Robust silver-mediated imidazolo-dC base pairs in metal DNA: dinuclear silver bridges with exceptional stability in double helices with parallel and antiparallel strand orientation

Selectivity of natural isoquinoline alkaloid assembler in programming poly(dA) into parallel duplex by polyvalent synergy

Ligand-induced assembly of disordered DNAs attracts much attention due to its potential action in transcription regulation and molecular switches-based sensors. Among natural isoquinoline alkaloids (NIAs), we screened out nitidine (NIT) as polyvalent-binding assembler to program poly(dA) into a parallel duplex assembly at neutral pH. The molecule planarity of NIAs was believed to be a determinant factor in programming the parallel poly(dA) assembly. Poly(dA) with more than six adenines can initiate the synergistic binding of NIT to generate the parallel assembly. It is expected that one A-A pair in duplex can bind one NIT molecule provided that poly(dA) is long enough, suggesting the pivotal role of the polyvalent synergy of NIT in programming the parallel poly(dA) assembly. A gold nanoparticles-based colorimetric method was also developed to screen NIT out of NIAs having the potential to construct the poly(dA) assembly. Our work will inspire more interest in developing polyadenine-based switches and sensors by concentrating NIT within the polyadenine parallel assembly.

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.

8-Furylimidazolo-2'-deoxycytidine: crystal structure, packing, atropisomerism and fluorescence

8-Furylimidazolo-2'-deoxycytidine (furImidC), C14H14N4O5, is a fluorescent analogue of 2'-deoxycytidine, also displaying the same recognition face. As a constituent of DNA, furImidC forms extraordinarily strong silver-mediated self-pairs. Crystal structure determination revealed that furImidC adopts two types of disordered residues: the sugar unit and the furyl moiety. The disorder of the sugar residue amounts to an 87:13 split. The disorder of the furyl ring results from axial chirality at the C8-C2'' bond connecting the nucleobase to the heterocycle. The two atropisomers are present in unequal proportions [occupancies of 0.69 (2) and 0.31 (2)], and the nucleobase and the furyl moiety are coplanar. Considering the atomic sites with predominant occupancy, an anti conformation with χ = - 147.2 (7)° was found at the glycosylic bond and the 2'-deoxyribosyl moiety shows a C2'-endo (S, 2T1) conformation, with P = 160.0°. A 1H NMR-based conformational analysis of the furanose puckering revealed that the S conformation predominates also in solution. In the solid state, two neighbouring furImidC molecules are arranged in a head-to-tail fashion, but with a notable tilt of the molecules with respect to each other. Consequently, one N-H...N hydrogen bond is found for neighbouring molecules within one layer, while a second N-H...N hydrogen bond is formed to a molecule of an adjacent layer. In addition, hydrogen bonding is observed between the nucleobase and the sugar residue. A Hirshfeld surface analysis was performed to visualize the intermolecular interactions observed in the X-ray study. In addition, the fluorescence spectra of furImidC were measured in solvents of different polarity and viscosity. furImidC responds to microenvironmental changes (polarity and viscosity), which is explained by a hindered rotation of the furyl residue in solvents of high viscosity.

Nucleoside Analogues with a Seven-Membered Sugar Ring: Synthesis and Structural Compatibility in DNA-RNA Hybrids

Herein we describe results on the pairing properties of synthetic DNA and RNA oligonucleotides that contain nucleotide analogues with a 7-membered sugar ring (oxepane nucleotides). Specifically, we describe the stereoselective synthesis of a set of three oxepane thymine nucleosides (OxT), their conversion to phosphoramidite derivatives, and their use in solid-phase synthesis to yield chimeric OxT-DNA and OxT-RNA strands. The different regioisomeric OxT phosphoramidites allowed for positional variations of the phosphate bridge and assessment of duplex stability when the oxepane nucleotides were incorporated in dsDNA, dsRNA, and DNA-RNA hybrids. Little to no destabilization was observed when two of the three regioisomeric OxT units were incorporated in the DNA strand of DNA-RNA hybrids, a remarkable result considering the dramatically different structure of oxepanes in comparison to 2'-deoxynucleosides. Extensive molecular modeling and dynamics studies further revealed the various structural features responsible for the tolerance of both OxT modifications in DNA-RNA duplexes, such as base-base stacking and sugar-phosphate H-bond interactions. These studies suggest that oxepane nucleotide analogues may find applications in synthetic biology, where synthetic oligonucleotides can be used to create new tools for biotechnology and medicine.

