Silver Ions in Non-canonical DNA Base Pairs: Metal-Mediated Mismatch Stabilization of 2'-Deoxyadenosine and 7-Deazapurine Derivatives with 2'-Deoxycytidine and 2'-Deoxyguanosine.

Novel silver-mediated dA-dC, dA*-dC, and dA*-dG base pairs were formed in a natural DNA double helix environment (dA* denotes 7-deaza-dA, 7-deaza-7-iodo-dA, and 7-cyclopropyl-7-deaza-dA). 7-Deazapurine nucleosides enforce silver ion binding and direct metal-mediated base pair formation to their Watson-Crick face. New phosphoramidites were prepared from 7-deaza-dA, 7-deaza-7-iodo-dA, and 7-cyclopropyl-7-deaza-dA, which contain labile isobutyryl protecting groups. Solid-phase synthesis furnished oligonucleotides that contain mismatches in near central positions. Increased thermal stabilities (higher Tm values) were observed for oligonucleotide duplexes with non-canonical dA*-dC and dA-dC pairs in the presence of silver ions. The stability of the silver-mediated base pairs was pH dependent. Silver ion binding was not observed for the dA-dG mismatch but took place when mismatches were formed between 7-deazaadenine and guanine. The specific binding of silver ions was confirmed by stoichiometric UV titration experiments, which proved that one silver ion is captured by one mismatch. The stability increase of canonical DNA mismatches might have an impact on cellular DNA repair.