2,6-Bis(functionalized) purines as metal-ion-binding surrogate nucleobases that enhance hybridization with unmodified 2'-O-methyl oligoribonucleotides

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.

Sequence dependence of Pd(II)-mediated base pairing by palladacyclic nucleobase surrogates

A C-nucleoside derivative of phenylpyridine or the respective palladacycle was incorporated at either 3'- or 5'-terminus of a short oligodeoxynucleotide. Hybridization properties of these modified oligonucleotides were studied in a fluorescence-based competition assay in addition to conventional UV melting temperature analysis and compared with those of a previously prepared analogue featuring the modified nucleoside in the middle of the sequence. With the unpalladated phenylpyridine oligonucleotides, UV melting temperature qualitatively correlated with the ability to displace a strand from a double helix in the competition assay, decreasing in the order 5' > 3' > middle. Corresponding results on the palladacyclic oligonucleotides were more difficult to interpret but both UV melting and competition experiments revealed a decrease in the duplex stability upon palladation in most cases. On the other hand, dependence of the UV melting temperature on the identity of the canonical nucleobase opposite to the modified nucleobase analogue was much more pronounced with the palladacyclic duplexes than with their unpalladated counterparts. Furthermore, UV melting profiles of the palladacyclic duplexes featured an additional transition at a temperature exceeding the melting temperature of the unmodified part of the duplex. Taken together, these results lend support to the idea of Pd(II)-mediated base pairs that are highly stable but incompatible with the geometry of a double helix.

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.

Metal-Mediated Base Pairing of Rigid and Flexible Benzaldoxime Metallacycles

Oligonucleotides incorporating a central C-nucleoside with either a rigid or flexible benzaldoxime base moiety have been synthesized, and the hybridization properties of their metallacyclic derivatives have been studied by UV melting experiments. In all cases, the metallated duplexes were less stable than their unmetallated counterparts, and the metallacyclic nucleobases did not show a clear preference for any of the canonical nucleobases as a base-pairing partner. With palladated oligonucleotides, increased flexibility translated to less severe destabilization, whereas the opposite was true for the mercurated oligonucleotides; this reflects the greater difficulties in accommodating a rigid PdII -mediated base pair than a rigid HgII -mediated base pair within the base stack of a double helix.

Effects of metal ions on thermal stabilities of DNA duplexes containing homo- and heterochiral mismatched base pairs: comparison of internal and terminal substitutions

The effects of metal ions on the stabilities of duplexes containing a _D-homochiral and heterochiral mismatched base pairs were studied. In some duplexes containing an internal mismatched base pair, significant stabilization by Hg^II and Ag^I ions was observed. While, in duplexes containing a terminal mismatched base pair, only the duplexes containing T-T and _LT-T mispairs were significantly stabilized by Hg^II ions, and the stabilities of the duplexes containing T-T and _LT-T mispairs exceeded those of the corresponding homochiral matched duplex. The results suggest that the formation of homo- and heterochiral T-Hg^II-T base pairs at duplex termini would be useful for the thermal and enzymatic stabilization of DNA-based nanodevice.

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.

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.

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.

Fluorescent Oligonucleotide Probes for Screening High-Affinity Nucleobase Surrogates

Double-helical oligonucleotide probes featuring a single-nucleotide gap opposed by one of the canonical nucleobases and flanked by the fluorescent nucleobase analogue pyrrolocytosine have been synthesized and titrated with Pd^II chelates of dipicolinamide and its N² ,N⁶ -dialkylated derivatives. The fluorometric titrations revealed greatly increased affinity of the Pd^II chelates for the nucleobases opposing the gap compared to the respective free nucleotides in solution. Owing to the constrained environment of the single-nucleotide gap, the relative stabilities of the various Pd^II -mediated base pairs were also significantly different from those previously reported at monomer level.

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.

