Fluorescent
Rhenium-Naphthalimide Conjugates as Cellular
Imaging Agents
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Abstract
A range of biologically
compatible, fluorescent rhenium-naphthalimide
conjugates, based upon the rhenium <i>fac</i>-tricarbonyl
core, has been synthesized. The fluorescent ligands are based upon
a N-functionalized, 4-amino-derived 1,8-naphthalimide core and incorporate
a dipicolyl amine binding unit to chelate Re(I); the structural variations
accord to the nature of the alkylated imide with ethyl ester glycine
(<b>L</b><sup><b>1</b></sup>), 3-propanol (<b>L</b><sup><b>2</b></sup>), diethylene glycol (<b>L</b><sup><b>3</b></sup>), and benzyl alcohol (<b>L</b><sup><b>4</b></sup>) variants. The species are fluorescent in the visible
region between 505 and 537 nm through a naphthalimide-localized intramolecular
charge transfer, with corresponding fluorescent lifetimes of up to
9.8 ns. The ligands and complexes were investigated for their potential
as imaging agents for human osteoarthritic cells and protistan fish
parasite <i>Spironucleus vortens</i> using confocal fluorescence
microscopy. The results show that the specific nature of the naphthalimide
structure serves to control the uptake and intracellular localization
of these imaging agents. Significant differences were noted between
the free ligands and complexes, with the Re(I) complex of <b>L</b><sup><b>2</b></sup> showing hydrogenosomal localization in <i>S. vortens</i>