1H-NMR studies of a monointercalating drug into a d(CpGpApTpCpG)2 minihelix

Comparison of the bis-intercalating complexes formed between either ditercalinium or a flexible analogue and d(CpGpCpG)2 or d(TpTpCpGpCpGpApA)2 minihelices: 1H- and 31P-NMR analyses

The 400-MHz 1H- and 162-MHz 31P-nmr have been used to study complexes constituted by (a) the d(TpTpCpGpCpGpApA)2 or the d(CpGpCpG)2 self-complementary oligonucleotides and (b) two bifunctional 7H-pyrido [4,3-c] carbazole dimer drugs, the antitumoral ditercalinium (NSC 366241), a dimer with a rigid bis-piperidine linking chain and its pharmacologically inactive analogue, a dimer with a flexible spermine-like linking chain. Nearly all proton and phosphorus signals have been assigned by two-dimensional (2D) nmr (correlated spectroscopy, homonuclear Hartmann-Hahn, nuclear Overhauser enhancement spectroscopy, 2D 31P (1H) heteronuclear correlated spectroscopy and 31P-31P chemical exchange experiments). Both drugs bis-intercalate into the two CpG sites. The complexes show small differences in the position of the 7H-pyrido [4,3-c] carbazole ring into the intercalation site and possibly in the ribose-phosphate backbone deformation. However, the inactive analogue exhibits a longer residence lifetime in octanucleotide than the ditercalinium does. All these results are discussed in terms of differences in dimer activities.

1H- and 31P-NMR studies of ditercalinium binding to a d(GCGC)2 and d(CCTATAGG)2 minihelices: a sequence specificity study

The structures of the complexes formed in aqueous solution between ditercalinium, a bis-intercalating drug, and both the self-complementary tetranucleotide d(GCGC)2 and octanucleotide d(CCTATAGG)2, have been investigated by 400-MHz 1H-nmr and 162-MHz 31P-nmr. All the nonexchangeable protons, as well as the exchangeable imino protons and the phosphorus signals, have been assigned. Both oligonucleotides have been shown to adopt a right-handed B-DNA type structure. The addition of ditercalinium to the oligonucleotides lead to the formation of complexes in slow exchange at the nmr time scale with the free helices. At all drug-to-helix ratios studied, the ditercalinium was found in the bound form, whereas free and complexed oligonucleotides were in slow exchange, allowing resonance assignments through two-dimensional chemical exchange experiments. for d(GCGC)2 the strong upfield shifts induced on all aromatic protons of both the bases and the drug by complexation with ditercalinium suggest an interaction by intercalation of the two rings. However, the loss of twofold symmetry upon binding, as well as the chemical shift variation of the drug proton signals of one of the chromophores with temperature and concentration, favor a model in which the drug-nucleotide complexes have one ring of the drug intercalated and the other stacked on top of the external base pair. The intermolecular contacts between drug protons and nucleotide protons give a defined geometry for complexation that is consistent with the proposed model. In contrast, with d(CCTATAGG)2 several drug-nucleotide complexes were formed and a large increase in line broadening was observed at high drug-to-DNA ratios, precluding a detailed analysis of these complexes. However, the large upfield shift in the imino proton resonances together with the shielding of the ditercalinium ring protons favor a model with bis-intercalation of ditercalinium. This model is supported by the downfield shift of at least 4 out of 14 phosphorus signals. The results are compared with those obtained on ditercalinium binding to the homologous sequences d(CGCG)2 and d(TTCGCGAA)2, and discussed in terms of sequence specificity.

Bisintercalation of ditercalinium into a d(CpGpApTpCpG)2 minihelix: a 1H- and 31P-NMR study

The structure of the complex formed in aqueous solution between ditercalinium, a bisintercalating drug, and the self-complementary hexanucleotide d(CpGpApTpCpG)2 is investigated by 400-MHz 1H-nmr and 162-MHz 31P-nmr. Whatever the drug to helix ratio, ditercalinium occurred in the bound form, whereas free and complexed hexanucleotide are in slow exchange. This allows unambiguous resonance assignment through two-dimensional chemical exchange experiments. The strong upfield shifts measured on most aromatic protons on both drug and bases as well as on DNA imino protons are consistent with bisintercalation of the dimer. Nuclear Overhauser effects observed between drug and nucleotide protons give a defined geometry for complexation, and suggest a DNA conformational change upon drug binding.