Synthesis, characterization and luminescence properties of dipyridin-2-ylamine ligands and their bis(2,2'-bipyridyl)ruthenium(II) complexes and labelling studies of papain from carica papaya

摘要

Two luminescent polypyridyl Ru~(II) complexes including dipyridin-2-ylamine (dpa) ligands functionalized by a maleimide group, namely [Ru(bpy)_2(1a-b)](PF_6)_2 (bpy = 2,2′-bipyridyl; 1a = 1-[4-(dipyridin-2-ylamino)butyl]-1H-pyrrole-2,5- dione; 1b = 1-[5-(dipyridin-2-ylamino)pentyl]-1H-pyrrole-2,5-dione), were synthesized, and the X-ray structure of [Ru(bpy)_2(1b)](PF _6)_2 was solved. The photophysical properties of these complexes and the starting dipyridin-2-ylamine ligands were studied. Upon excitation at their maximum of absorption, the dpa ligands exhibited weak luminescence because of quenching by the maleimide group. Conversely, the complexes displayed noticeable luminescence, with an emission wavelength at 600 nm that originated from a metal-to-ligand charge-transfer (MLCT) triplet state. Reaction of the ligands and the complexes with the cysteine endoproteinase papain was shown to occur at the single free cysteine (Cys25) as expected by the usual reactivity of maleimides. The resulting bioconjugates displayed luminescence assigned to the attached fluorophore, and luminescence enhancement was observed with respect to the starting reagents. The circular dichroism spectrum of one of the papain-Ru~(II) bioconjugates displayed a typical bisignate band in the near-UV range, indicating that the reaction of papain with the rac complex appeared to be stereoselective in favour of the Δ enantiomer. Reaction of dipyridin-2-ylamine complexes of Ru~(II) functionalized with a maleimide moiety with papain occurred in a stereoselective fashion and yielded bioconjugates displaying enhanced luminescence with respect to the starting materials.