51V NMR chemical shifts calculated from QM/MM models of vanadium chloroperoxidase

V-51 NMR chemical shifts calculated from QM/MM-optimized (QM = quantum mechanical; MM = molecular mechanical) models of vanadium-dependent chloroperoxidase (VCPO) are presented. An extensive number of protonation states for the vanadium cofactor (active site of the protein) and a number of probable positional isomers for each of the protonation states are considered. The size of the QM region is increased incrementally to observe the convergence behavior of the V-51 NMR chemical shifts. A total of 40 models are assessed by comparison to experimental solid-state V-51 NMR results recently reported in the literature. Isotropic chemical shifts are found to be a poor indicator of the protonation state; however, anisotropic chemical shifts and the nuclear quadrupole tensors appear to be sensitive to changes in the proton environment of the vanadium nuclei. This detailed investigation of the V-51 NMR chemical shifts computed from QM/MM models provides further evidence that the ground state is either a triply protonated (one axial water and one equatorial hydroxyl group) or a doubly protonated vanadate moiety in VCPO. Particular attention is given to the electrostatic and geometric effects of the protein environment. This is the first study to compute anisotropic NMR chemical shifts from QM/MM models of an active metalloprotein for direct comparison with solid-state MAS NMR data. This theoretical approach enhances the potential use of experimental solid-state NMR spectroscopy for the structural determination of metalloproteins.