DNA minor groove recognition by bis-benzimidazole analogues of Hoechst 33258:: insights into structure-DNA affinity relationships assessed by fluorescence titration measurements

Fluorescence titration measurements have been used to examine the binding interaction of a number of analogues of the bis-benzimidazole DNA minor groove binding agent Hoechst 33258 with the decamer duplex d(GCAAATTTGC)(2). The method of continuous variation in ligand concentration (Job plot analysis) reveals a 1:1 binding stoichiometry for all four analogues; binding constants are independent of drug concentration (in the range [ligand] = 0.1-5 mu M). The four analogues studied were chosen in order to gain some insight into the relative importance of a number of key structural features for minor groove recognition, namely (i) steric bulk of the N-methylpiperazine ring, (ii) ligand hydrophobicity, (iii) isohelicity with the DNA minor groove and (iv) net ligand charge. This was achieved, first, by replacing the bulky, non-planar N-methylpiperazine ring with a less bulky planar charged imidazole ring permitting binding to a narrower groove, secondly, by linking the N-methylpiperazine ring to the phenyl end of the molecule to give the molecule a more linear, less isohelical conformation and, finally, by introducing a charged imidazole ring in place of the phenolic OH making it dicationic, enabling the contribution of the additional electrostatic interaction and extended conformation to be assessed. Delta G values were measured at 20 degrees C in the range -47.6 to -37.5 kJ mol(-1) and at a number of pH values between 5.0 and 7.2. We find a very poor correlation between Delta G values determined by fluorescence titration and effects of ligand binding on DNA melting temperatures, concluding that isothermal titration methods provide the most reliable method of determining binding affinities. Our results indicate that the bulky N-methylpiperazine ring imparts a large favourable binding interaction, despite its apparent requirement for a wider minor groove, which others have suggested arises in a large part from the hydrophobic effect. The binding constant appears to be insensitive to the isohelical arrangement of the constituent rings which in these analogues gives the same register of hydrogen bonding interactions with the floor of the groove.