Conformational tuning of magnetic interactions in metal-DNA complexes

Triggering Copper(II)-Mediated Base Pair Formation by Light

Metal-mediated base pairs enable a site-specific incorporation of transition metal ions into nucleic acid structures. The resulting nucleic acid-metal complex conjugates are of interest in the context of functionalized nucleic acids, as they bear metal-based functionality. It is desirable to devise nucleic acids with an externally triggered metal-binding affinity, as this may allow regulating this functionality. Toward this end, a caged deoxyribonucleoside analog H^NPP was devised for the site-specific binding of copper(II) ions upon irradiation by light, based on the ligand 3-hydroxy-2-methylpyridin-4(1H)-one (H) and the photocleavable 2-(2-nitrophenyl)propoxy protecting group (NPP). The formation of both H-Cu(II)-H homo base pairs and H-Cu(II)-X hetero base pairs (involving a second artificial deoxyribonucleoside X, based on imidazole-4-carboxylate) was achieved upon irradiation of DNA duplexes bearing the respective H^NPP:H^NPP or H^NPP:X mispairs in the presence of copper(II) ions. The H-Cu(II)-X pair shows an exceptional DNA duplex stabilization of up to 43 °C upon its formation, exceeding that of the H-Cu(II)-H pair. It therefore represents one of the most stabilizing Cu(II)-mediated base pairs reported so far. Our findings expand the scope of light-triggered metal-mediated base pair formation by introducing a copper(II)-binding ligand.

Ferromagnetic TM2BC (TM = Cr, Mn) monolayers for spintronic devices with high Curie temperature

Transition metal boro-carbide (TM2BC) structures crystallize in the layered orthorhombic structure in their bulk phases. In this study, however, we find that TM2BC (TM = Cr, Mn) prefer a tetragonal (t) crystal structure in their monolayer phases due to the occurrence of strong sp2 bonds between the metal and B/C atoms instead of sp3 + sp2 bonds which exist in the orthorhombic phase. The calculated energy difference between the orthorhombic and the tetragonal structures based on density functional theory (DFT) is more than 1 eV per unit cell. In addition, t-Cr2BC and t-Mn2BC monolayers are dynamically and thermally stable with their magnetic metal electronic structures. For further investigations, we combine our DFT calculations with the Monte Carlo simulations and find that both t-TM2BC monolayers show ferromagnetic properties. The calculated Curie temperatures are 846 K and 128 K for t-Cr2BC and t-MnBC sheets, respectively. In addition, we examine the magnetic anisotropy energies (MAE) of t-TM2BC monolayers and find that both structures prefer out-of-plane as the easy axis magnetization direction and the applied electric field can easily modulate the MAE of the monolayers. Our theoretical calculations reveal that t-TM2BC (TM = Cr, Mn) sheets have great potential for the future design of controllable spintronic devices with their tunable MAE properties.

Short DNA Oligonucleotide as a Ag+ Binding Detector

Ag⁺ has been known to mediate several natural metallo-base pairs. Based on the unique structural information of a short 8-mer DNA strand (5'-GCACGCGC-3') induced by Ag⁺, we constructed several fluorescent DNA beacons for the detection of Ag⁺ according to the increase in the fluorescence emission on Ag⁺ binding. This Ag⁺ detection assay is quick, sensitive, and easy to adapt and can function in a wide range of temperatures from 5 to 65 °C.

A Novel DNA Helical Wire Containing HgII -Mediated T:T and T:G Pairs

Numerous applications of metal-mediated base pairs (metallo-base-pairs) to nucleic acid based nanodevices and genetic code expansion have been extensively studied. Many of these metallo-base-pairs are formed in DNA and RNA duplexes containing Watson-Crick base pairs. Recently, a crystal structure of a metal-DNA nanowire with an uninterrupted one-dimensional silver array was reported. We now report the crystal structure of a novel DNA helical wire containing Hg^II -mediated T:T and T:G base pairs and water-mediated C:C base pairs. The Hg-DNA wire does not contain any Watson-Crick base pairs. Crystals of the Hg-DNA wire, which is the first DNA wire structure driven by Hg^II ions, were obtained by mixing the short oligonucleotide d(TTTGC) and Hg^II ions. This study demonstrates the potential of metallo-DNA to form various structural components that can be used for functional nanodevices.

