Mode of binding of camptothecins to double helix oligonucleotides

We report an NMR study on the interaction of topotecan (Tpt) and other camptothecins (Cpts) with several double helix and single strand oligonucleotides. The results obtained by (31)P NMR spectroscopy, nuclear Overhauser experiments (NOE) and molecular dynamics (MD) simulations show that Cpt drugs do not intercalate into the double helix, as suggested by many authors. Phosphorus NMR spectra indicated that no deformation occurs at any level of the phosphodiester backbone, while 2D NOESY experiments allowed the detection of several contacts between the aromatic protons of Cpts and those of the double helix. Models of the drug/oligonucleotide complexes, built on the basis of NOE data, show that the drug is located at the end of the double helix, by stacking the A and B rings with the guanine or cytidine of the terminal CG base pairs, with a preference for the 3[prime or minute]-terminal end sites. Cpts interact with double strand, as well as with single strand oligomers, as can be seen from the NMR shift variation observed on the drug protons; but this shielding effect cannot be an evidence of intercalation, as it is largely due to external non-specific interactions of the positively charged drug with the negatively charged ionic surface of the oligonucleotide. The molecular weight of one of the complexes was obtained from the correlation time value. The conformational behaviour of the DNA fragment d(CGTACG)(2) was studied by MD simulations on a ns time scale in the presence of water molecules and Na(+) ions. Different models were examined and the deformations induced on the phosphodiester backbone by molecules that are known to intercalate, were monitored by MD simulations.