DNA Binding by a New Metallointercalator that Contains a Proflavine Group Bearing a Hanging Chelating Unit

The new bifunctional molecule 3,6‐diamine‐9‐[6,6‐bis(2‐aminoethyl)‐1,6‐diaminohexyl]acridine (D), which is characterised by both an aromatic moiety and a separate metal‐complexing polyamine centre, has been synthesised. The characteristics of D and its ZnII complex ([ZnD]) (protonation and metal‐complexing constants, optical properties and self‐aggregation phenomena) have been analysed by means of NMR spectroscopy, potentiometric, spectrophotometric and spectrofluorimetric techniques. The equilibria and kinetics of the binding process of D and [ZnD] to calf thymus DNA have been investigated at I=0.11 M (NaCl) and 298.1 K by using spectroscopic methods and the stopped‐flow technique. Static measurements show biphasic behaviour for both D–DNA and [ZnD]–DNA systems; this reveals the occurrence of two different binding processes depending on the polymer‐to‐dye molar ratio (P/D). The binding mode that occurs at low P/D values is interpreted in terms of external binding with a notable contribution from the polyamine residue. The binding mode at high P/D values corresponds to intercalation of the proflavine residue. Stopped‐flow, circular dichroism and supercoiled‐DNA unwinding experiments corroborate the proposed mechanism. Molecular‐modelling studies support the intercalative process and evidence the influence of NH+⋅⋅⋅O interactions between the protonated acridine nitrogen atom and the oxygen atoms of the polyanion; these interactions play a key role in determining the conformation of DNA adducts. A new proflavine‐based intercalator and its ZnII complex form DNA adducts according to intercalative and groove‐binding processes, as shown by means of spectroscopic methods and stopped‐flow measurements. Molecular modelling studies support both binding processes and provide evidence for the influence of NH+⋅⋅⋅O interactions between the protonated acridine nitrogen and the oxygen atoms of the polyanion; this promotes the formation of DNA adducts in which the intercalator penetrates perpendicularly to the base pair hydrogen bonds (see picture).