Control of electron-transfer and DNA binding properties by the tolyl spacer group in viologen linked acridines

Abstract

Novel water soluble viologen and pyridinium linked tolylacridines 1a,b and 2a,b were synthesized and their photophysical and DNA binding properties including the photoinduced electron-transfer reactions were investigated. When compared to the cases of the model tolylacridines 3a,b and the pyridinium linked derivatives 2a,b, the singlet excited states of 1a and 1b were efficiently quenched in water and methanol. Intramolecular quenching rate constants (k_ET) calculated in water are found to be 1.2 × 10¹⁰ and 8.8 × 10¹⁰ s⁻¹ for 1a and 1b and 1.4 × 10⁸ and 0.9 × 10⁸ s⁻¹ for 2a and 2b, respectively, suggesting thereby that the viologen moiety quenches the fluorescence of the acridine chromophore efficiently when compared to the case of the pyridinium moiety. From the intermolecular electron-transfer studies, it was observed that the singlet and triplet excited states of the acridine chromophore are capable of donating an electron to the viologen moiety. DNA binding studies indicated that the p-tolylacridine derivatives 1a and 2a exhibit strong binding to DNA with binding constants of 1.0 × 10⁵ and 3.3 × 10⁵ M⁻¹, respectively, whereas the o-tolylacridine derivatives 1b and 2b showed negligible affinity for DNA. The rate constants for the static quenching of 1a and 2a by DNA (k_DNA) are found to be 7 × 10⁹ and 3 × 10⁹ s⁻¹, respectively, indicating that 1a is an efficient DNA oxidizing agent. Nanosecond laser flash photolysis studies of these systems in aqueous solutions did not show any transients. However, in the presence of DNA, 1a gave transient absorption due to the reduced methyl viologen radical cation. These results demonstrate that the tolyl spacer group in these systems constitutes an interesting variation which controls both the electron transfer and DNA binding properties, and hence, such molecules and derivatives thereof can have potential application as probes for nucleic acids and as DNA cleaving agents.