A luminescent lipid mimetic iridium(III) N-heterocyclic carbene complex for membrane labelling

Labelling phospholipid membranes with luminophores without altering the biophysical characteristics of the system is particularly challenging due to the small size of the phospholipid molecules and the sensitivity of membrane properties to the presence of fused heterocyclic molecules. Here the design and synthesis of a luminescent lipid mimetic Ir(III) N-heterocyclic carbene complex of the form [Ir(ppy)2(C^N)] (where ppy = 2-(phenyl)-pyridine and C^N is a N-heterocyclic carbene ligand) conjugated to stearic acid is described. This complex was synthesised by the reaction of an acetate functionalised Ir(III) precursor complex with tert-butyl N-(2-aminoethyl)carbamate (mono-BOC protected ethylene diamine) and after deprotection of the amine group this complex was coupled to stearic acid using the peptide coupling reagent 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). The photophysical properties of the synthesised complexes were evaluated and they showed blue-green luminescence in the range of 514–520 nm. Fluorescence microscopy studies showed that the lipid mimetic complex successfully incorporated into liposomes composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), while dynamic light scattering (DLS) and differential scanning calorimetry (DSC) studies showed that the complex had negligible influence on the biophysical properties of the liposomes. A luminescent lipid mimetic iridium(III) N-heterocyclic carbene complex conjugated to stearic acid has been prepared. Fluorescence microscopy in combination with dynamic light scattering and differential scanning calorimetry studies showed that the complex successfully incorporated into the membrane of liposomes without significantly altering the liposome biophysical properties. [Display omitted] •Luminescent lipid mimetic iridium(III) N-heterocyclic carbene complex synthesised•Photophysical characterization of lipid mimetic complex and precursors•Incorporation of lipid mimetic complex into liposomes composed of phospholipids•Lipid mimetic complex has a negligible influence on the liposome biophysical properties.