Stability Constant Determination of Chiral Ligands with Eu(III) Used as Potential Probes for Biomolecules

In this study, a selection of lanthanide(III)-based systems were characterized in order to expand the tool set for circularly polarized luminescence-based systems used as probes for biomolecules. The RR and SS ligands were complexed with Eu(III) ion and fully characterized in terms of stability constants. This study also confirmed that the steric hindrances resulting from the helical wrapping of the meso-ligand molecules around the lanthanide(III) ion dictate the formation of stable 1:3 species.

We hypothesize that the formation of 3:1, 2:1, and 1:1 (ligand:metal) complexes with N,N’-(R,R-1-phenylethyl), N,N’-(S,S-1-phenylethyl)-2,6-pyridine dicarboxamide with lanthanide(III) metal would be achieved. Difficulties with the formation of the meso-ligand, N,N’-(R,S-1-phenylethyl)-2,6-pyridine dicarboxamide, may be due to the opposite stereochemistry of each chiral carbon present (steric hindrances upon complexation). Determination of the stepwise stability constants for these metal complexes will aid in our chiral recognition studies of simple biomolecular systems.

Stability constants were determined spectrophotometrically by time-resolved luminescence and absorption titrations with Eu(III) using a custom automatic titration system. The stability constants obtained for Eu(N,N’-(R,R-1-phenylethyl)-2,6-pyridine dicarboxamide)₃ are 1:1 = 8.01, 2:1 = 15.94, 3:1 = 23.73 which we believe mirror the analog Eu(III) complex with the SS ligand. Stability constants for the meso complex have been difficult to obtain because we are unsure of the speciation. We are currently modifying the conditions to determine if the tris complex can be formed. Further investigation will be done with Tb(III) and Gd(III) to fully characterize our lanthanide(III)-containing systems.