51V NMR Chemical Shifts Calculated from QM/MM Models of Peroxo Forms of Vanadium Haloperoxidases

QM/MM models of the peroxo forms of vanadium-containing haloperoxidases (VHPOs) are critically assessed in terms of active site geometries, hydrogen bonds within the active site, isotropic and anisotropic V-51 NMR chemical shifts, and TD-DFT excitation energies. The geometric stability within the active site of the protein is comparable to the respective native forms, as indicated by low standard deviations in bond lengths across a number of local minima sampled along MID trajectories. There is a significant calculated upfield shift in delta(V-51) upon formation of the peroxo from the respective native forms for both the vanadium-containing chloroperoxidase (VCPO) and vanadium-containing bromoperoxidase (VBPO) models, which is in qualitative agreement with V-51 NMR experiments of VBPO in solution. The models show appreciable differences between the anisotropic chemical shifts of the different protonation states of the peroxo form of VHPO. The most likely candidates for the peroxo forms of the VHPO enzymes appear to be unprotonated or have a single proton on either of the equatorial oxygen ligands, based on QM/MM modeling in combination with X-ray, V-51 NMR, and UV-vis data.