| Researches about the conducting ability of natural DNA have obtained controversy results:insulators, semi-conductors, conductors or even super-conductors. As revealed, the conductivity of natural DNA greatly rely on its sequence, environment and connect atoms with electrode and also other factors. Great space exists between its practical applications as molecular wires and theoretical research. In recent years, the modification of DNA and improvement of its conductivity have been under intensive study, for example, artificial base derivatives and analogs (hole travels along DNA through the stacking interactions between joint pairs). Among various modifications, metal-mediated pairs and size-expanded bases are in highlight. On the basis of previous works, for DNA modification and improvement, a series of significant work has been carried out in our lab and gained interesting conclusions on these issues. The primary innovations can be described as follows.(1) Multi-Copper-Mediated DNA Base Pairs Acting as Suitable Building Blocks for the DNA-based Nanowires:It has been proven that DNA can be turned into semiconductor or even conductor by suitable transition metal modifications. Then, researches about M-base and M-DNA have been under intensive study. There have been many single-metal-mediated pairs but not multiple-metal-mediated pairs. We predict that more metal should have larger impact on pairs. Hence, a three-copper-mediated guanine-cytosine (G3CuC) and a two-copper-mediated adenine-thymine (A2CuT) base pairs were designed by equi-stoichiometric H-by-Cu replacements in our paper, and theoretically explored and thoroughly discussed. Geometrically, G3CuC and A2CuT have great resemblances to the natural GC and AT with a size-expansion of about 1.0 A due to the larger radii of Cu(I). Their significantly larger binding energies promise them to be structurally suitable for DNA helix construction. Electronically, the equi-number H-by-Cu replacement not only leads to considerable reductions of the HOMO-LUMO gaps and ionization potentials, but also enhances transverse electronic communication within isolated G3CuC and A2CuT pairs, revealed by the charge-transfer transitions in the UV absorption spectra of G3CuC and A2CuT. G3CuC possesses the smallest HOMO-LUMO gaps. It can be reasonably concluded that the multi-Cu-mediated G3CuC and A2CuT pairs are promising candidates for building blocks of the Cum-DNA nanowires and Cum-DNA would have better conducting ability than natural DNAs. Interestingly, from electrical aspect, the relationship between G3CuC and A2CuT is similar to that between natural GC and AT, providing useful information for aim-directed designs of DNA-based wires. This work would open a new prospective for rational design of the DNA-based molecular wires by multi-metal incorporation.(2) Rational Design of Ring-outer-stretched Analogues of G and A: Size-expanded bases are one kind of artificial bases and they origin from fusion of natural base and an aromatic ring, for example, x-base, y-base, yy-base and hetero-ring bases and so on. Related studies also showed these size-expanded bases can selectively pairing with their corresponding natural partner bases and featured narrower HOMO-LUMO gaps, lower ionization potentials (IP), or being fluorescent. That is attributed to the expanded aromatic regions by fusion of an aromatic ring, and it is an effective way to improve base properties by insertion of an aromatic ring. However, it should be pointed out that relative to the natural ones xbase, ybases or the hetero-ring-inserted bases are all size-expanded, and also are the pairs of them with their natural partners. Consequently, single or partial replacement of natural bases by the corresponding expanded bases would induce structural distortion of sugar-phosphate backbone and impair the stable and double-helical DNA. This highly demands that designed bases should be electronically improved and meanwhile of natural sizes. Coincidently, Isao Saito et al designed a DNA wire possessing a extremely high hole transport ability by replacing the natural A bases between two GGG runs by its derivatives by fusion of a aromatic ring onto at N7-C8 site from the outside. Just like other artificial bases with larger aromatic areas, all of these structurally ring-outer-stretched A-derivatives have smaller HOMO-LUMO gaps, lower ionization potentials and also suppressed oxidative degradation. It inspired us to re-consider whether there would be a way to improve the electronic properties of natural DNA bases while preserving their natural sizes/whether there can be a way to avoid the passive effect while improving the electronic properties. It is thought that an aromatic ring attached to bases from the outside should be an answer for it. Inspired by it, herein, we designed a series of derivatives of natural A and G bases by fusions of aromatic rings onto them at N7-C8 from the outside, producing benzodeazapurine (BDG and BDA), H2b-methoxy-substituted benzodeazapurine (MDG and MDA) and C2b-N-substituted benzodeazapurine (PDG and PDA). They are shorted as *G and *A. Structurally, they preserve the same sizes of natural purine bases, enhancing their arbitrary to randomly incorporate into DNAs. Meanwhile, they have the preference to selectively pair with their natural counterpart C and T bases and form stable DNA duplexes. Electronically, they are all improved, with narrowed HOMO-LUMO gaps and lowered IPs, enhancing their ability to produce holes and hole-transfer ability. Among the three modification methods, MDG and MDA are most effective, followed by BDG and BDA, while PDG and PDA are relatively worst but still better than natural G and A bases. As the HOMO energy levels concerned, MDA is the best candidate of natural adenine, while PDA cannot be applied because it probably even blockages charge traveling. In the whole, the G-analogs and A-analogs are electronically improved, as good as x-bases and y-bases, while being of natural size and with more decoration opportunities. |
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