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.

Reactions of Pd(II) with chelate-tethered 2,6-diaminopurine derivatives: N3-coordination and reaction of the purine system.

Alkyldiamine-tethered derivatives of 2,6-diaminopurine, ethylenediamine-N9-propyl-2,6-diaminopurine, L1, and ethylenediamine-N9-ethyl-2,6-diaminopurine, L2, react with Pd(II) to give N3-coordinated complexes. However, the exact nature of the resulting complex is dependent on the reaction conditions. With PdCl(2)(MeCN)(2) in MeCN/H(2)O the expected [PdCl(N3-2,6-DAP-alkyl-en)](+) complex, 1, is formed with L1 chelating the metal center in a tridentate manner through the diamine function and N3 of the purine base. However, under the same conditions the shorter, ethyl-tethered, L2 gives a complex dication, 2, containing a tetradentate ligand forming simultaneously 5-, 6-, and 7-membered chelate rings. This resulting acetamidine, derived by addition to coordinated MeCN, appears to be the first such case involving the 2-amino group of a purine. The ethyl-analogue of 1, [PdCl(N3-2,6-DAP-Et-en)](+) 3, was prepared by reaction of L2 with K(2)PdCl(4) in aqueous media.