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.

Enzymatic Formation of an Artificial Base Pair Using a Modified Purine Nucleoside Triphosphate

The expansion of the genetic alphabet with additional, unnatural base pairs (UBPs) is an important and long-standing goal in synthetic biology. Nucleotides acting as ligands for the coordination of metal cations have advanced as promising candidates for such an expansion of the genetic alphabet. However, the inclusion of artificial metal base pairs in nucleic acids mainly relies on solid-phase synthesis approaches, and very little is known about polymerase-mediated synthesis. Herein, we report the selective and high yielding enzymatic construction of a silver-mediated base pair (dIm^C-Ag^I-dPur^P) as well as a two-step protocol for the synthesis of DNA duplexes containing such an artificial metal base pair. Guided by DFT calculations, we also shed light into the mechanism of formation of this artificial base pair as well as into the structural and energetic preferences. The enzymatic synthesis of the dIm^C-Ag^I-dPur^P artificial metal base pair provides valuable insights for the design of future, more potent systems aiming at expanding the genetic alphabet.

Heterochiral DNA with Complementary Strands with α-d and β-d Configurations: Hydrogen-Bonded and Silver-Mediated Base Pairs with Impact of 7-Deazapurines Replacing Purines

Heterochiral DNA with hydrogen-bonded and silver-mediated base pairs have been constructed using complementary strands with nucleosides with α-d or β-d configuration. Anomeric phosphoramidites were employed to assemble the oligonucleotides. According to the Tm values and thermodynamic data, the duplex stability of the heterochiral duplexes was similar to that of homochiral DNA, but mismatch discrimination was better in heterochiral DNA. Replacement of purines by 7-deazapurines resulted in stable parallel duplexes, thereby confirming Watson-Crick-type base pairing. When cytosine was facing cytosine, thymine or adenine residues, duplex DNA formed silver-mediated base pairs in the presence of silver ions. Although the CD spectra of single strands with α-d configuration display mirror-like shapes to those with the β-d configuration, the CD spectra of the hydrogen-bonded duplexes and those with a limited number of silver pairs show a B-type double helix almost indistinguishable from natural DNA. Nonmelting silver ion-DNA complexes with entirely different CD spectra were generated when the number of silver ions was equal to the number of base pairs.

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.

Silver-Mediated Homochiral and Heterochiral α-dC/β-dC Base Pairs: Synthesis of α-dC through Glycosylation and Impact of Consecutive, Isolated, and Multiple Metal Ion Pairs on DNA Stability

Isolated and consecutive heterochiral α-dC- base pairs have been incorporated into 12-mer oligonucleotide duplexes at various positions, thereby replacing Watson-Crick pairs. To this end, a new synthesis of the α-d anomer of dC has been developed, and oligonucleotides containing α-dC residues have been synthesized. Silver-mediated base pairs were formed upon the addition of silver ions. Furthermore, we have established that heterochiral α-dC-dC base pairs can approach the stability of a Watson-Crick pair, whereas homochiral dC-dC pairs are significantly less stable. A positional change of the silver-mediated base pairs affects the duplex stability and reveals the nearest-neighbor influence. When the number of silver ions was equivalent to the number of duplex base pairs (12), non-melting silver-rich complexes were formed. Structural changes have been supported by circular dichroism (CD) spectra, which showed that the B-DNA structure was maintained whilst the silver ion concentration was low. At high silver ion concentration, silver-rich complexes displaying different CD spectra were formed.

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.

