Chiral-at-Metal Silver-Mediated Base Pairs: Metal-Centred Chirality versus DNA Helical Chirality.

When covalently incorporating ligands capable of forming chiral metal complexes into a DNA oligonucleotide duplex, an enantiospecific formation of metal-mediated base pairs is possible. We have been investigating the chirality of the silver-mediated base pair P-Ag -P (P, 1H-imidazo[4,5-f][1,10]phenanthroline) depending on the number of consecutive P : P pairs within a series of duplexes. Towards this end, both enantiomers of the nucleoside analogue 3-(1H-imidazo[4,5-f][1,10]phenanthrolin-1-yl)propane-1,2-diol comprising an acyclic backbone were introduced into DNA duplexes, resulting in diastereomeric metal-mediated base pairs. The same chiral-at-metal complex is formed inside the duplex for up to five neighbouring P-Ag -P pairs, irrespective of whether (S)-P or (R)-P is used. With six silver-mediated base pairs, the chirality of the metal complex is inverted for (S)-P but not for (R)-P. This indicates an intricate balance of what determines the configuration of the metal complex, the intrinsically preferred metal-centred chirality or the DNA helical chirality.