Silver(I) Coordination in Silver(I)-Mediated Homo Base Pairs of 6-Pyrazolylpurine in DNA Duplexes Involves the Watson-Crick Edge

DNA duplexes comprising 6-(1H-pyrazol-1-yl)-9H-purine (6PP), 1-deaza-6PP ((1D)6PP), 7-deaza-6PP ((7D)6PP) and 1,7-dideaza-6PP ((1,7D)6PP) 2 '-deoxyribonucleosides, respectively, were investigated towards their ability to form metal-mediated base pairs in the presence of Ag-I. In 6PP and (7D)6PP, the Ag-I ion can coordinate to the nucleobase via the endocyclic N1 nitrogen atom, that is, via the Watson-Crick edge. In contrast, this nitrogen atom is not available in (1D)6PP and (1,7D)6PP, so that in (1D)6PP an Ag-I coordination is only possible via the Hoogsteen edge (N7). Reference duplexes with either adenine:adenine mispairs or canonical adenine:thymine base pairs were used to investigate the impact of the pyrazolyl moiety on the Ag-I-binding properties. To determine the thermal and structural duplex stabilities in the absence or presence of Ag-I, all duplexes were examined by UV and circular dichroism spectroscopic studies. These investigations shed light on the question of whether N1- or N7-coordination is preferred in purine-based metal-mediated base pairs.