Metal-mediated Base Pairs Research Articles

A QM/MM refinement of an experimental DNA structure with metal-mediated base pairs

A series of hybrid quantum mechanical/molecular mechanical (QM/MM) calculations was performed on models of a DNA duplex with artificial silver(I)-mediated imidazole base pairs. The optimized structures were compared to the original experimental NMR structure (Nat. Chem. 2 (2010) 229–234). The metal⋯metal distances are significantly shorter (~0.5Å) in the QM/MM model than in the original NMR structure. As a result, argentophilic interactions are feasible between the silver(I) ions of neighboring metal-mediated base pairs. Using the computationally determined metal⋯metal distances, a re-refined NMR solution structure of the DNA duplex was obtained. In this new NMR structure, all experimental constraints remain fulfilled. The new NMR structure shows less deviation from the regular B-type conformation than the original one. This investigation shows that the application of QM/MM models to generate additional constraints to be used during NMR structural refinements represents an elegant approach to obtaining high-resolution NMR structures.

6-Substituted purines containing thienyl or furyl substituents as artificial nucleobases for metal-mediated base pairing

The artificial nucleobases 6-(2'-thienyl)-purine (6TP) and 6-(2'-furyl)-purine (6FP) have been investigated in terms of their applicability in metal-mediated base pairs. In principle, these nucleobases should be capable of providing an S,N- and O,N-coordination environment, respectively. Two metal complexes have been synthesized and structurally characterized, using the N9-methylated derivatives of 6TP and 6FP as model nucleobases. The silver(I) complex [Ag(9-Me6TP)(CH3CN)(NO3)]n is of polymeric nature. The thienyl substituent is not involved in metal binding. In contrast, the dinuclear copper(II) complex [Cu(9-Me6FP)Cl2(OH2)]2 clearly shows that the furyl substituent can participate in the coordination of the metal ion. Oligonucleotide double helices comprising a 6TP:6TP or a 6FP:6FP mispair are slightly stabilized in the presence of silver(I). However, a distinction between the formation of a metal-mediated base pair and an unspecific binding event cannot be finally made. Based on these studies, it is proposed that the formation of Hoogsteen-type metal-mediated base pairs should be the more promising alternative compared with the originally anticipated Watson-Crick-type metal-mediated base pairs.

Structural, Electronic, and Optical Properties of Metallo Base Pairs in Duplex DNA: A Theoretical Insight

Using density functional theory calculations, we investigated the structural, energetic, electronic, and optical properties of recently synthesized duplex DNA containing metal-mediated base pairs. The studied duplex DNA consists of three imidazole (Im) units linked through metal (Im-M-Im, M = metal) and four flanking A:T base pairs (two on each side). We examined the role of artificial base pairing in the presence of two distinctive metal ions, diamagnetic Ag(+) and magnetic Cu(2+) ions, on the stability of duplex DNA. We found that metal-mediated base pairs form stable duplex DNA by direct metal ion coordination to the Im bases. Our results suggest a higher binding stability of base pairing mediated by Cu(2+) ions than by Ag(+) ions, which is attributed to a larger extent of orbital hybridization. We furthermore found that DNA modified with Im-Ag(+)-Im shows the low-energy optical absorption characteristic of π-π*orbital transition of WC A:T base pairs. On the other hand, we found that the low-energy optical absorption peaks for DNA modified with Im-Cu(2+)-Im originate from spin-spin interactions. Additionally, this complex exhibits weak ferromagnetic coupling between Cu(2+) ions and strong spin polarization, which could be used for memory devices. Moreover, analyzing the role of counter ions (Na(+)) and the presence of explicit water molecules on the structural stability and electronic properties of the DNA duplex modified with Im-Ag(+)-Im, we found that the impact of these two factors is negligible. Our results are fruitful for understanding the experimental data and suggest a potential route for constructing effective metal-mediated base pairs in duplex DNA for optoelectronic applications.

