Solution structure and behaviour of Δ-cis-α-[Ru(R,R-picchxnMe2)(phi)]2+ by NMR spectroscopy and molecular modelling

Abstract

The self-association of the DNA metalloprobe Δ-cis-α-[Ru(R,R-picchxnMe₂)(phi)]²⁺ (α-phi) in aqueous solution has been investigated using ¹H NMR spectroscopy and molecular modelling. The concentration dependence of proton chemical shifts of the complex gave initial indications of a self-associated species, while its structural isomer Δ-cis-β-[Ru(R,R-picchxnMe₂)(phi)]²⁺ (β-phi) showed no such dependence. 2D-COSY and 2D-ROESY experiments were used for the complete assignment of the proton resonances of both isomers and allowed a qualitative determination of the self-association of the α isomer through the detection of intermolecular ROEs. NMR spectroscopy can also be effectively used to differentiate Δ- and Λ-diastereomers. In addition, we show, by pulsed field gradient longitudinal eddy-current delay (PFGLED) NMR spectroscopy, that α-phi self-associates at higher concentrations with an effective molecular weight at 25 mM three times that at 2.5 mM. This apparent oligomerisation was not observed for the β-isomer.