Dynamics and thermodynamics of the counterion effect in a 7H-pyridocarbazole dimer (ditercalinium). Hypothesis of a nonbisintercalative binding mode to calf thymus DNA at high drug/base ratio.

Ditercalinium, a 7H-pyridocarbazole dimer designed to bisintercalate into DNA, forms tight ion pairs in water with inorganic and organic anions. The thermodynamics and kinetics of the acetate-ditercalinium pairing has been investigated by means of T-jump spectroscopy. The formation of the pair has a constant estimated to 1000 M-1 and proceeds via a fast two-step mechanism with a relaxation time of 12 microseconds (acetate pH 5) to 50 microseconds (cacodylate, pH 7.5) involving an intermediate solvent-separated ion pair. A strong association of ditercalinium to cardiolipid has been observed and is expected to be involved in the respiratory chain inhibition induced by ditercalinium (unpublished results). Direct estimates of the binding constants of the drug to calf thymus DNA were obtained by means of UV titrations at high drug/base ratio (greater than 0.17). The maximum number of binding sites per base both at pH 5 and pH 7.5 was found to be 0.22, a value consistent with monointercalation as expected from the prediction of Shafer's model for the interaction of bifunctional ligands to DNA. This work also supports the hypothesis that significant ionic binding may account for the ditercalinium/DNA interaction at high base/drug ratios (0.2).