Probing Metal-Ion Purine Interactions at DNA Minor-Groove Sites

The effect of the 2-amino group on metal ion binding at the N3-position of a purine base has been investigated using chelate-tethered derivatives. Reactions of diamine-tethered 2,6-diaminopurine (DAP) with divalent d-block metal ions Cu(II) and Cd(II) confirm that binding can occur, but this is much less prevalent than with adenine. In this regard DAP is similar to guanine where we have previously observed a general lack of N3-binding by divalent metal ions compared to adenine (e.g., Houlton et al., Angew. Chem., Int. Ed. 2000, 39, 2360; Chem.Eur. J. 2000, 6, 4371). For the univalent d-block metals ions, Cu(I) and Ag(I), binding to adenine N3 is not observed in the solid state, as shown by reactions with dithioether-tethered adenine derivatives. Instead, depending on stoichiometry of the reaction, discrete (with metal/ligand ratio 1:2) or polymeric (with metal/ligand ratio 1:1) complexes were isolated and characterized by single crystal X-ray methods. In the former the nucleobases are pendant and involved in base-pair interactions, with both Watson−Crick···Watson−Crick and Hoogsteen···Hoogsteen type pairings present. For the coordination polymers a rather unexpected influence of the tether length on the site of nucleobase binding is found for bridging ligand binding modes involving the chelating diamine and the adeninyl group. Polymer chains derived with the shorter ethyl tether show binding at the N7 site of adeninyl, while binding at N1 is found in the longer propyl chain length.