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The spectroscopic and photophysical properties of ruthenium polypyridyl polypeptide conjugates of the
type [Ru(bpy)2PIC-Argn]n+2+, where bpy is 2,2-bipyridyl (bpy), PIC is 2-(4-carboxyphenyl)imidazo[4,5-
f][1,10]phenanthroline and PIC-Argn is this ligand peptide bonded to polyarginine where n is 5 or 8, is
described. The resonance Raman spectroscopy of the peptide conjugated complex and parent are strongly
pH dependent and demonstrate a switch of lowest energy charge transfer transition between bpy and pic
ligands as s function of pH. The pKa of the imidazole ring on the complex is obtained from resonance
Raman spectroscopy as 7.8 ± 0.2. The luminescence lifetime of the complex is strongly oxygen
dependent and a Stern-Volmer plot of O2 quenching for [Ru(bpy)2(PIC-Arg8)]10+ yielded a KSV value of
2300 ± 420 M-1 which was independent of pH over the range 2 to 11. The complexes, because of their
large Stokes shifts can, uniquely, be used under identical conditions of probe concentration and excitation
wavelength for resonance Raman and luminescence cellular imaging. Cellular imaging was conducted
using SP2 myeloma cells which confirmed that the [Ru(bpy)2(PIC-Arg8)]10+ is readily taken up by
mammalian cells although the parent and pentarginine analogues are not membrane permeable.
Preliminary examples of multi-parameter imaging using these probes were presented. Resonance Raman
maps of [Ru(bpy)2(PIC-Arg8)]10+ within living myeloma cells showed on the basis of spectral
discrimination, attributed to pH, three distinct regions of the cell could be identified, ascribed to the
nucleus, the cytoplasm and the membranes. Luminescence lifetime imaging showed quite large variations
in the probe lifetime within the living cell which was tentatively ascribed to variation in O2 concentration
about the cell. Preliminary estimates of O2 concentration were made and it was found that the membranes,
both inner and outer are the most O2 rich regions of the cell. Overall, we propose that such peptide
labeled luminescent metal are potentially a valuable addition to cellular imaging by providing tools for
multiplexed analysis of the cell environment.
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