Construction and characterization of metal ion-containing DNA nanowires for synthetic biology and nanotechnology

DNA is an attractive candidate for integration into nanoelectronics as a biological nanowire due to its linear geometry, definable base sequence, easy, inexpensive and non-toxic replication and self-assembling properties. Recently we discovered that by intercalating Ag+ in polycytosine-mismatch oligonucleotides, the resulting C-Ag+-C duplexes are able to conduct charge efficiently. To map the functionality and biostability of this system, we built and characterized internally-functionalized DNA nanowires through non-canonical, Ag+-mediated base pairing in duplexes containing cytosine-cytosine mismatches. We assessed the thermal and chemical stability of ion-coordinated duplexes in aqueous solutions and conclude that the C-Ag+-C bond forms DNA duplexes with replicable geometry, predictable thermodynamics, and tunable length. We demonstrated continuous ion chain formation in oligonucleotides of 11-50 nucleotides (nt), and enzyme ligation of mixed strands up to six times that length. This construction is feasible without detectable silver nanocluster contaminants. Functional gene parts for the synthesis of DNA- and RNA-based, C-Ag+-C duplexes in a cell-free system have been constructed in an Escherichia coli expression plasmid and added to the open-source BioBrick Registry, paving the way to realizing the promise of inexpensive industrial production. With appropriate design constraints, this conductive variant of DNA demonstrates promise for use in synthetic biological constructs as a dynamic nucleic acid component and contributes molecular electronic functionality to DNA that is not already found in nature. We propose a viable route to fabricating stable DNA nanowires in cell-free and synthetic biological systems for the production of self-assembling nanoelectronic architectures.

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.

Flexibility and stabilization of HgII-mediated C:T and T:T base pairs in DNA duplex

Owing to their great potentials in genetic code extension and the development of nucleic acid-based functional nanodevices, DNA duplexes containing Hg^II-mediated base pairs have been extensively studied during the past 60 years. However, structural basis underlying these base pairs remains poorly understood. Herein, we present five high-resolution crystal structures including one first-time reported C–Hg^II–T containing duplex, three T–Hg^II–T containing duplexes and one native duplex containing T–T pair without Hg^II. Our structures suggest that both C–T and T–T pairs are flexible in interacting with the Hg^II ion with various binding modes including N3–Hg^II–N3, N4–Hg^II–N3, O2–Hg^II–N3 and N3–Hg^II–O4. Our studies also reveal that the overall conformations of the C–Hg^II–T and T–Hg^II–T pairs are affected by their neighboring residues via the interactions with the solvent molecules or other metal ions, such as Sr^II. These results provide detailed insights into the interactions between Hg^II and nucleobases and the structural basis for the rational design of C–Hg^II–T or T–Hg^II–T containing DNA nanodevices in the future.

Computational design of three Cu-induced triangular pyrimidines based DNA motifs with improved conductivity