Thermodynamic and structural properties of the specific binding between Ag+ ion and C:C mismatched base pair in duplex DNA to form C–Ag–C metal-mediated base pair

Metal ion-nucleic acid interactions have attracted considerable interest for their involvement in structure formation and catalytic activity of nucleic acids. Although interactions between metal ion and mismatched base pair duplex are important to understand mechanism of gene mutations related to heavy metal ions, they have not been well-characterized. We recently found that the Ag(+) ion stabilized a C:C mismatched base pair duplex DNA. A C-Ag-C metal-mediated base pair was supposed to be formed by the binding between the Ag(+) ion and the C:C mismatched base pair to stabilize the duplex. Here, we examined specificity, thermodynamics and structure of possible C-Ag-C metal-mediated base pair. UV melting indicated that only the duplex with the C:C mismatched base pair, and not of the duplexes with the perfectly matched and other mismatched base pairs, was specifically stabilized on adding the Ag(+) ion. Isothermal titration calorimetry demonstrated that the Ag(+) ion specifically bound with the C:C base pair at 1:1 molar ratio with a binding constant of 10(6) M(-1), which was significantly larger than those for nonspecific metal ion-DNA interactions. Electrospray ionization mass spectrometry also supported the specific 1:1 binding between the Ag(+) ion and the C:C base pair. Circular dichroism spectroscopy and NMR revealed that the Ag(+) ion may bind with the N3 positions of the C:C base pair without distorting the higher-order structure of the duplex. We conclude that the specific formation of C-Ag-C base pair with large binding affinity would provide a binding mode of metal ion-DNA interactions, similar to that of the previously reported T-Hg-T base pair. The C-Ag-C base pair may be useful not only for understanding of molecular mechanism of gene mutations related to heavy metal ions but also for wide variety of potential applications of metal-mediated base pairs in various fields, such as material, life and environmental sciences.

Metal-Mediated DNA Base Pairing: Alternatives to Hydrogen-Bonded Watson–Crick Base Pairs

With its capacity to store and transfer the genetic information within a sequence of monomers, DNA forms its central role in chemical evolution through replication and amplification. This elegant behavior is largely based on highly specific molecular recognition between nucleobases through the specific hydrogen bonds in the Watson-Crick base pairing system. While the native base pairs have been amazingly sophisticated through the long history of evolution, synthetic chemists have devoted considerable efforts to create alternative base pairing systems in recent decades. Most of these new systems were designed based on the shape complementarity of the pairs or the rearrangement of hydrogen-bonding patterns. We wondered whether metal coordination could serve as an alternative driving force for DNA base pairing and why hydrogen bonding was selected on Earth in the course of molecular evolution. Therefore, we envisioned an alternative design strategy: we replaced hydrogen bonding with another important scheme in biological systems, metal-coordination bonding. In this Account, we provide an overview of the chemistry of metal-mediated base pairing including basic concepts, molecular design, characteristic structures and properties, and possible applications of DNA-based molecular systems. We describe several examples of artificial metal-mediated base pairs, such as Cu(2+)-mediated hydroxypyridone base pair, H-Cu(2+)-H (where H denotes a hydroxypyridone-bearing nucleoside), developed by us and other researchers. To design the metallo-base pairs we carefully chose appropriate combinations of ligand-bearing nucleosides and metal ions. As expected from their stronger bonding through metal coordination, DNA duplexes possessing metallo-base pairs exhibited higher thermal stability than natural hydrogen-bonded DNAs. Furthermore, we could also use metal-mediated base pairs to construct or induce other high-order structures. These features could lead to metal-responsive functional DNA molecules such as artificial DNAzymes and DNA machines. In addition, the metallo-base pairing system is a powerful tool for the construction of homogeneous and heterogeneous metal arrays, which can lead to DNA-based nanomaterials such as electronic wires and magnetic devices. Recently researchers have investigated these systems as enzyme replacements, which may offer an additional contribution to chemical biology and synthetic biology through the expansion of the genetic alphabet.

Raman spectroscopic detection of the T-Hg II-T base pair and the ionic characteristics of mercury.

Developing applications for metal-mediated base pairs (metallo-base-pair) has recently become a high-priority area in nucleic acid research, and physicochemical analyses are important for designing and fine-tuning molecular devices using metallo-base-pairs. In this study, we characterized the HgII-mediated T-T (T-HgII-T) base pair by Raman spectroscopy, which revealed the unique physical and chemical properties of HgII. A characteristic Raman marker band at 1586 cm−1 was observed and assigned to the C4=O4 stretching mode. We confirmed the assignment by the isotopic shift (18O-labeling at O4) and density functional theory (DFT) calculations. The unusually low wavenumber of the C4=O4 stretching suggested that the bond order of the C4=O4 bond reduced from its canonical value. This reduction of the bond order can be explained if the enolate-like structure (N3=C4-O4−) is involved as a resonance contributor in the thymine ring of the T-HgII-T pair. This resonance includes the N-HgII-bonded state (HgII-N3-C4=O4) and the N-HgII-dissociated state (HgII+ N3=C4-O4−), and the latter contributor reduced the bond order of N-HgII. Consequently, the HgII nucleus in the T-HgII-T pair exhibited a cationic character. Natural bond orbital (NBO) analysis supports the interpretations of the Raman experiments.