Pd(2+)-mediated base pairing in oligonucleotides

Two short glycol nucleic acid (GNA) oligonucleotides, having either a terminal or an intrachain nucleobase replaced by the pyridine-2,6-dicarboxamide chelate of Pd(2+), have been synthesized and their hybridization properties studied by melting temperature measurements. In the termini of a double-stranded oligonucleotide, the Pd(2+) chelates provided dramatic stabilization of the duplex relative to its metal-free counterpart, in all likelihood owing to formation of Pd(2+)-mediated base pairs between pyridine-2,6-dicarboxamide and the opposing nucleobase. In contrast, no stabilization was observed when the Pd(2+) chelate was placed in the middle of the chain. Furthermore, the results could not be reproduced by adding a Pd(2+) salt in situ to the dilute oligonucleotide solutions but the palladated oligonucleotides had to be synthesized and purified prior to the hybridization studies. This behavior, presumably attributable to the relatively slow ligand-exchange reactions of Pd(2+), differs greatly from what is usually observed with more labile metal ions. The present results offer an explanation for the failure of previous attempts to incorporate Pd(2+)-mediated base pairs into oligonucleotides.

Fluorescence probing of metal-ion-mediated hybridization of oligonucleotides

The structure-dependent fluorescence of pyrrolocytosine has been harnessed to quantify the affinity of metal-ion-chelating oligonucleotides for their native counterparts.

Formation of mixed-ligand complexes of Pd2+ with nucleoside 5'-monophosphates and some metal-ion-binding nucleoside surrogates

Formation of mixed-ligand Pd²⁺ complexes between canonical nucleoside 5'-monophosphates and five metal-ion-binding nucleoside analogs has been studied by ¹H-NMR spectroscopy to test the ability of these nucleoside surrogates to discriminate between unmodified nucleobases by Pd²⁺-mediated base pairing. The nucleoside analogs studied included 2,6-bis(3,5-dimethylpyrazol-1-yl)-, 2,6-bis(1-methylhydrazinyl)- and 6-(3,5-dimethylpyrazol-1-yl)-substituted 9-(β-d-ribofuranosyl)purines 1–3, and 2,4-bis(3,5-dimethylpyrazol-1-yl)- and 2,4-bis(1-methylhydrazinyl)-substituted 5-(β-d-ribofuranosyl)-pyrimidines 4–5. Among these, the purine derivatives 1-3 bound Pd²⁺ much more tightly than the pyrimidine derivatives 4, 5 despite apparently similar structures of the potential coordination sites. Compounds 1 and 2 formed markedly stable mixed-ligand Pd²⁺ complexes with UMP and GMP, UMP binding favored by 1 and GMP by 2. With 3, formation of mixed-ligand complexes was retarded by binding of two molecules of 3 to Pd²⁺.

Imidazolo-dC metal-mediated base pairs: purine nucleosides capture two Ag(+) ions and form a duplex with the stability of a covalent DNA cross-link

8-Phenylimidazolo-dC ((ph) ImidC, 2) forms metal-mediated DNA base pairs by entrapping two silver ions. To this end, the fluorescent "purine" 2'-deoxyribonucleoside 2 has been synthesised and converted into the phosphoramidite 6. Owing to the ease of nucleobase deprotonation, the new Ag(+) -mediated base pair containing a "purine" skeleton is much stronger than that derived from the pyrrolo- [3,4-d]pyrimidine system ((ph) PyrdC, 1). The silver-mediated (ph) ImidC-(ph) ImidC base pair fits well into the DNA double helix and has the stability of a covalent cross-link. The formation of such artificial metal base pairs might not be limited to DNA but may be applicable to other nucleic acids such as RNA, PNA and GNA as well as other biopolymers.

Metal-ion-mediated base pairing between natural nucleobases and bidentate 3,5-dimethylpyrazolyl-substituted purine ligands

