Synthesis, characterization, and biological studies of emissive rhenium-glutamine conjugates

Two new rhenium complexes containing pyridine-triazole (pyta) and quinoline-triazole (quinta) ligands with attached glutamine-targeting agents have been characterized and tested for uptake in the HT-29 human colon adenocarcinoma cell line. The glutamine moiety in Re(CO)3Br(pyta) (1) and Re(CO)3Br(quinta) (2) remains pendant in solution. Both complexes exhibit absorptions in the 300-400-nm range with metal-to-ligand charge transfer (MLCT) character, as predicted by time-dependent density functional theory calculations. Geometrical analysis by theoretical calculations provides information on the cationic complexes 1 (+) and 2 (+) resulting from aquo for halide ligand exchange under aqueous conditions. The emissive properties of both complexes were studied under aqueous conditions, and the measured quantum yields were 0.46 % for 1 (+) and 0.18 % for 2 (+). The large Stokes shifts and oxygen sensitivity of the emission suggest a (3)MLCT process for both complexes. Cell studies in the HT-29 cell line demonstrate that both complexes are nontoxic over a large concentration range (0-1.4 mM). Preliminary uptake studies show that 2 (+), but not 1 (+), displays significant concentration-dependent uptake at 3 and 24 h.