Efficient Synthesis of 1-Deaza-6-methoxypurine-N9-β-2′-deoxyribonucleoside and Related DNA Building Blocks

An improved synthesis of the 2′-deoxyribonucleoside containing 1-deaza-6-methoxypurine as artificial nucleobase is described. By optimizing the conditions for the deprotection of p-toluoyl-protected 1-deaza-6-nitropurine 2′-deoxyribonucleoside, the isolated yield of the title compound could be increased from 15% to 90%. In addition, the synthesis and characterization of the corresponding 5′-(4,4′-dimethoxytrityl) (DMT) protected phosphoramidite, a potentially useful building block for the synthesis of artificial oligonucleotides, is reported. The title compound offers interesting hydrogen bond donor and metal-binding properties for its application in metal-mediated base pairs.

Contiguous Metal‐Mediated Base Pairs Comprising Two AgIIons

The incorporation of transition-metal ions into nucleic acids by using metal-mediated base pairs has proved to be a promising strategy for the site-specific functionalization of these biomolecules. We report herein the formation of Ag(+)-mediated Hoogsteen-type base pairs comprising 1,3-dideaza-2'-deoxyadenosine and thymidine. By defunctionalizing the Watson-Crick edge of adenine, the formation of regular base pairs is prohibited. The additional substitution of the N3 nitrogen atom of adenine by a methine moiety increases the basicity of the exocyclic amino group. Hence, 1,3-dideazaadenine and thymine are able to incorporate two Ag(+) ions into their Hoogsteen-type base pair (as compared with one Ag(+) ion in base pairs with 1-deazaadenine and thymine). We show by using a combination of experimental techniques (UV and circular dichroism (CD) spectroscopies, dynamic light scattering, and mass spectrometry) that this type of base pair is compatible with different sequence contexts and can be used contiguously in DNA double helices. The most stable duplexes were observed when using a sequence containing alternating purine and pyrimidine nucleosides. Dispersion-corrected density functional theory calculations have been performed to provide insight into the structure, formation and stabilization of the twofold metalated base pair. They revealed that the metal ions within a base pair are separated by an Ag···Ag distance of about 2.88 Å. The Ag-Ag interaction contributes some 16 kcal mol(-1) to the overall stability of the doubly metal-mediated base pair, with the dominant contribution to the Ag-Ag bonding resulting from a donor-acceptor interaction between silver 4d-type and 4s orbitals. These Hoogsteen-type base pairs enable a higher functionalization of nucleic acids with metal ions than previously reported metal-mediated base pairs, thereby increasing the potential of DNA-based nanotechnology.

Positive cooperativity of the specific binding between Hg2+ ion and T:T mismatched base pairs in duplex DNA

Metal-mediated base pairs by the interaction between metal ions and artificial bases in oligonucleotides have been developed for their potential applications in nanotechnology. We recently found that a natural T:T mismatched base pair bound with Hg2+ ion to form a novel T–Hg–T base pair. Here, we examined the thermodynamic properties of the binding between Hg2+ and each of the single and double T:T mismatched base pair duplex DNAs by isothermal titration calorimetry. Hg2+ specifically bound with the T:T mismatched base pair at 1:1 molar ratio with 106M−1 binding constant, which was significantly larger than those for nonspecific metal ion–DNA interactions. In the Hg2+–double T:T mismatched base pair interaction, the affinity for the second Hg2+ binding was significantly larger than that for the first Hg2+ binding. The positively cooperative binding may be favorable to align multiple Hg2+ in duplex DNA for the application of the metal-mediated base pairs in nanotechnology.

Thermodynamic Properties of the Specific Binding Between Ag+ Ions and C:C Mismatched Base Pairs in Duplex DNA

Metal-mediated base pairs formed by the interaction between metal ions and artificial bases in oligonucleotides have been developed for potential applications in nanotechnology. We recently found that a natural C:C mismatched base pair bound to an Ag+ ion to generate a novel metal-mediated base pair in duplex DNA. Preparation of the novel C-Ag-C base pair involving natural bases is more convenient than that of metal-mediated base pairs involving artificial bases because time-consuming base synthesis is not required. Here, we examined the thermodynamic properties of the binding between the Ag+ ion and each of single and double C:C mismatched base pair in duplex DNA by isothermal titration calorimetry. The Ag+ ion specifically bound to the C:C mismatched base pair at a 1:1 molar ratio with 106 M−1 binding constant, which was significantly larger than those for nonspecific metal ion–DNA interactions. The specific binding between the Ag+ ion and the single C:C mismatched base pair was mainly driven by the positive dehydration entropy change and the negative binding enthalpy change. In the interaction between the Ag+ ion and each of the consecutive and interrupted double C:C mismatched base pairs, stoichiometric binding at a 1:1 molar ratio was achieved in each step of the first and second Ag+ binding. The binding affinity for the second Ag+ binding was similar to that for the first Ag+ binding. Stoichiometric binding without interference and negative cooperativity may be favorable for aligning multiple Ag+ ions in duplex DNA for applications of the metal-mediated base pairs in nanotechnology.

