms.

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.

Silver(I)-mediated base pairing involving an S-glycosidic GNA nucleoside analogue

The 4S-Ag(I)-C base pair (4S, 3-((2-(methylthio)pyrimidin-4-yl)thio)propane-1,2-diol; C, deoxycytidine) represents the first metal-mediated base pair comprising an S-glycosidic nucleoside analogue. We report here the synthesis of the phosphoramidite suitable for the automated solid-phase synthesis of DNA oligonucleotides containing 4S and its silver(I)-binding ability. The DNA duplexes comprising a 4S:C mispair exhibit a large thermal stabilization upon the addition of one equivalent of silver ions, giving rise to the formation of the above-mentioned silver(I)-mediated base pair. By formally replacing the sulfur atom in the glycosidic bond by an oxygen atom, i.e., by applying 3-((2-(methylthio)pyrimidin-4-yl)oxy)propane-1,2-diol (4 O) as the artificial nucleoside analogue, the participation of this atom as a donor atom in silver(I)-mediated base pairing is shown to be neglectable.Supplemental data for this article is available online at.

A fast and automated separation and quantification method for bromine speciation analyzing bromide and 5-bromo-2'-deoxyuridine in enzymatically digested DNA samples via ion chromatography-inductively coupled plasma-mass spectrometry

A fast and automated separation and quantification method for bromide and the artificial nucleoside 5-bromo-2'-deoxyuridine (5-BrdU) via hyphenation of ion exchange chromatography (IC) and inductively coupled plasma-mass spectrometry (ICP-MS) is presented. The analysis of these two species is relevant to monitor the transfer of electrons along metal-mediated DNA base pairs. Charge transfer in DNA is of high interest for the implementation in nanotechnological applications like molecular wires. 5-BrdU as part of the DNA sequence releases bromide upon one electron reduction after efficient electron transfer along the DNA. The concentrations of 5-BrdU and bromide in enzymatically digested DNA samples can therefore be used as a marker for the efficiency of electron transfer along the DNA helix. A large number of samples was analyzed using an automated IC system. This platform enables time-efficient external calibration by inline dilution of a stock solution. Due to the fast separation of the two bromine species in less than 90 s, the developed method is suitable for screening applications with a multitude of samples. Despite the isobaric interferences and a low degree of ionization for bromine detection via ICP-MS the method has a limit of detection (LOD) of 30 ng/L which is approximately an order of magnitude lower than a comparable method using reversed phase high performance liquid chromatography (RP-HPLC) and ICP-MS.

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.

Enzymatic Construction of Artificial Base Pairs: The Effect of Metal Shielding

Th formation of metal base pairs is a versatile method for the introduction of metal cations into nucleic acids that has been used in numerous applications including the construction of metal nanowires, development of energy, charge-transfer devices and expansion of the genetic alphabet. As an alternative, enzymatic construction of metal base pairs is an alluring strategy that grants access to longer sequences and offers the possibility of using such unnatural base pairs (UBPs) in SELEX experiments for the identification of functional nucleic acids. This method remains rather underexplored, and a better understanding of the key parameters in the design of efficient nucleotides is required. We have investigated the effect of methylation of the imidazole nucleoside (dIm^nMe TP) on the efficiency of the enzymatic construction of metal base pairs. The presence of methyl substituents on dImTP facilitates the polymerase-driven formation of dIm^4Me -Ag^I -dIm and dIm^2Me TP-Cr^III -dIm base pairs. Steric factors rather than the basicity of the imidazole nucleobase appear to govern the enzymatic formation of such metal base pairs. We also demonstrate the compatibility of other metal cations rarely considered in the construction of artificial metal bases by enzymatic DNA synthesis under both primer extension reaction and PCR conditions. These findings open up new directions for the design of nucleotide analogues for the development of metal base pairs.

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.

Anomeric 5-Aza-7-deaza-2'-deoxyguanosines in Silver-Ion-Mediated Homo and Hybrid Base Pairs: Impact of Mismatch Structure, Helical Environment, and Nucleobase Substituents on DNA Stability

Nucleoside configuration (α-d vs. β-d), nucleobase substituents, and the helical DNA environment of silver-mediated 5-aza-7-deazaguanine-cytosine base pairs have a strong impact on DNA stability. This has been demonstrated by investigations on oligonucleotide duplexes with silver-mediated base pairs of α-d and β-d anomeric 5-aza-7-deaza-2'-deoxyguanosines and anomeric 2'-deoxycytidines incorporated in 12-mer duplexes. To this end, a new synthetic protocol has been developed to access the pure anomers of 5-aza-7-deaza-2'-deoxyguanosine by glycosylation of either the protected nucleobase or its salt followed by separation of the glycosylation products by crystallization and chromatography. Thermal stability measurements were performed on duplexes with α-d/α-d and β-d/β-d homo base pairs or α-d/β-d and β-d/α-d hybrid pairs within two sequence environments, positions 6 or 7, of oligonucleotide duplexes. The respective T_m stability increases observed after silver ion addition differ significantly. Homo base pairs with β-d/β-d or α-d/α-d nucleoside combinations are more stable than α-d/β-d hybrid base pairs. The positional switch of silver-ion-mediated base pairs has a significant impact on stability. Nucleobase substituents introduced at the 5-position of the dC site of silver-mediated base pairs affect base pair stability to a minor extent. Our investigation might lead to applications in the construction of bioinspired nanodevices, in DNA diagnostics, or metal-DNA hybrid materials.

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'-O,4'-C-Methylene-Bridged Nucleic Acids Stabilize Metal-Mediated Base Pairing in a DNA Duplex

The 2'-O,4'-C-methylene-bridged or locked nucleic acid (2',4'-BNA/LNA), with an N-type sugar conformation, effectively improves duplex-forming ability. 2',4'-BNA/LNA is widely used to improve gene knockdown in nucleic acid based therapies and is used in gene diagnosis. Metal-mediated base pairs (MMBPs), such as thymine (T)-Hg^II -T and cytosine (C)-Ag^I -C have been developed and used as attractive tools in DNA nanotechnology studies. This study aimed to investigate the application of 2',4'-BNA/LNA in the field of MMBPs. 2',4'-BNA/LNA with 5-methylcytosine stabilized the MMBP of C with Ag^I ions. Moreover, the 2',4'-BNA/LNA sugar significantly improved the duplex-forming ability of the DNA/DNA complex, relative to that by the unmodified sugar. These results suggest that the sugar conformation is important for improving the stability of duplex-containing MMBPs. The results indicate that 2',4'-BNA/LNA can be applied not only to nucleic acid based therapies, but also to MMBP technologies.

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.

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.