On the Enzymatic Formation of Metal Base Pairs with Thiolated and pKa -Perturbed Nucleotides

The formation of artificial metal base pairs is an alluring and versatile method for the functionalization of nucleic acids. Access to DNA functionalized with metal base pairs is granted mainly by solid-phase synthesis. An alternative, yet underexplored method, envisions the installation of metal base pairs through the polymerization of modified nucleoside triphosphates. Herein, we have explored the possibility of using thiolated and pK_a -perturbed nucleotides for the enzymatic construction of artificial metal base pairs. The thiolated nucleotides S2C, S6G, and S4T as well as the fluorinated analogue 5FU are readily incorporated opposite a templating S4T nucleotide through the guidance of metal cations. Multiple incorporation of the modified nucleotides along with polymerase bypass of the unnatural base pairs are also possible under certain conditions. The thiolated nucleotides S4T, S4T, S2C, and S6G were also shown to be compatible with the synthesis of modified, high molecular weight single-stranded (ss)DNA products through TdT-mediated tailing reactions. Thus, sulfur-substitution and pK_a perturbation represent alternative strategies for the design of modified nucleotides compatible with the enzymatic construction of metal base pairs.

Highly sensitive and selective mercury sensor based on mismatched base pairing with dioxT

A highly selective and sensitive sensor for mercury was designed based on a new fluorescent nucleobase, dioxT. Its metal-sensing ability was investigated using mismatched dioxT-T and dioxT-C base pairing. The sensor exhibited a high sensitivity (quenching efficiency, 80%, 1 : 1 binding mode) and selectivity upon the addition of mercury ions.

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.

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.

Coordination-induced structural changes of DNA-based optical and electrochemical sensors for metal ions detection

Metal ions play a critical role in human health and abnormal levels are closely related to various diseases. Therefore, the detection of metal ions with high selectivity, sensitivity and accuracy is particularly important. This article highlights and comments on the coordination-induced structural changes of DNA-based optical, electrochemical and optical-electrochemical-combined sensors for metal ions detection. Challenges and potential solutions of DNA-based sensors for the simultaneous detection of multiple metal ions are also discussed for further development and exploitation.

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.

Covalently Palladated Oligonucleotides Through Oxidative Addition of Pd0

An 11-mer oligonucleotide incorporating a central (2-iodobenzoylamino)methyl residue has been synthesized and palladated by oxidative addition of Pd₂ (dba)₃ . UV melting profiles of the duplexes formed by the palladated oligonucleotide with its natural complements were biphasic and the higher melting temperatures (T_m ) exhibited considerable hysteresis. CD spectra, in turn, resembled those of canonical B-type double helices. Two-step denaturation, with the "low-T_m " melting involving only canonical base pairs and the "high-T_m " melting involving also dissociation of a Pd^II -mediated base pair, appears the most likely explanation for the observed UV melting profiles. As the latter step in all cases takes place at a higher temperature than denaturation of natural duplexes of the same length, the putative Pd^II -mediated base pairs are stabilizing.

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.

Towards the enzymatic formation of artificial metal base pairs with a carboxy-imidazole-modified nucleotide

The identification of synthetic nucleotides that sustain the formation of orthogonal, unnatural base pairs is an important goal in synthetic biology. Such artificial synthons have been used for the generation of semi-synthetic organisms as well as functional nucleic acids with enhanced binding properties. The enzymatic formation of artificial metal-base pairs is a vastly underexplored and alluring alternative to existing systems. Here, we report the synthesis and biochemical characterization of 1‑(2-deoxy‑β‑d‑ribofuranosyl) imidazole‑4‑carboxylate nucleoside triphosphate (dIm^CTP) which is equipped with a carboxylic acid moiety on the imidazole moiety in order to increase the coordination environment to [2 + 2] and [2 + 1]. A clear metal dependence was observed for the single incorporation of the modified nucleotide into DNA by the DNA polymerase from Thermus aquaticus (Taq). The presence of Ag^I in primer extension reactions conducted with combinations of 1‑(2‑deoxy‑β‑d‑ribofuranosyl) imidazole nucleoside triphosphate (dImTP) and dIm^CTP supported the unusual [2 + 1] coordination pattern. The efficiency of the tailing reactions mediated by the terminal deoxynucleotidyl transferase (TdT) was markedly improved when using dIm^CTP instead of dImTP. Even though products with multiple modified nucleotides were not observed, the appendage of additional metal binding ligands on the imidazole nucleobase appears to be a valid approach to improve the biochemical properties of modified triphosphates in the context of an expansion of the genetic alphabet with metal base pairs.

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.

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.

