Distance dependence of electron transfer in acridine-intercalated DNA

The photoinduced electron-transfer reaction in a DNA helix has been studied using 9-amino-6-chloro-2-methoxyacridine (ACMA) selectively intercalated at an internucleotide site of a DNA. The photoexcited ACMA is used as an electron acceptor, and guanine in DNA as an electron donor. ACMA-DNA conjugates possessing guanine at 5' or 3' direction(s) with different distance(s) from the intercalated ACMA have been prepared for a systematic examination of the distance dependence of the electron-transfer reaction in a DNA helix. In DNA consisting of only the (dA-dT) base pair, the fluorescence of the singlet-excited ACMA decays in a mono-exponential manner with a longest Lifetime of 22.8 ns. The fluorescence lifetime of the excited ACMA decreases with incorporation of guanosine depending on the distance between the ACMA and guanine. When an ACMA and a guanine are directly stacked, the decay Lifetime markedly decreases, showing a forward electron-transfer rate, k(fet), of approximate to 10(10) s(-1). Treating the kinetic data according to k(fet) = Aexp(-beta R), where R is the separation distance in Angstrom, gives a beta value of 1.47 Angstrom(-1). Moreover, as a model compound for ACMA, the redox potential of quinacrine dihydrate bi (tetrafluoroborate) has been measured by cyclic voltammetry in acetonitrile. Comparison of the redox potential with those of nucleobases has revealed that only guanine can quench the fluorescence of ACMA and a free-energy change Delta G degrees = -0.46 eV has been evaluated for this electron-transfer reaction. (C) 1999 Elsevier Science S.A. All rights reserved.