AG(I) MODIFIED BASE-PAIRS INVOLVING COMPLEMENTARY (G, C) AND NONCOMPLEMENTARY (A, C) NUCLEOBASES - ON THE POSSIBLE STRUCTURAL ROLE OF AQUA LIGANDS IN METAL-MODIFIED NUCLEOBASE PAIRS

Two Ag(I) complexes containing the model nucleobases 1-methylcytosine (1-MeC), 9-methyladenine (9-MeA), and 7,9-dimethylguanine (7,9-DimeG) have been prepared and studied: [Ag(1-MeC-N3)(9-MeA-N7)(H2O)]NO3 (2) crystallizes in the space group P2(1)/a, a = 11.167 (2) angstrom, b = 13.437 (2) angstrom, c = 11.520 (2) angstrom, beta = 109.79 (2)-degrees, V = 1626 (i) angstrom3, z = 4. Ag has a severely distorted trigonal-planar coordination geometry with two strong bonds (2.128 (2) angstrom and 2.120 (2) angstrom) to the nitrogens of the nucleobases and a weak bond (2.664 (2) angstrom) to a water molecule. The N-Ag-N vector is markedly nonlinear (angle at Ag 165.8 (1)-degrees). Both nucleobases and the water molecule are virtually coplanar. Intramolecular H bonding is between O(2) of 1-MeC and N(6) of 9-MeA (3.053 (3) angstrom) as well as between N(4) of 1-MeC and the aqua ligand (2.894 (4) angstrom). Structural details of the structure of 2 clearly demonstrate that the water molecule is an integral part of the "metal-modified" base pair. A second mixed-nucleobase complex of composition [Ag(1-MeC)(7,9-DimeG)NO3]2.[(1-MeC)(7,9-DimeGH)PF6].10H2O (3) has been prepared and characterized by elemental analysis and H-1 NMR spectroscopy. On the basis of the structure of 2 an alternative model to existing hypotheses on Ag-DNA interactions is put forward which considers the "insertion" of a metal-aqua entity into an existing base pair.