HgII binds to C-T mismatches with high affinity

Binding reactions of HgII and AgI to pyrimidine-pyrimidine mismatches in duplex DNA were characterized using fluorescent nucleobase analogs, thermal denaturation and 1H NMR. Unlike AgI, HgII exhibited stoichiometric, site-specific binding of C-T mismatches. The on- and off-rates of HgII binding were approximately 10-fold faster to C-T mismatches (kon ≈ 105 M-1 s-1, koff ≈ 10-3 s-1) as compared to T-T mismatches (kon ≈ 104 M-1 s-1, koff ≈ 10-4 s-1), resulting in very similar equilibrium binding affinities for both types of 'all natural' metallo base pairs (Kd ≈ 10-150 nM). These results are in contrast to thermal denaturation analyses, where duplexes containing T-T mismatches exhibited much larger increases in thermal stability upon addition of HgII (ΔTm = 6-19°C), as compared to those containing C-T mismatches (ΔTm = 1-4°C). In addition to revealing the high thermodynamic and kinetic stabilities of C-HgII-T base pairs, our results demonstrate that fluorescent nucleobase analogs enable highly sensitive detection and characterization of metal-mediated base pairs - even in situations where metal binding has little or no impact on the thermal stability of the duplex.

5-Aza-7-deaza-2'-deoxyguanosine and 2'-Deoxycytidine Form Programmable Silver-Mediated Base Pairs with Metal Ions in the Core of the DNA Double Helix

5-Aza-7-deaza-2'-deoxyguanosine (dZ) forms a silver-mediated base pair with dC. The metal ion pair represents a mimic of the H-bonded Watson-Crick dG-dC pair. The modified nucleoside displays a similar shape as the parent 2'-deoxyguanosine from which it can be constructed by transposition of nitrogen-7 to the bridgehead position-5. It lacks the major groove binding site as the positional change moves the dG- acceptor position from nitrogen-7 to nitrogen-1. As a shape mimic of dG, it fits nicely in the DNA double helix. The purine-pyrimidine dZ-dC hetero pair shows a relationship to the pyrimidine-pyrimidine dC-dC homo base pair. The dZ-dC pair forms a mismatch in the absence of silver ions and matches after addition of metal ions. Base-pair formation was verified on self-complementary 6-mer duplexes and 12-mer DNA with random composition by UV-dependent T_m measurements. Modified silver-mediated and hydrogen-bonded canonical base pairs can coexist. The dZ-Ag⁺ -dC base pair is slightly less stable than the dG-dC pair, shows sequence dependence, and consumes one or two silver ions. These properties make the dZ-Ag⁺ -dC pair suitable for programmable incorporation of silver ions in DNA which cannot be achieved by canonical base pairs. If the silver ion content is higher than the total number of base pairs the duplexes turn into very stable structures in which all base pairs are considered to be in the silver-mediated pairing mode.

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.

Gemcitabine, Pyrrologemcitabine, and 2'-Fluoro-2'-Deoxycytidines: Synthesis, Physical Properties, and Impact of Sugar Fluorination on Silver Ion Mediated Base Pairing

The stability of silver-mediated "dC-dC" base pairs relies not only on the structure of the nucleobase, but is also sensitive to structural modification of the sugar moiety. 2'-Fluorinated 2'-deoxycytidines with fluorine atoms in the arabino (up) and ribo (down) configuration as well as with geminal fluorine substitution (anticancer drug gemcitabine) and the novel fluorescent phenylpyrrolo-gemcitabine (^ph PyrGem) have been synthesized. All the nucleosides display the recognition face of naturally occurring 2'-deoxycytidine. The nucleosides were converted into phosphoramidites and incorporated into 12-mer oligonucleotides by solid-phase synthesis. The addition of silver ions to DNA duplexes with a fluorine-modified "dC-dC" pair near the central position led to significant duplex stabilization. The increase in stability was higher for duplexes with fluorinated sugar residues than for those with an unchanged 2'-deoxyribose moiety. Similar observations were made for "dC-dT" pairs and to a minor extent for "dC-dA" pairs. The increase in silver ion mediated base-pair stability was reversed by annulation of a pyrrole ring to the cytosine moiety, as shown for 2'-fluorinated ^ph PyrGem in comparison with phenylpyrrolo-dC (^ph PyrdC). This phenomenon results from stereoelectronic effects induced by fluoro substitution, which are transmitted from the sugar moiety to the silver ion mediated base pairs. The extent of the effect depends on the number of fluorine substituents, their configuration, and the structure of the nucleobase.