The potential of three modified purine bases, namely, 6-(3,5-dimethylpyrazol-1-yl)purine, 2-(3,5-dimethylpyrazol-1-yl)hypoxanthine, and 2-(3,5-dimethylpyrazol-1-yl)adenine, for metal-ion-mediated base pairing within an oligonucleotide environment has been investigated. The respective modified nucleosides were incorporated in the middle of 9-mer 2'-O-methyl oligonucleotides and the hybridization of these modified oligonucleotides with their unmodified counterparts studied by UV and CD spectrometry in the absence and presence of Cu(2+) or Zn(2+). All of the modified oligonucleotides formed more stable duplexes in the presence of divalent metal ions than in the absence thereof, but with different preferences for the complementary oligonucleotide. The oligonucleotide incorporating 2-(3,5-dimethylpyrazol-1-yl)hypoxanthine readily accepted any of the natural nucleobases opposite to this modified base regardless of whether Cu(2+) or Zn(2+) was used as the bridging metal ion. The other two oligonucleotides, on the other hand, were much more discriminating, exhibiting markedly elevated Tm values only in the presence of Cu(2+) and only when certain natural nucleobases were paired with the modified one. The origin of the selectivity (or promiscuity) of the metal-ion-mediated base pairing is discussed in terms of the ability of the modified nucleobases, as well as their natural counterparts, to serve as anionic ligands.

Second generation silver(I)-mediated imidazole base pairs

The imidazole-Ag(I)-imidazole base pair is one of the best-investigated artificial metal-mediated base pairs. We show here that its stability can be further improved by formally replacing the imidazole moiety by a 2-methylimidazole or 4-methylimidazole moiety. A comparison of the thermal stability of several double helices shows that the addition of one equivalent of Ag(I) leads to a 50% larger increase in the melting temperature when a DNA duplex with methylated imidazole nucleosides is applied. This significant effect can likely be attributed to a better steric shielding of the metal ion within the metal-mediated base pair.

Interaction of Pd(2+) complexes of 2,6-disubstituted pyridines with nucleoside 5'-monophosphates

To learn more about the underlying principles of metal-ion-mediated recognition of nucleic acid bases, PdCl(+) complexes of six 2,6-disubstituted pyridines, viz. pyridine-2,6-dicarboxamide, its N(2),N(6)-dimethyl and N(2),N(6)-diisopropyl derivatives, 6-carbamoylpyridine-2-carboxylic acid, 6-aminomethylpyridine-2-carboxamide and its N(2)-methyl derivative, were prepared and their interaction with nucleoside 5'-monophosphate (NMP) was studied by (1)H NMR spectroscopy in D2O at pH7.2. The binding sites within the nucleobases were assigned on the basis of Pd(2+) induced changes in chemical shifts of the base moiety proton resonances. The mole fractions of NMPs engaged in mono- or dinuclear Pd(2+) complexes were determined at various concentrations by comparing the intensities of the aromatic and anomeric protons of the complexed and uncomplexed NMPs. Some of the pyridine complexes showed moderate discrimination between the NMPs.

A family of "click" nucleosides for metal-mediated base pairing: unravelling the principles of highly stabilizing metal-mediated base pairs

A family of artificial nucleosides has been developed by applying the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition. Starting from 2-deoxy-β-D-glycosyl azide as a common precursor, three bidentate nucleosides have been synthesized. The 1,2,3-triazole involved in all three nucleobases is complemented by 1,2,4-triazole (TriTri), pyrazole (TriPyr), or pyridine (TriPy). Molecular structures of two metal complexes indicate that metal-mediated base pairs of TriPyr may not be fully planar. An investigation of DNA oligonucleotide duplexes comprising the new "click" nucleosides showed that they can bind Ag(I) to form metal-mediated base pairs. In particular the mispair formed from TriPy and the previously established imidazole nucleoside is significantly stabilized in the presence of Ag(I). A comparison of different oligonucleotide sequences allowed the determination of general factors involved in the stabilization of nucleic acids duplexes with metal-mediated base pairs.

Ag+-mediated DNA base pairing: extraordinarily stable pyrrolo-dC-pyrrolo-dC pairs binding two silver ions

6-Substituted pyrrolo-dC-pyrrolo-dC mismatches selectively capture silver ions to form extraordinarily stable metal-mediated base pairs. One single modification in a 12-mer duplex causes a Tm increase of 36.0 °C relative to the metal-free mismatched duplex. Spectrophotometric titrations as well as ESI mass spectra confirmed the binding of two silver ions per base pair. The Ag(+)-mediated base pairs may permit the construction of metal-responsive DNA with a very high silver loading.