Hydration and solution structure of d(CGCAAATTTGCG)₂ and its complex with propamidine from NMR and molecular modelling

The hydration of the d(CGCAAATTTGCG)₂ duplex and its complex with a propamidine reporter ligand has been examined in aqueous solution by two-dimensional NMR at two spectrometer frequencies and three temperatures. Quantitative analysis of ROESY and NOESY cross-peaks showed effective correlation times of ~0.5 ns at 283 K for DNA-water interactions in the major groove. In some cases the sign of the NOE inverts on changing either the temperature or spectrometer frequency. Larger effective correlation times of ~1 ns were observed for water interactions with A5(H2) and A6(H2) atoms located in the minor groove. Interproton NOEs and changes in chemical shifts showed that propamidine binds in the minor groove 5'-AATTT region of the host duplex, but does not displace waters adjacent to either A5(H2) or A6(H2). In the complex, the effective correlation times of these waters increase more than two-fold, possibly as a result of stabilisation due to H-bonded interaction with the amidine groups of the ligand. Hydration of the bound molecule was also found, suggesting that water may contribute to the DNA binding process for bis(amidine) drugs. Structure refinement by a NOE-restrained dynamic annealing procedure revealed that ligand binding is non-centrosymmetric with respect to the duplex, in accordance with the energetically favoured 5'-ATT (≡5'-AAT) sites predicted by analytical molecular modelling. In particular, the bound propamidine spans 3-4 base pairs in the A6-T7-T8 tract and makes close H-bonded contacts with A(N3/O4') acceptors positioned close to the minor groove floor. The refined NMR structure for the DNA-propamidine complex is compared with that determined recently using X-ray crystallographic methods.