Neutralizing copper(II) complex charge through sulfonation: no1py2pyS as a new chelator for 64Cu radiopharmaceuticals

We report the synthesis and comprehensive characterization of a novel sulfonated TACN-based ligand, no1py2pyS (L 2S ), designed to combine high aqueous solubility, neutral complex charge, and strong Cu(II) affinity. L 2S was synthesized via an orthoamide pathway coupled with regioselective sulfonation of picolyl pendants. Protonation equilibria were investigated using 1 H NMR and UV–Vis spectroscopies. The Cu(II) complex ([CuL 2S ]) formed rapidly under acidic (1 M HCl) and physiological (PBS, pH 7) conditions and remained stable across a wide pH range (0−12), even under highly acidic environment (5 M HCl). DFT calculations revealed two diastereoisomeric forms, Δ(λλλ) and Λ(λλλ), with a negligible free energy difference (0.04 kcal mol −1 ), suggesting significant population of both forms at room temperature. However, EPR spectroscopy indicated the predominance of the Δ(λλλ)/Λ(δδδ) enantiomeric pair (87%), which displays a Jahn-Teller distorted octahedral geometry. X-ray crystallography validated the structure, revealing macrocyclic and pendant arm coordination ( N 6 donor set) and sulfonate-mediated copper bridging. Cyclic voltammetry demonstrated quasi-reversible redox behavior, a reduction potential ( E 1/2 = −614 mV vs. Ag/AgCl/ 3 M KCl) below the physiological threshold and a reduced risk of demetallation, as no complex dissociation was observed. Finally, radiolabeling studies with [ 64 Cu]Cu 2+ showed quantitative incorporation (>99%) under mild conditions (pH 4.5, RT, 10 min) at low concentration (10 −6 M), and high stability in human serum for up to 2 h. Taken together, these findings identify L 2S as a promising platform for the development of copper-based radiopharmaceuticals.