Theoretical Insights into Metallo-DNA Nanowires

Expanding the versatility of nucleic acids has always been a compelling research focus within the field of molecular engineering. Specifically, the interaction between metal ions and DNA has been widely explored in the past five decades. A notable strategy for modifying DNA involves demonstrating the specific binding of Hg²⁺ ions to T:T mismatched base pairs in double-stranded DNA (dsDNA). Similarly, the specificity of Ag⁺ ions in C:C base pairs has also been reported. More recently, advancements have been made in illustrating the transmetalation of DNA with mercury, silver, and gold in various naturally occurring and synthetically engineered DNA bases. As demonstrated in previous studies, these mono-/di-coordinated base pairs can be utilized as a template to build long metallo-DNA nanowires. However, the underlying charge transport mechanism of these complex metal-DNA compounds is largely unexplored.
In this work, we have elucidated the underlying conduction mechanism in both short and extended metal-DNA complexes, focusing specifically on metallo-DNA nanowires composed of C:Ag⁺:C building blocks. Our computational analysis provides insight into three key aspects: (i) the eigenstates of the nanowire (ii) the coupling strength (hopping integral) between consecutive building blocks and (iii) the delocalization of frontal orbitals (HOMO). We employed density functional theory (DFT)-based charge transport calculations with a mixed functional approach i.e., LANDL2Z (B3LYP) for metal (non-metal) atoms with 6-31(d,p) basis set in addition to LANL2 pseudopotential for metal atoms. We observe that for longer metallo-DNA nanowires HOMO energy is primarily delocalized over the metal atoms. This finding contradicts our earlier studies, where we reported metal atoms predominantly contributing to LUMO energy. Further analysis underscores the critical role of metallo-DNA nanowire length in significantly influencing the charge transport properties. Specifically, we observed that nanowires comprising more than three building blocks exhibit substantial HOMO orbital delocalization on Ag⁺ atoms, a trend also evident in longer nanowires with up to 20 building blocks. In contrast, LUMO orbitals are consistently found to be localized on the DNA bases. Collectively, these findings strongly suggest that charge transport in these systems is mediated by metal atoms, rather than the DNA bases. Following up, we noticed a striking increase in coupling between successive base pairs in metalated DNA nanowires compared to their non-metalated counterparts. More precisely, the electronic coupling between adjacent metallo-DNA building blocks at the HOMO level is computed to be 150 meV, which is notably 40% higher than that of non-metalated poly-GC nanowires. The enhanced coupling signifies better carrier transport through these biological nanowires, as larger hopping integrals help mitigate the impact of fluctuations on carrier transmission. Additionally, low-lying LUMO energy levels mediated by Ag⁺ exhibit electronic couplings exceeding 350 meV between neighboring metallo-DNA building blocks. We posit that the substantial coupling between metal atoms will enhance the robustness of charge transport in these nanowires as corroborated by Green's function-based transport calculations.
Our findings provide a promising design framework for tuning the electrical conductivity of these nanowires, which could spur future experimental endeavors. Moreover, the computational insights gained from our study can serve as a foundation for developing tight-binding models of metallo-DNA nanowires, further driving exploratory research into these intriguing molecular systems.