Interest in using DNA as a building block for nanoelectronic sensors and devices stems from its efficient hole-conducting properties and the relative ease with which it can be organized into predictable nanometre-sized two- and three-dimensional structures. However, because a hole migrates along DNA through the highest occupied molecular orbital of the guanine bases, its conductivity decreases as the adenine-thymine base-pair content increases. This means that there are limitations on what sequences can be used to construct functional nanoelectronic circuits, particularly those rich in adenine-thymine pairs. Here we show that the charge-transfer efficiency can be dramatically increased in a manner independent of guanine-cytosine content by adjusting the highest occupied molecular orbital level of the adenine-thymine base pair to be closer to that of the guanine-cytosine pair. This is achieved by substituting the N7 nitrogen atom of adenine with a C-H group to give 7-deazaadenine, which does not disturb the complementary base pairing observed in DNA.
A positive charge migrates along DNA mainly via a series of short-range charge transfer (CT) processes between G-C base pairs, which have relatively high HOMO levels. As such, the CT efficiency sharply decreases with the insertion of A-T base pairs between the G-C base pairs. We have previously demonstrated that the CT efficiency through DNA can be dramatically increased by using deazaadenine (Z), an analogue of A, to adjust the HOMO levels of the A-T base pairs closer to those of the G-C base pairs (Nat. Chem. 2009, 1, 156). In the present study, we have expanded this approach to show that the CT efficiency can also be increased by replacing A bases with diaminopurine (D).
By decreasing the HOMO energy gap between the base-pairs to increase the charge conductivity of DNA, the positive charge photochemically generated in DNA can be made to migrate along the π-way of DNA over long distances to form a long-lived charge-separated state. By fine-tuning the kinetics of the charge-transfer reactions, we designed a functionalized DNA system in which absorbed photon energy is converted into chemical energy to form I-I covalent bonds, where more than 100 I(2) molecules were produced per functionalized DNA. Utilizing the fact that charge-transfer kinetics through DNA is sensitive to the presence of a single mismatch that causes the perturbation of the π-stacks, single nucleotide polymorphisms (SNPs) in genomic sequences were detected by measuring the photon energy conversion efficiency in DNA by a conventional starch iodine method.
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