It is not possible to compare the calculated bond lengths (Table ) with the X-ray crystallographic data found in the PDB because these data have a large error of 0.24 Å. However, the Extended X-ray Absorption Fine Structure (EXAFS) regions obtained at pH 6.0 at 77 K from vanadium K-edge X-ray Absorption spectroscopy led to more precise bond lengths ( 0.02 Å). Namely, two V-O bonds are 1.69 Å, one V-O bond is 1.54 Å, and one V-O bond is 1.93 Å. Based on the calculated bond lengths, one can conclude that this short bond length of 1.54 Å must correspond to an unprotonated O4. Furthermore, the longer bond length of 1.93 Å corresponds to either mono-or diprotonated O4. The bond lengths of 1.69 Å in this spectroscopic data correspond to an unprotonated or monoprotonated oxygen. Table . Calculated bond lengths (in Å) and bond angles (O4-V-N5) for optimized structures (QM Region I) where 1-23 are the input structure as well as bond lengths and bond angle for the X-ray crystal structure data (1IDQ) of VCPO.