Chimeric GNA/DNA metal-mediated base pairs

DNA double helices comprising chimeric GNA/DNA metal-mediated base pairs have been synthesized and characterized (GNA = glycol nucleic acid). The possibility to combine different nucleic acid backbones within one metal-mediated base pair expands the applicability of metal-functionalized nucleic acids.

Synthesis of covalently linked parallel and antiparallel DNA duplexes containing the metal-mediated base pairs T–Hg(ii)–T and C–Ag(i)–C

DNA duplexes fixed in anti-parallel and parallel orientations by introducing covalent linkages have been synthesized and metal ions, Hg(II) and Ag(I), were incorporated into pyrimidine-pyrimidine base pairs.

On the role of mercury in the non-covalent stabilisation of consecutive U–HgII–U metal-mediated nucleic acid base pairs: metallophilic attraction enters the world of nucleic acids

Metal atoms with a closed-shell electronic structure and positive charge as for example the Au(I), Pt(II), Ag(I), Tl(I) or Hg(II) atoms do not in some compounds repel each other due to the so-called metallophilic attraction (P. Pyykkö, Chem. Rev., 1997, 97, 597-636). Here we highlight the role of the Hg(II)Hg(II) metallophilic attraction between the consecutive metal-mediated mismatched base pairs of nucleic acids. Usually, the base stacking dominates the non-covalent interactions between steps of native nucleic acids. In the presence of metal-mediated base pairs these non-covalent interactions are enriched by the metal-base interactions and the metallophilic attraction. The two interactions arising due to the metal linkage of the mismatches were found in this study to have a stabilizing effect on nucleic acid structure. The calculated data are consistent with recent experimental observations. The stabilization due to the metallophilic attraction seems to be a generally important concept for the nucleic acids containing heavy metals with short contacts.

HgII Ion Specifically Binds with T:T Mismatched Base Pair in Duplex DNA

Metal-mediated base pair formation, resulting from the interaction between metal ions and artificial bases in oligonucleotides, has been developed for its potential application in nanotechnology. We have recently found that the T:T mismatched base pair binds with Hg(II) ions to generate a novel metal-mediated base pair in duplex DNA. The thermal stability of the duplex with the T-Hg-T base pair was comparable to that of the corresponding T:A or A:T. The novel T-Hg-T base pair involving the natural base thymine is more convenient than the metal-mediated base pairs involving artificial bases due to the lack of time-consuming synthesis. Here, we examine the specificity and thermodynamic properties of the binding between Hg(II) ions and the T:T mismatched base pair. Only the melting temperature of the duplex with T:T and not of the perfectly matched or other mismatched base pairs was found to specifically increase in the presence of Hg(II) ions. Hg(II) specifically bound with the T:T mismatched base pair at a molar ratio of 1:1 with a binding constant of 10(6) M(-1), which is significantly higher than that for nonspecific metal ion-DNA interactions. Furthermore, the higher-order structure of the duplex was not significantly distorted by the Hg(II) ion binding. Our results support the idea that the T-Hg-T base pair could eventually lead to progress in potential applications of metal-mediated base pairs in nanotechnology.

Solution structure of a DNA double helix with consecutive metal-mediated base pairs

Metal-mediated base pairs represent a powerful tool for the site-specific functionalization of nucleic acids with metal ions. The development of applications of the metal-modified nucleic acids will depend on the availability of structural information on these double helices. We present here the NMR solution structure of a self-complementary DNA oligonucleotide with three consecutive imidazole nucleotides in its centre. In the absence of transition-metal ions, a hairpin structure is adopted with the artificial nucleotides forming the loop. In the presence of Ag(i) ions, a duplex comprising three imidazole-Ag(+)-imidazole base pairs is formed. Direct proof for the formation of metal-mediated base pairs was obtained from ¹J(¹⁵N,¹⁰⁷/¹⁰⁹Ag) couplings upon incorporation of ¹⁵N-labelled imidazole. The duplex adopts a B-type conformation with only minor deviations in the region of the artificial bases. This work represents the first structural characterization of a metal-modified nucleic acid with a continuous stretch of metal-mediated base pairs.

