Dppz ligand

When the ruthenium-modified oligomer is annealed to its unmetal-lated complement, the metal complex intercalates and intense luminescence is observed (77). By contrast, the ruthenium-modified oligonucleotide alone or in the presence of noncomplementary single-stranded DNA displays little luminescence. These results are consistent with previous studies luminescence is observed in aqueous solution only when the stacked bases of a DNA helix provide a platform for intercalation of the dppz ligand. 

Table III shows that the luminescent lifetimes and the relative luminescent intensities for the covalently bound duplex and its noncovalent analogue are similar. As with [Ru(phen)2(dppz)]2+, a biexponential decay in emission is observed for the ruthenated oligonucleotide hybridized to its complement. A small shift in the wavelength of maximum emission is also observed compared to the noncovalent complex. This shift likely reflects the sensitivity in emission to the stacking of the oriented dppz ligand a dependence of the maximum emission wavelength on base...

Ru(tpy)(dppz)OH2 (Fig. 12) decreases as the concentration of the buffer cation is increased (55). From Eq. (1), we can determine the binding parameters for comparison to those of classical intercalators, such as ethidium bromide. The measured values for K, which reflect the contribution to the binding affinity that does not arise from electrostatic forces, are essentially identical for ethidium (AG° = -6.5 kcal/ mol) and Ru(tpy)(dppz)OH2 (-5.9 kcal/mol). Thus, this experiment shows rigorously that the intercalation ability of the coordinated dppz ligand is the same as for a traditional intercalator. 

Fig. 12. (A) X-ray crystal structure of Ru(tpy)(dppz)OH2 taken from Gupta et al. (123). Copyright 1992 VCH Publishers. (B) The dppz ligands undergo extensive rr-stacking in the crystal, which is consistent with the high affinity of Ru(tpy)(dppz)OH2 for intercalation.

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