Characterisation of bifunctional ruthenium(II) complexes, potential DNA photo-probes. Presence of folded and unfolded conformers

Abstract

Novel bifunctional ruthenium(II) complexes, [Ru(TAP)₂(POQ-Nmet)]²⁺ and [Ru(BPY)₂(POQ-Nmet)]²⁺ (1a, 2a), containing a metallic and an organic moiety, have been prepared as photoprobes and photoreagents of DNA (TAP = 1,4,5,8-tetraazaphenanthrene, POQ-Nmet = 5-[6-(7-chloroquinolin-4-yl)-3-thia-6-azaheptanamido]-1,10-phenanthroline). The ES mass spectrometry and ¹H NMR data in organic solvents indicate that the quinoline moiety exists in both the protonated and non-protonated form. Moreover, the comparison of the NMR data with those of the corresponding monofunctional complexes (without quinoline) evidences that [Ru(TAP)₂(POQ-Nmet)]²⁺ and [Ru(BPY)₂(POQ-Nmet)]²⁺ are unfolded when the quinoline unit is protonated whereas deprotonation permits folding of the molecule. In the folded state the spatial proximity of the electron donor (the organic moiety) and electron acceptor (the metallic moiety) in [Ru(TAP)₂(POQ-Nmet)]²⁺ favours intramolecular photo-induced electron transfer, which has been shown in a previous study to be responsible for the very low luminescence of 1a in non-protonating solutions. The restoration of the luminescence by protonation of the quinoline moiety as observed previously is in agreement with the unfolding of the molecule demonstrated in this work. The existence of such folding–unfolding processes related to protonation is crucial for studies of 1a with DNA.