Metal-mediated base pairing within the simplified nucleic acid GNA

Hydroxypyridone and pyridopurine homo- and hetero-base pairs have been investigated in the context of duplex GNA (glycol nucleic acid). Phosphoramidites for automated GNA solid phase synthesis were synthesized economically in a few steps starting from commercially available enantiopure glycidol. Similar to their behavior in DNA, the hydroxypyridone and pyridopurine homo-base pairs display a metal-dependent base pairing, with the hydroxypyridone base pair exhibiting a preference for copper(II) ions and the pyridopurine a preference for nickel(II) ions. However, these metallo-base pairs show modulated properties in GNA with respect to metal-dependent pairing stabilities and metal selectivities. Most interestingly, the hydroxypyridone homo-base pair and hydroxypyridone-pyridopurine hetero-base pair are particularly well accommodated in the GNA duplex and form copper(II)-dependent base pairs that are more stable compared to a Watson-Crick A:T base pair at the same position by nearly 20 degrees C and 24 degrees C, respectively. The structure of the copper(II)-hydroxypyridone homo-base pair is discussed based on a recent metallo-GNA duplex crystal structure.

Soft Metal-Mediated Base Pairing with Novel Synthetic Nucleosides Possessing an O,S-Donor Ligand

Metal-mediated base pairing with artificial ligand-bearing nucleosides allows site-selective metal incorporation inside DNA duplexes. In particular, this strategy has provided a general way of discrete, heterogeneous metal arrays in a programmable manner. To increase the kind of metallo-building blocks, we have newly synthesized two artificial nucleosides which have an O, S-donor ligand as the nucleobase moiety, mercaptopyridone ( M) and hydroxypyridinethione ( S). These nucleosides were found to efficiently form metal-mediated base pairs with soft transition metal ions such as Pd (2+) and Pt (2+).

Structural and physicochemical features on the metal-mediated base pairs

The chemical structure of the mercury-mediated T-T pair (T-Hg(I)I-T) was determined with (15)N NMR spectroscopy. In order to determine the chemical structure of the T-Hg(I)I-T pair, (15)N-(15)N J-coupling across a metal center (2JNN) was employed. Notably, this is the first observation of (2)J(NN) in a biological macromolecule (DNA duplex). This pairing mode was found to be a irregular metal ion-binding mode for DNA and RNA molecules, in which imino proton-metal exchange processes are included. Accordingly, (2)J(NN) is highly important for the determination of the chemical structures of metal-mediated base pairs.

15N−15N J-Coupling Across HgII: Direct Observation of HgII-Mediated T−T Base Pairs in a DNA Duplex

N−N J-coupling across a metal center (2JNN) was clearly detected in a biological macromolecule (DNA duplex) for the first time. By using 2JNN, the base pairing mode of mercury-mediated T−T pairs (T−HgII−T) was definitely determined. This pairing mode was found to be a novel metal ion-binding mode for DNA and RNA molecules, in which imino proton−metal exchange processes are included. Accordingly, 2JNN is highly important for the determination of the chemical structures of metal-mediated base pairs.

Differential reactivity of α and β 2′-deoxyribonucleosides towards protonation and metalation

The reactivity of artificial 2′-deoxyribonucleosides, designed as nucleoside surrogates in metal-mediated base pairs, towards protonation and metalation has been shown to be dependent on the choice of the anomer. The α nucleosides comprising the aglycones imidazole, 1,2,4-triazole, benzimidazole and imidazo[4,5- b]pyridine are more basic than the respective β nucleosides as was shown by a combined experimental and theoretical approach. The Δp K a values observed experimentally are in the range of 0.19 ± 0.03 to 0.41 ± 0.07 (with the error representing three times the standard deviation of the mean value). An independent confirmation of this differential reactivity was obtained from density functional theory (DFT) calculations using 1,2,4-triazole nucleoside as an example. The result of these calculations is in good agreement with the experimental data (Δp K a = 0.16 vs. 0.21 ± 0.07). The stability of the respective metal ion complexes of the anomeric 1,2,4-triazole nucleosides follows the same trend as that of the respective protonated nucleosides: Those of the α nucleoside are more stable than those of the β nucleoside (Δlog β 2 = 0.6 ± 0.2 for the 2:1 complex with Ag +; Δlog β 1 = 0.51 ± 0.07 for the 1:1 complex with Hg 2+). These slightly different reactivities will be useful for fine-tuning the metal-ion binding behavior of oligonucleotides containing metal-mediated base pairs.