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 Base Pairs in DNA Incorporating Purines, 7-Deazapurines, and 8-Aza-7-deazapurines: Impact of Reduced Nucleobase Binding Sites and an Altered Glycosylation Position

Formation of silver-mediated DNA was studied with oligonucleotides incorporating 8-aza-7-deazapurine, 7-deazapurine, and purine nucleosides. The investigation was performed on non-self-complementary duplexes with one or two modifications and self-complementary duplexes with an alternating dA-dT motif. Homo base pairs as well as base pair mismatches of dA analogues with dC and Watson-Crick pairs with dT were studied by stoichiometric silver ion titration and T_m measurements. N⁸ -Glycosylated 8-aza-7-deazaadenine forms silver-ion-mediated base pairs capturing two silver ions (low silver content) whereas regularly glycosylated 8-aza-7-deazapurine, 7-deazapurine (c⁷ A_d ), and dA do not form comparable structures. Stable silver-mediated "dA-dC" base pair mismatches were detected for all nucleosides. Two silver ions per base pair are bound by 8-aza-7-deazapurine whereas c⁷ A_d binds only one silver ion. The situation is different when the equivalents of silver ions were increased to the number of total base pairs. Surprisingly, in 12-mer duplexes as well as in related 25-mer duplexes every base pair consumed one silver ion.

A metal-mediated base pair that discriminates between the canonical pyrimidine nucleobases

A nucleoside analogue comprising the ligand 1H-imidazo[4,5-f][1,10]phenanthroline (P) was applied to develop a molecular beacon capable of discriminating the canonical nucleobases cytosine and thymine. The beacon is based on the formation of a stable Ag+-mediated base pair between P and cytosine, whereas the presence of Ag+ strongly destabilizes nucleic acids comprising an artificial base pair between P and thymine. Metal-mediated base pair formation was investigated by temperature-dependent UV spectroscopy and CD spectroscopy and complemented by extensive DFT calculations. The molecular beacon significantly extends the application spectrum of nucleic acids with metal-mediated base pairs. It is of potential use in the detection of single-nucleotide polymorphisms.

MercuryII-mediated base pairs in DNA: unexpected behavior in metal ion binding and duplex stability induced by 2′-deoxyuridine 5-substituents

DNA accepts small substituents at the 5-position of 2′-deoxyuridine residues within mercury ion mediated dU–Hg^II–dU base pairs, while triple bonds interact with mercury ions and those with space demanding aromatic side chains block metal ion mediated base pair formation.

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.

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.

Silver Ions in Non-canonical DNA Base Pairs: Metal-Mediated Mismatch Stabilization of 2'-Deoxyadenosine and 7-Deazapurine Derivatives with 2'-Deoxycytidine and 2'-Deoxyguanosine

Novel silver-mediated dA-dC, dA*-dC, and dA*-dG base pairs were formed in a natural DNA double helix environment (dA* denotes 7-deaza-dA, 7-deaza-7-iodo-dA, and 7-cyclopropyl-7-deaza-dA). 7-Deazapurine nucleosides enforce silver ion binding and direct metal-mediated base pair formation to their Watson-Crick face. New phosphoramidites were prepared from 7-deaza-dA, 7-deaza-7-iodo-dA, and 7-cyclopropyl-7-deaza-dA, which contain labile isobutyryl protecting groups. Solid-phase synthesis furnished oligonucleotides that contain mismatches in near central positions. Increased thermal stabilities (higher Tm values) were observed for oligonucleotide duplexes with non-canonical dA*-dC and dA-dC pairs in the presence of silver ions. The stability of the silver-mediated base pairs was pH dependent. Silver ion binding was not observed for the dA-dG mismatch but took place when mismatches were formed between 7-deazaadenine and guanine. The specific binding of silver ions was confirmed by stoichiometric UV titration experiments, which proved that one silver ion is captured by one mismatch. The stability increase of canonical DNA mismatches might have an impact on cellular DNA repair.

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.

A fluorescent surrogate of thymidine in duplex DNA

DMAT is a new fluorescent thymidine mimic composed of 2′-deoxyuridine fused to dimethylaniline.

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.