DNA minor groove recognition by a tetrahydropyrimidinium analogue of hoechst 33258: NMR and molecular dynamics studies of the complex with d(GGTAATTACC)2

Hoechst 43254 (H43254), a 2,3,4,5-tetrahydropyrimidin-1-ium analogue of the bis-benzimidazole minor groove binding agent Hoechst 33258 (H33258), has been studied by NMR and restrained molecular dynamics in its complex with d(GGTAATTACC)2. We investigate the origin of the enhanced complex stability afforded by the replacement of the N-methylpiperazine ring of H33258 with the tetrahydropyrimidinium ring of H43254, the latter presenting the opportunity for specific minor groove-directed recognition through a pyrimidinium NH. A set of 25 drug-DNA NOEs define the binding site with some precision and are used as part of the structural analysis using restrained molecular dynamics simulations considering explicit solvation and the treatment of electrostatic interactions using the particle mesh Ewald method within AMBER 4.1. Starting with three different initial structures with the drug located at different sites in the groove (pairwise RMSD 4.3-12.6 A) we arrive at three very similar structures (pairwise RMSD 0.80-1.34 A) representing one converged binding site at the centre of the AATT tract. Two of the three structures show the tetrahydropyrimidinium ring to be suitably positioned for an -NH to adenine N3 hydrogen bond suggesting that electrostatic interactions may play an important role in the enhanced affinity as well as imparting additional A-T specificity. The NMR data show that the pyrimidinium NH interaction is dynamic since signal averaging from the two sides of the ring indicate rapid rotations in the bound form.