Novel DNA triangular pyrimidine derivatives are designed by metal decoration through replacement of H by Cu in the Watson–Crick hydrogen bond region. The DFT method is used to examine the coordination of triangle-arranged Cu with three pyrimidines in nonplanar three-bladed turbine geometries. The Cu···Cu cuprophilic bonds are ascribed to the partially occupied d orbitals without direct molecular orbital (MO) interactions. Four-center bonds depend on Cu–N/O bonds, which are contributed by p orbitals of N/O atoms along or perpendicular to the bond axis. The activity of frontier MOs is modulated, leading to the decrease of gaps, ionization potentials (IPs), and electron affinities (EAs) desired for the improvement of conductivity. The hole trapping ability is assured by virtue of the spin density distributed on Cu. On average, the single electron density is located on π orbitals of three aromatic base rings. There is paramagnetic electron delocalization on the inner d orbitals of triangle region. The analysis of electron localization function ELF-π and electrostatic potential maps reveals that the outer strong π–π stacking interaction together with the inner d orbital channel enable effective transduction of electrical signals along the Cu–DNA nanowires. The 3Cu-induced triangular pyrimidines have important potential applications as structural motifs of molecular electronic devices.

Intensified effects of multi-Cu modification on the electronic properties of the modified base pairs containing hetero-ring-expanded pyrimidine bases

Novel DNA base pair derivatives (A2CunU, A2CunC, G3CunU, and G3CunC) are designed by aromatic expansion of pyrimidine bases with four kinds of hetero-rings (denoted by nC and nU, n = 1, 2, 3, and 4) and metal-decoration through Cu replacement of hydrogens in the Watson-Crick hydrogen bond region. Their structures and properties are calculated for examining the cooperative effects of the two modification ways. The calculated results reveal that multiple Cu decoration makes up the deficiencies of size-expansion, and exhibits not only increase of structural stability and reduction of ionization potentials, but also ideal shrink of the HOMO-LUMO gaps, notable enhancement of interbase coupling as well as remarkable redshifts of π → π* transitions for all M-x modified base pairs. The decrease extents of the gaps and ionization potentials follow the same order G3CunU > G3CunC > A2CunU > A2CunC, and in each series (denoted by different n), the gaps, ionization potentials and first π → π* transition energies have an order of 4 < 1 < 2 < 3. The Cu d orbitals function as bridges for π electron delocalization on the conjugated aromatic rings of two bases, leading to an enhancement of transverse electronic communication, as verified by spin density delocalization, orbital composition changes, redshift of the π → π* transition and also advocated by the electron-sharing indexes such as delocalization index, Mayer bond orders and multicenter bonding. Electron localization function ELF-π isosurfaces above the molecular plane further suggested that effective longitudinal conduction is closely relevant with the bicyclic domain involving good electron delocalization and strong π-π stacking between layers. This work presents theoretical evidence for the cooperative effects of metal decoration and ring-expansion modifications on the electronic properties of the modified base pairs and also proves that the base pairs designed here could be competent building blocks for the DNA-based nanowires with improved electron activity and excellent conductivity.

Magnetostructural Dynamics from Hubbard-U Corrected Spin-Projection: [2Fe-2S] Complex in Ferredoxin

A Hubbard-corrected spin-projected two-determinant approach, EBS+Uscf, is introduced to treat low-spin ground states of antiferromagnetically coupled transition metal complexes. In addition to providing access to total energies, forces, and ab initio simulations, it allows one to readily compute Heisenberg's exchange coupling J(t) on the fly. By studying the binuclear [2Fe-2S] cofactor in a metalloprotein, Anabaena Fd, within this consistent nonempirical procedure in combination with a QM/MM framework, it is illustrated that spin-projection, self-interaction corrections, thermal fluctuations, and protein matrix shifts are crucial in obtaining ⟨J⟩ close to the experiment.

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

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

Synthesis and properties of a Cu(II) complexing pyrazole ligandoside in DNA

The development of metal base pairs is of immense importance for the construction of DNA nanostructures. Here we report the synthesis of a biaryl pyrazole-phenol nucleoside that forms in DNA a stable self-pair upon complexation of a Cu(II) ion. A sequence with five consecutive pyrazole nucleotides allows the complexation of five Cu(II) ions in a row.

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.

