was mutated to the adduct, and the model fitted and refined as above. Because the PDB validation report did not evaluate the geometry of adduct 6, ligand distortions in the bound ligands were tabulated in Coot and used to calculate Z scores, comparing ideal values and standard deviations from GRADE with modeled values for 1a, 1b and 6 (Additional file 7). Diffraction precision indexes were calculated using Online_DPI . Protein structures were aligned and illustrated using Pymol , and annotated