A new arene–Ru based supramolecular coordination complex for efficient binding and selective sensing of green fluorescent protein

A new dipyridyl ligand is encoded with 120° angularity between its coordination vectors by using a central pyridine carboxamide scaffold to orient two 4-(pyridin-4-ylethynyl)phenyl moieties. The N,N′-bis(4-(pyridin-4-ylethynyl)phenyl)pyridine-2,6-dicarboxamide ligand undergoes self-assembly with a diruthenium arene complex to furnish a [2 + 2] metallacycle with a wedge-like structure. The metallacycle binds to the enhanced green fluorescent protein (EGFP) variant of GFP, resulting in steady-state spectral changes in UV-Vis absorption and emission experiments. These studies indicate that the metallacycle induces conformation changes to the EGFP, disrupting the tripeptide chromophore. Furthermore, gel electrophoresis, circular dichroism and atomic force microscopy studies indicate that binding ultimately leads to aggregation of the protein. Computational investigations indicate a favorable interaction, predominantly between the metallacycle and the Arg168 residue of the EGFP. An interaction with Arg168 and related residues was previously observed for an emission-attenuating antibody, supporting that these interactions induce changes to the photophysical properties of EGFP by disrupting the tripeptidechromophore in a similar manner. Additionally, we have also described the quenching study of the reporter GFP protein in vivo by a new metal complex using reflected fluorescence microscopy. We anticipate that such metal complexes which can passively diffuse into the cells in vivo can serve as potential tools in molecular and drug targeting based biological studies.