Excited-State Proton Transfers in 9-Aminoacridine Carboxamides

Excited-state proton transfers have been investigated for several isomeric and homologous N-((dimethylamino)-alkyl)-9-aminoacridine carboxamides in a time-correlated single photon counting fluorescence study. The pH behavior of these DNA intercalators' steady-state absorption and fluorescence spectra has also been investigated. While the excited-state pK sub(a)* for removal of a proton from the singly protonated species is close to the ground-state pK sub(a) in the parent molecule 9-aminoacridine, the presence of the carboxamide substituent renders pK sub(a)* much lower than pK sub(a) for the corresponding deprotonation in the present compounds. This fact, along with the presence of an additional deprotonation site at the distal N atom of the side chain, endows these molecules with complex pH dependence in their excited-state kinetics. Triexponential model functions are generally required to fit the experimental fluorescence decays. The pH dependence of the component lifetimes and preexponential factors can be simulated with a kinetic model for sequential deprotonations involving four aminoacridine species in both the ground and fluorescing states. These results provide a baseline study for comparisons with these intercalators' excited-state kinetics in DNA.