Photophysical properties of N-alkylated azahelicene derivatives as DNA intercalators: counterion effects

In this work, three compounds having the same organic moiety (N-methyl-5-azahelicenium salts) but different counterions (I⁻, NO₃⁻ and COOCF₃⁻) have been investigated in buffered aqueous solutions and in the presence of DNA to give information on the counterion effects on the binding. In particular, the absorption spectra, fluorescence quantum yields and fluorescence lifetimes in aqueous solution for free organic molecules have been determined by steady-state and time-resolved spectrofluorimetric measurements. The obtained values are compared with those of the chromophores in the presence of increasing concentrations of DNA. The results allow determination of the association constants (K_a) and the number of base couples per chromophore molecule (n) by means of the McGhee Von Hippel model. The binding parameters are strongly affected by the nature of counterions since the highest K_a value was determined for the compound having COOCF₃⁻; on the other hand the NO₃⁻ derivative is able to interact with the highest number of binding sites. The morphology and structural properties of the DNA–chromophore complexes were investigated by circular dichroism (CD) and atomic force microscopy (AFM). The data revealed that I⁻ and COOCF₃⁻ derivatives preferentially form intercalation complexes, while the NO₃⁻ salt is able to form intercalation and grove binding complexes at the same time.