Direct detection and characterization of bioinorganic peroxo moieties in a vanadium complex by 17O solid-state NMR and density functional theory

Abstract

Electronic and structural properties of short-lived metal-peroxido complexes, which are key intermediates in many enzymatic reactions, are not fully understood. While detected in various enzymes, their catalytic properties remain elusive because of their transient nature, making them difficult to study spectroscopically. We integrated ¹⁷O solid-state NMR and density functional theory (DFT) to directly detect and characterize the peroxido ligand in a bioinorganic V(V) complex mimicking intermediates non-heme vanadium haloperoxidases. ¹⁷O chemical shift and quadrupolar tensors, measured by solid-state NMR spectroscopy, probe the electronic structure of the peroxido ligand and its interaction with the metal. DFT analysis reveals the unusually large chemical shift anisotropy arising from the metal orbitals contributing towards the magnetic shielding of the ligand. The results illustrate the power of an integrated approach for studies of oxygen centers in enzyme reaction intermediates.

Keywords: (17)O solid state NMR; Density functional theory calculations; Peroxido ligand; Reactive intermediates.

Figure 1.

Crystal structures of (a) NH₄[VO(¹⁷O₂)(H₂O)(C₅H₃N(COO)₂] [5], (b) peroxo intermediate of vanadium chloroperoxidase (pdb code: 1IDU) [4] and, (c) the superoxo intermediate of homoprotocatechuate dioxygenase (pdb code: 2IGA) [3].

Figure 2.

¹⁷O NMR spectra of HS003 at 19.96 T. (a) QCPMG experiment,¹⁷O MAS spectra (colored lines) together with the best-fit simulated traces (gray). (b) Hahn echo experiment at 20 kHz MAS and, (c) Hahn echo experiment at 40 kHz MAS. The simulations of the experimental spectra were performed using the best-fit parameters as given in Table 1. The inset shows the structure of HS003 with the isotopically ¹⁷O labeled oxygen atoms.

Figure 3.

Left: Relative orientation of the quadrupolar and the chemical shift tensors in the molecular frame of HS003. Right: Contour plots of selected occupied (left) and unoccupied (right) molecular orbitals contributing to the chemical shift tensor of the peroxido ligand in HS003 from DFT calculations. The dashed lines indicate symmetry allowed transformations of the angular momentum that can introduce non-zero paramagnetic shielding. The red/green lobes indicate regions of positive/negative parity.