Kinetically Inert Bispidol-Based Cu(II) Chelate for Potential Application to (64/67)Cu Nuclear Medicine and Diagnosis

Inorg Chem. 2015 May 4;54(9):4431-44. doi: 10.1021/acs.inorgchem.5b00207. Epub 2015 Apr 13.

Abstract

A family of 2,4-pyridyl-disubstituted bispidol derivatives bearing methylene carboxylic acid ethyl esters (L1-L3), methylene carboxylic acids (L4 and L5), or methylenethiophene (L6) groups were synthesized. In water, all ligands form rigid 1:1 complexes in the presence of Zn(II) in which the bicycle adopts a chair-chair conformation (cis isomer), as observed by (1)H NMR and, in the case of ligand L1, by an X-ray diffraction crystal structure. Interestingly, addition of Zn(II) ions on ligand L1 induces a metal-mediated selective hydrolysis of the ethyl esters. This selective hydrolysis was not observed upon addition of other cations such as Na(+), Mg(+), and Ca(2+). Reduction of the central ketone was achieved to prevent ring opening via retro Diels-Alder reactions and to afford highly stable and water-soluble ligands (L4, L5, L6). The complexation properties of L4 and L6 were studied in solution, with a particular interest for ligand L4. Fast complexation occurs in strongly acidic media (pH = 1), with a high affinity toward Cu(II) (log KCuL4 = 19.2(3), pCu = 17.0 at pH 7.4, pCu = -log[Cufree], [Cu] = 1 × 10(-6) M, [L] = 1 × 10(-5) M) and high selectivity versus Co(II), Ni(II), and Zn(II), as shown by the values of the binding constants obtained from potentiometric and spectrophotometric titrations. Reversible redox potential with E1/2 = -430 mV (vs normal hydrogen electrode) was measured. The complex was found to be fairly inert from acid-assisted dissociation experiments in 5 M HClO4 (t1/2 = 110 d at 25 °C).

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Carboxylic Acids / chemistry*
  • Coordination Complexes / chemical synthesis*
  • Copper / chemistry*
  • Copper Radioisotopes
  • Crystallography, X-Ray
  • Cycloaddition Reaction
  • Humans
  • Kinetics
  • Ligands
  • Magnetic Resonance Spectroscopy
  • Molecular Conformation
  • Nuclear Medicine / instrumentation
  • Nuclear Medicine / methods
  • Potentiometry
  • Radiopharmaceuticals
  • Thiophenes / chemistry*
  • Zinc / chemistry

Substances

  • Carboxylic Acids
  • Coordination Complexes
  • Copper Radioisotopes
  • Ligands
  • Radiopharmaceuticals
  • Thiophenes
  • Copper
  • Zinc