Synthesis, Characterization, and Glutathionylation of Cobalamin Model Complexes [Co(N4PyCO₂Me)Cl]Cl₂ and [Co(Bn-CDPy3)Cl]Cl₂

Synthetic Co­(III) complexes containing N5 donor sets undergo glutathionylation to generate biomimetic species of glutathionylcobalamin (GSCbl), an important form of cobalamin (Cbl) found in nature. For this study, a new Co­(III) complex was synthesized derived from the polypyridyl pentadentate N5 ligand N4PyCO₂Me (<b>1</b>). The compound [Co­(N4PyCO₂Me)­Cl]­Cl₂ (<b>3</b>) was characterized by X-ray crystallography, UV–vis, IR, ¹H NMR, and ¹³C NMR spectroscopies and mass spectrometry (HRMS). Reaction of <b>3</b> with glutathione (GSH) in H₂O generates the biomimetic species [Co­(N4PyCO₂Me)­(SG)]²⁺ (<b>5</b>), which was generated <i>in situ</i> and characterized by UV–vis and ¹H NMR spectroscopies and HRMS. ¹H NMR and UV–vis spectroscopic data are consistent with ligation of the cysteine thiolate of GSH to the Co­(III) center of <b>5</b>, as occurs in GSCbl. Kinetic analysis indicated that the substitution of chloride by GS^– occurs by a second-order process [<i>k</i>₁ = (10.1 ± 0.7) × 10^–2 M^–1 s^–1]. The observed equilibrium constant for formation of <b>5</b> (<i>K</i>_obs = 870 ± 50 M^–1) is about 3 orders of magnitude smaller than for GSCbl. Reaction of the Co­(III) complex [Co­(Bn-CDPy3)­Cl]­Cl₂ (<b>4</b>) with GSH generates glutathionylated species [Co­(Bn-CDPy3)­(GS)]²⁺ (<b>6</b>), analogous to <b>5</b>. Glutathionylation of <b>4</b> occurs at a similar rate [<i>k</i>₂ = (8.4 ± 0.5) × 10^–2 M^–1 s^–1], and the observed equilibrium constant (<i>K</i>_obs = 740 ± 47 M^–1) is slightly smaller than for <b>5</b>. Glutathionylation showed a significant pH dependence, where rates increased with pH. Taken together, these results suggest that glutathionylation is a general reaction for Co­(III) complexes related to Cbl