The DNA binding of the ΔΔ-, ΔΛ-, and ΛΛ-[{Ru(Me₂bpy)₂}₂(μ-bpm)]⁴⁺

¹H NMR spectroscopy has been used to study the dodecanucleotide binding of the rac (ΔΔ- and ΛΛ-) and meso (ΔΛ stereoisomers of the dinuclear species [{Ru(Me₂bpy)₂}₂(μ-bpm)]⁴⁺ (Me₂bpy = 4,4'-dimethyl-2,2'-bipyridine; bpm = 2,2'-bipyrimidine). The addition of the ΔΔ- and ΛΛ-enantiomers to the dodecanucleotide d(CAATCCGGATTG)₂ induced different shifts for the resonances from each enantiomer. On binding, the meso diastereoisomer exhibited twice the number of resonances observed for the free metal complex, identified as two sets of resonances with shifts similar to those induced by the ΔΔ- and ΛΛ-enantiomers of the rac form. In NOESY spectra of d(CAATCCGGATTG)₂ bound with each stereoisomer, NOE's from the metal complex were observed to the dodecanucleotide H1' and H4' protons, indicating that [{Ru(Me₂bpy)₂}₂(μ-bpm)]⁴⁺ bound in the dodecanucleotide minor groove. The strongest NOE's from the metal complex to d(CAATCCGGATTG)₂ were observed for the ΛΛ-isomer. The observed pattern of intermolecular NOE's indicated that the ΛΛ-isomer bound at the central CC/GG site as well as at the terminal CA/GT sequence of the dodecanucleotide. By contrast, NOE data of the ΛΛ-isomer associated with a second dodecanucleotide d(CAATCGCGATTG)₂ indicated that the ΛΛ-isomer bound at the AAT/ATT sequence as well as at the terminal base pairs in this case. It is proposed that [{Ru(Me₂bpy)₂}₂(μ-bpm)]⁴⁺ will only bind in the DNA minor groove: however, as the metal complex is not easily accommodated in the groove because of its size, it will only bind at sites where the minor groove is more open