Total enantioselectivity in the DNA binding of the dinuclear ruthenium(II) complex [{Ru(Me2bpy)2}2(μ-bpm)]4+ {bpm=2,2′-bipyrimidine; Me2bpy=4,4′-dimethyl-2,2′-bipyridine}

The binding of the three stereoisomers (DeltaDelta-, LambdaLambda- and DeltaLambda-) of the dinuclear ruthenium(II) complex [{Ru(Me(2)bpy)(2)}(2)(mu-bpm)](4+) {Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine; bpm = 2,2'-bipyrimidine} to a tridecanucleotide containing a single adenine bulge has been studied by H-1 NMR spectroscopy. The addition of the DeltaDelta-isomer to d(CCGAGAATTCCGG)(2) induced significant chemical shift changes for the base and sugar resonances of the residues at the bulge site (G(3)A(4)G(5)/C11C10), whereas small shifts were observed upon addition of the enantiomeric LambdaLambda-form. NOESY spectra of the tridecanucleotide bound with the DeltaDelta-isomer revealed intermolecular NOE's between the metal complex and the nucleotide residues at the bulge site, while only weak NOE's were observed to terminal residues to the LambdaLambda-form. Competitive binding studies were performed where both enantiomers were simultaneously added to the tridecanucleotide, and for all ratios of the two stereoisomers the DeltaDelta-isomer remained selectively bound at the bulge site with the LambdaLambda-enantiomer localised at the terminal regions of the tridecanucleotide. The meso-diastereoisomer (DeltaLambda) was found to bind to the tridecanucleotide with characteristics intermediate between the DeltaDelta- and LambdaLambda-enantiomers of the rac form. Two distinct sets of metal complex resonances were observed, with one set having essentially the same shift as the free metal complex, whilst the other set of resonances exhibited significant shifts. The NOE data indicated that the meso-diastereoisomer does not bind as selectively as the DeltaDelta-isomer, with NOE's observed to a greater number of nucleotide residues compared to the DeltaDelta-form. This study provides a rare example of total enantioselectivity in the binding of an inert transition metal complex to DNA, produced by the shape recognition of both ruthenium(II) centres.