Structural and Solution Characterization of Mononuclear Vanadium(IV) Complexes That Help To Elucidate the Active Site Structure of the Reduced Vanadium Haloperoxidases

The complexes [VO(H₂O)ada] (1), [VO(H₂O)Hheida] (2), and [VO(H₂O)aeida] (3) (H₂ada, N-(carbamoylmethyl)-iminodiacetic acid; H₃heida, N-(2-hydroxyethyl)iminodiacetic acid; H₂aeida, N-(2-aminoethyl)iminodiacetic acid) were synthesized and crystallographically characterized. Crystallographic parameters for 1·2H₂O: monoclinic, space group P2₁/c (No. 14), a = 7.327(2) Å, b = 23.386(7) Å, c = 7.258(3) Å, α = 90°, β = 110.95(2)°, γ = 90°, V = 1204.6(7) ų, Z = 4, R1 = 0.0353, and wR² = 0.0848. Crystallographic parameters for 2·H₂O: orthorhombic, space group Pbca (No. 61), a = 10.512(2) Å, b = 11.727(2) Å, c = 16.719(5) Å, α = 90°, β = 90°, γ = 90°, V = 2060.6(8) ų, Z = 8, R1 = 0.0297, and wR² = 0.0758. Crystallographic parameters for 3: monoclinic, space group P2₁/c (No. 14), a = 6.785(1) Å, b = 9.714(2) Å, c = 14.959(2) Å, α = 90°, β = 95.12(1)°, γ = 90°, V = 982.2(3) ų, Z = 4, R1 = 0.0298, and wR² = 0.0762. In each structure, the tetradentate ligand is disposed so that the tertiary nitrogen is bound trans to the vanadyl oxo, and the rest of the donors occupy equatorial coordination positions. In solution, the structural integrity of these compounds is maintained as observed by UV/visible and EPR spectroscopies, and axial ligation by nitrogen is inferred on the basis of ESEEM spectroscopy. The implications of this study with respect to understanding the coordination environment of VO²⁺ in the reduced, inactive form of vanadium bromoperoxidase (VBrPO) are discussed, and it is proposed that significant changes in the coordination environment of vanadium in VBrPO occur upon its reduction, which may provide a plausible explanation for its irreversible inactivation.