Direct observation of hole transfer through double-helical DNA over 100 Å

Abstract

Mechanism of photo-induced electron transfer and the subsequent hole transfer in DNA has been studied extensively, but so far we are not aware of any reliable report of the observation of the long-distance hole-transfer process. In this article, we demonstrate the results of direct observation for the long-distance hole transfer in double-helical DNA over 100 Å with time-resolved transient absorption measurements. DNA conjugated with naphthalimide (NI) and phenothiazine (PTZ) (which worked as electron-acceptor and donor molecules, respectively) at both 5 ends was synthesized to observe the hole-transfer process. Site-selective charge injection into G by means of the adenine-hopping process was accomplished by excitation of NI with a 355-nm laser flash. Transient absorption around 400 nm, which was assigned to the NI radical anion, was observed immediately after the irradiation of a laser flash, indicating that the charge separation between NI and the nearest G occurred. Then, the transient absorption of the PTZ radical cation (PTZ •؉ ) at 520 nm was emerged, which was attributed to the hole transfer through DNA to the PTZ site. By monitoring the time profiles of the transient absorption of PTZ •؉ for NI-A6-(GA)n-PTZ and NI-A6-(GT)n-PTZ (n ‫؍‬ 2, 3, 4, 6, 8, 12) (base sequences correspond to those for DNA modified with NI), the long-distance hole-transfer process from G to PTZ, which occurred in the time scale of microsecond to millisecond, was observed directly. By assuming an average distance of 3.4 Å between base-pairs, total distance reaches 100 Å for n ‫؍‬ 12 sequences. Our results clearly show the direct observation of the long-distance hole transfer over 100 Å.D ouble-helical DNA, which has unique structural properties, is a promising candidate for molecular electronic devices and a scaffold for two-and three-dimensional nanostructures (1, 2). Mechanistic studies of charge transfer in DNA have attracted considerable attention because of their relevance to the development of DNA molecular wires (3) and the involvement in DNA oxidative lesion and strand cleavage (4, 5). There are many mechanistic studies for the ''one-dimensional'' conductivity in the double helix (6-8). Factors controlling charge-transfer properties in DNA (such as electronic coupling between donor and acceptor, structural dynamics, reorganization energy, etc.) have been discussed (9-14), and the mechanism for charge transfer, especially hole transfer over long distances in DNA, is the subject of experimental and theoretical studies.Generally, experimental studies of charge transfer in DNA have been performed by time-resolved measurements and electrophoresis analysis (15)(16)(17)(18)(19). A model of a multistep holehopping mechanism, which was proposed by Giese and coworkers (20), is the most widely adopted. An alternative mechanism in which delocalized hole migrates by means of a polaron-like mechanism was proposed by Schuster and coworkers (18). Strand-cleavage studies using PAGE analysis have provided valuable information about the seque...

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