Crystal structure of metallo DNA duplex containing consecutive Watson-Crick-like T-Hg(II)-T base pairs

The metallo DNA duplex containing mercury-mediated T-T base pairs is an attractive biomacromolecular nanomaterial which can be applied to nanodevices such as ion sensors. Reported herein is the first crystal structure of a B-form DNA duplex containing two consecutive T-Hg(II)-T base pairs. The Hg(II) ion occupies the center between two T residues. The N3-Hg(II) bond distance is 2.0 Å. The relatively short Hg(II)-Hg(II) distance (3.3 Å) observed in consecutive T-Hg(II)-T base pairs suggests that the metallophilic attraction could exist between them and may stabilize the B-form double helix. To support this, the DNA duplex is largely distorted and adopts an unusual nonhelical conformation in the absence of Hg(II). The structure of the metallo DNA duplex itself and the Hg(II)-induced structural switching from the nonhelical form to the B-form provide the basis for structure-based design of metal-conjugated nucleic acid nanomaterials.

Structural and magnetic properties of a variety of transition metal incorporated DNA double helices

By using density functional theory calculations, the structural, energetic, magnetic, and optical properties for a variety of transition metal (M = Mn, Fe, Co, Ni and Cu) ions incorporated modified-DNA (M-DNA) double helices has been investigated. The DNA is modified with either hydroxypyridone (H) or bis(salicylaldehyde)ethylenediamine (S-en) metalated bases. We find the formation of extended M-O network leading to the ferromagnetic interactions for the case of H-DNA for all the metal ions. More ordered stacking arrangement was found for S-en-DNA. We calculate the exchange coupling constant (J) considering Heisenberg Hamiltonian for quantitative description of magnetic interactions. The ferromagnetic and antiferromagnetic interactions are obtained by varying different transition metal ions. The extent of the magnetic interaction depends on the number of transition metal ions. Optical profiles show peaks below 2 eV, a clear signature of spin-spin coupling.

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.

Alternative DNA base pairing through metal coordination

Base-pairing in the naturally occurring DNA and RNA oligonucleotide duplexes is based on π-stacking, hydrogen bonding, and shape complementarity between the nucleobases adenine, thymine, guanine, and cytosine as well as on the hydrophobic-hydrophilic balance in aqueous media. This complex system of multiple supramolecular interactions is the product of a long-term evolutionary process and thus highly optimized to serve its biological functions such as information storage and processing. After the successful implementation of automated DNA synthesis, chemists have begun to introduce artificial modifications inside the core of the DNA double helix in order to study various aspects of base pairing, generate new base pairs orthogonal to the natural ones, and equip the biopolymer with entirely new functions. The idea to replace the hydrogen bonding interactions with metal coordination between ligand-like nucleosides and suitable transition metal ions culminated in the development of a plethora of artificial base-pairing systems termed "metal base-pairs" which were shown to strongly enhance the DNA duplex stability. Furthermore, they show great potential for the use of DNA as a molecular wire in nanoscale electronic architectures. Although single electrons have proven to be transmitted by natural DNA over a distance of several base pairs, the high ohmic resistance of unmodified oligonucleotides was identified as a serious obstacle. By exchanging some or all of the Watson-Crick base pairs in DNA with metal complexes, this problem may be solved. In the future, these research efforts are supposed to lead to DNA-like materials with superior conductivity for nano-electronic applications. Other fields of potential application such as DNA-based supramolecular architecture and catalysis may be strongly influenced by these developments as well. This text is meant to illustrate the basic concepts of metal-base pairing and give an outline over recent developments in this field.

Transverse electric field modulated tunneling magnetoresistance in a DNA molecular device

Quantum spin-dependent transport in a ferromagnetic(FM)/DNA/ferromagnetic(FM) device is theoretically investigated based on the lattice Green function method and the Landauer-Büttiker theory. The effect of a transverse electric field on magnetoresistance (MR) of the device is investigated. It is predicted that either the direction or strength of the transverse electric field can change the MR of the device. We suggest a possible application of modulating MR of the FM/DNA/FM device by a transverse electric field.

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.