Light-induced formation of silver(I)-mediated base pairs in DNA: Possibilities and limitations

Modulating aptamer function by copper(II)-mediated base pair formation

Two aptamers, one for ATP and one for arginine, were modified using an artificial 2'-dexoyribonucleoside based on the nucleobase surrogate imidazole-4-carboxylate. This synthetic nucleoside substitute does not engage in hydrogen bonding but is capable of forming Cu(II)-mediated base pairs instead. Hence, the addition of Cu(II) can be used to influence the ability of the aptamer derivatives to adopt the correct fold necessary for binding their respective target molecule. As a result, aptamer function can be modulated via the addition of Cu(II). The extent of modulation ability depends on the identity of the aptamer and on the exact location of the artificial nucleosides within the oligonucleotide sequence.

Tuning DNA Supramolecular Polymers by the Addition of Small, Functionalized Nucleobase Mimics

Nucleobase mimicking small molecules able to reconfigure DNA are a recently discovered strategy that promises to extend the structural and functional diversity of nucleic acids. However, only simple, unfunctionalized molecules such as cyanuric acid and melamine have so far been used in this approach. In this work, we show that the addition of substituted cyanuric acid molecules can successfully program polyadenine strands to assemble into supramolecular fibers. Unlike conventional DNA nanostructure functionalization, which typically end-labels DNA strands, our approach incorporates functional groups into DNA with high density using small molecules and results in new DNA triple helices coated with alkylamine or alcohol units that grow into micrometer-long fibers. We find that small changes in the small molecule functional group can result in large structural and energetic variation in the overall assembly. A combination of circular dichroism, atomic force microscopy, molecular dynamics simulations, and a new thermodynamic method, transient equilibrium mapping, elucidated the molecular factors behind these large changes. In particular, we identify substantial DNA sugar and phosphate group deformations to accommodate a hydrogen bond between the phosphate and the small-molecule functional groups, as well as a critical chain length of the functional group which switches this interaction from intra- to interfiber. These parameters allow the controlled formation of hierarchical, hybrid DNA assemblies simply through the addition and variation of small, functionalized molecules.

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.

Incorporation of a metal-mediated base pair into an ATP aptamer - using silver(I) ions to modulate aptamer function

For the first time, a metal-mediated base pair has been used to modulate the affinity of an aptamer towards its target. In particular, two artificial imidazole 2’-deoxyribonucleosides (Im) were incorporated into various positions of an established ATP-binding aptamer (ATP, adenosine triphosphate), resulting in the formation of three aptamer derivatives bearing Im:Im mispairs with a reduced ATP affinity. A fluorescence spectroscopy assay and a binding assay with immobilized ATP were used to evaluate the aptamer derivatives. Upon the addition of one Ag(I) ion per mispair, stabilizing Im–Ag(I)–Im base pairs were formed. As a result, the affinity of the aptamer derivative towards ATP is restored again. The silver(I)-mediated base-pair formation was particularly suitable to modulate the aptamer function when the Im:Im mispairs (and hence the resulting metal-mediated base pairs) were located close to the ATP-binding pocket of the aptamer. Being able to trigger the aptamer function opens new possibilities for applications of oligonucleotides.

Dynamic Structure and Stability of DNA Duplexes Bearing a Dinuclear Hg(II)-Mediated Base Pair

Quantum mechanical (QM) and hybrid quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations of a recently reported dinuclear mercury(II)-mediated base pair were performed aiming to analyse its intramolecular bonding pattern, its stability, and to obtain clues on the mechanism of the incorporation of mercury(II) into the DNA. The dynamic distance constraint was employed to find initial structures, control the dissociation process in an unbiased fashion and to determine the free energy required. A strong influence of the exocyclic carbonyl or amino groups of neighbouring base pairs on both the bonding pattern and the mechanism of incorporation was observed. During the dissociation simulation, an amino group of an adenine moiety of the adjacent base pair acts as a turnstile to rotate the mercury(II) ion out of the DNA core region. The calculations provide an important insight into the mechanism of formation of this dinuclear metal-mediated base pair and indicate that the exact location of a transition metal ion in a metal-mediated base pair may be more ambiguous than derived from simple model building.