DNA binding of Λ- and Δ-cis-β-[Ru(RR-picchxn)(phen)]2+ studied by NMR and flow linear dichroism spectroscopy

Two novel substitutionally-inert diastereomeric ruthenium(II) cations of the form Λ- and Δ-cis-β-[Ru(RR-picchxn)(phen)]²⁺, where RR-picchxn is N,N'-dimethyl-N,N'-di(2-picolyl)-1R, 2R-diaminocyclohexane and phen is 1,10- phenanthroline, have been studied with respect to their interactions with duplex DNA. NMR investigations show that both diastereomers bind to the oligonucleotide [d(CGCGATCGCG)]₂ in the fast exchange regime and that binding predominantly takes place in the minor groove of the oligonucleotide, but that the governing interactions are significantly different for the two A and A forms. Linear dichroism data support the latter interpretation, in that the relative orientations of cis-β-[Ru(RR-picchxn)(phen)]²⁺ to calf thymus DNA also are observed to differ for the Δ and Λ diastereomers. Interpretation of these data indicates the A form to be bound with the planar phen ligand closely parallel to the DNA base-pairs, but the average orientation of the phen ligand in the A form deviates significantly from a parallel alignment.