Hole Transfer Kinetics of DNA

Not long after the discovery of the double-helical structure of DNA in 1952, researchers proposed that charge transfer along a one-dimensional pi-array of nucleobases might be possible. At the end of the 19905 researchers discovered that a positive charge (a hole) generated in DNA migrates more than 200 angstrom along the structure, a discovery that ignited interest in the charge-transfer process in DNA. As a result, DNA became an interesting potential bottom-up material for constructing nanoelectronic sensors and devices because DNA can form various complex two-dimensional and three-dimensional structures, such as smiley faces and cubes. From the fundamental aspects of the hole transfer process, DNA is one of the most well-studied organic molecules with many reports on the synthesis of artificial nucleobase analogues. Thus, DNA offers a unique system to study how factors such as the HOMO energy and molecular flexibility affect hole transfer kinetics. Understanding the hole transfer mechanism requires a discussion of the hole transfer rate constants (k(HT)). This Account reviews the k(HT) values determined by our group and by Lewis and Wasielewski's group, obtained by a combination of the synthesis of modified DNA and time-resolved spectroscopy. DNA consists of G/C and A/T base pairs; the HOMO localizes on the purine bases G and A, and G has a lower oxidation potential and a higher energy HOMO. Typically, long-range hole transfer proceeded via sequential hole transfer between G/C's. The kinetics of this process in DNA sequences, including those with mismatches, is reproducible via kinetic modeling using the determined k(HT) for each hole transfer step between G/C's. We also determined the distance dependence parameter (beta), which describes the steepness of the exponential decrease of k(HT). Because of this value, >0.6 angstrom(-1) for hole transfer in DNA, DNA itself does not serve as a molecular wire. Interestingly, hole transfer proceeded exceptionally fast for some sequences in which G/C's are located close to each other, an observation that we cannot explain by a simple sequential hole transfer between G/C's but rather through hole delocalization over the nudeobases. To further investigate and refine the factors that affect k(HT), we examined various artificial nucleobases. We clearly demonstrated that k(HT) depends strongly on the HOMO energy gap between the bases (Delta(HOMO)), and that k(HT) can be increased with decreasing Delta(HOMO). We reduced Delta(HOMO) between the two type of base pairs by replacing adenines (A's) with deazaadenines (z)A's) or diaminopurines (D's) and showed that the hole transfer rate through the G/C and A/T mix sequence increased by more than 3 orders of magnitude. We also investigated how DNA flexibility affects k(HT). Locked nucleic add (LNA) modification, which makes DNA more rigid, lowered k(HT) by more than 2 orders of magnitude. On the other hand, 5-Me-2'-deoxyzebularine (B) modification, which increases DNA flexibility, increased k(HT) by more than 1 order of magnitude. These new insights in hole transfer kinetics obtained from modified DNAs may aid in the design of new molecular-scale conducting materials.