Binding of 3-aminoacridinium and 7-aminoquinolinium monocations to double-stranded DNA

Abstract

The complexations of the singly charged cations of 3-aminoacridine and 7-aminoquinoline by double-stranded, calf thymus DNA were studied by electronic absorption spectrophotometry. At high ratios of total DNA phosphate concentration to total probe concentration (the region associated with intercalative binding), four DNA phosphate units are associated with each bound monocation. However, a binding isotherm based on four DNA phosphate groups in a single binding site does not yield a reproducible equilibrium constant for the binding process. Rather, the sequestration of the monocations by two binding sites, each containing two DNA phosphate units, yields a satisfactory equilibrium expression. This result suggests that the intercalative mode of binding by DNA is not a simple one-step process, which is in agreement with previous kinetic studies. In the region of low DNA phosphate concentration to monocation concentration (the external binding region), the binding is described adequately by a linear isotherm.