DNA-Binding Properties of Non-Intercalating Water-Soluble Organometallic Ir(III) Luminophores

Abstract

A series of Ir(III) complexes, [Ir(C^N)2(en)]+ (where C^N = 2-phenyl-benzo[d]thiazolyl cyclometalating ligand; en = ethylene diamine), is reported with structural variation via a substituent (H, Me, OMe, Cl, OCF3) at the coordinated phenyl ring. The complexes were soluble in aqueous buffer, with solubility limits correlating inversely with the predicted logP. The complexes display efficient visible absorption at 400–500 nm (ε ∼ 5000 M−1 cm−1) due to charge-transfer transitions and are triplet emitters in aerated buffer (λem = 529–540 nm; lifetimes up to 0.763 µs; Φem ≤ 12%). Each complex was investigated, via computational and biophysical experiments, in the context of DNA binding. According to UV-visible titrations, the cationic complexes bind to DNA with apparent affinities ranging from 6 × 104 to 5 × 105 M−1 with apparent binding site sizes between 0.4 and 1.0 base pairs. Isothermal titration calorimetry (ITC) showed that complexes [Ir(L1-3)2(en)]Cl bind to DNA in two types of binding sites, viz., a high-affinity (107–108 M−1) binding site with characteristics of minor or major groove binding and a low-affinity binding site (105 M−1) with characteristics of non-specific binding to negatively charged DNA, with binding supported by hydrophobic interactions between complexes.