Targeted cell imaging properties of a deep red luminescent iridium(iii) complex conjugated with a c-Myc signal peptide

A nuclear localisation sequence (NLS) peptide, PAAKRVKLD, derived from the human c-Myc regulator gene, has been functionalised with a long wavelength (lambda(ex) = 550 nm; lambda(em) = 677 nm) cyclometalated organometallic iridium(iii) complex to give the conjugate Ir-CMYC. Confocal fluorescence microscopy studies on human fibroblast cells imaged after 18-24 h incubation show that Ir-CMYC concentrations of 80-100 mu M promote good cell uptake and nuclear localisation, which was confirmed though co-localisation studies using Hoechst 33342. In comparison, a structurally related, photophysically analogous iridium(iii) complex lacking the peptide sequence, Ir-PYR, showed very different biological behaviour, with no evidence of nuclear, lysosomal or autophagic vesicle localisation and significantly increased toxicity to the cells at concentrations >10 mu M that induced mitochondrial dysfunction. Supporting UV-visible and circular dichroism spectroscopic studies show that Ir-PYR and Ir-CMYC display similarly low affinities for DNA (ca. 10(3) M-1), consistent with electrostatic binding. Therefore the translocation and nuclear uptake properties of Ir-CMYC are attributed to the presence of the PAAKRVKLD nuclear localisation sequence in this complex.