The calculated lengths of the V-O bonds and the V-N bonds in the optimized structures with QM Region I are reported in Table . (The calculated bond lengths for the optimized structures found with QM Region II are given in Table of the Supplementary Material; these bond lengths do not significantly differ from the results given in Table using QM Region I.) In particular, the trigonal bipyramidal configuration is not rigidly maintained, and the bond angle between axial oxygen (O4), vanadium and the nitrogen (N5) of His496 is bent away from 180 ° by as much 48 ° in one case. Furthermore, protonation has the effect of lengthening the V-O bond by about 0.2 Å per hydrogen added. Unprotonated oxygens have a V-O bond length in the range of 1.598-1.691 Å; monoprotonated oxygens have a V-O bond length in the range of 1.732-1.944 Å; diprotonated oxygens have a V-O bond length in the range of 1.971-2.337 Å.