Ru(phen)_2dppz^(2+) Luminescence: Dependence on DNA Sequences and Groove-Binding Agents

Abstract

Emission of Δ-Ru(phen)_2dppz^(2+) bound to nucleic acid polymers of different sequence has been investigated by time-resolved luminescence spectroscopy and the effect of major and minor groove DNA binding agents on the luminescence profile of the complex evaluated. In the presence of a 1:1 mixture of poly d(AT) and poly d(GC), the excited-state decay of Δ-Ru(phen)_2dppz^(2+) can be described by a linear combination of the decay profiles in the presence of poly d(AT) and poly d(GC) independently. This analysis indicates that ∼85% of the complexes are bound to poly d(AT) and that the metallointercalator preferentially occupies AT sites in mixed-sequence polymers such as calf thymus or T4 DNA. When rac-Ru(phen)_2dppz^(2+) bound to [d(5‘-GAGTGCACTC-3‘)_2] is titrated with the major groove intercalator Δ-α-[Rh[(R,R)-Me_2trien]phi]^(3+), the ruthenium emission yield decreases while the absorbance of the π−π^* transition centered on the dppz ligand increases, until saturation behavior is observed at a 1:1 Rh/duplex ratio. These titrations indicate that Ru(phen)_2dppz^(2+) is displaced from the major groove by the rhodium complex. In contrast, for rac-Ru(phen)_2dppz^(2+) bound to poly d(AT), addition of the minor groove binding agent distamycin produces an increase in ruthenium emission which saturates at ∼1 distamycin/5 bp, consistent with the double helix being able to accommodate major and minor groove binders simultaneously. Distamycin has no effect on the emission of Ru(phen)_2dppz^(2+) emission bound to poly d(GC). These photophysical studies establish a sequence preference in binding to DNA by Ru(phen)_2dppz^(2+) as well as providing support for the original assignment by NMR of ruthenium intercalation from the major groove side of the DNA helix.