Complex with S-X77. We obtained crystal structures in presence of the two enantiomers respectively at resolution 1.55 Å for enantiomer 1, and at resolution 1.47 Å for enantiomer 2. As shown in Fig. , we could prove that enantiomer 1 had the R configuration by the unambiguous electron density reproducing the result obtained with the racemic mixture. In the crystal structure obtained in presence of enantiomer 2, instead we saw small blobs of electron density that could be modeled with a DMSO and water molecules (Fig. ). With the refinement of the structure obtained in presence of enantiomer S, small positive blobs of not modelable Fo-Fc were left. We repeated the crystallization experiments of both enantiomers using the highest reachable concentration, taking into account the DMSO tolerance of the protein. Crystallization trials were set up in presence of 5 mM inhibitor and the crystals diffracted at resolutions of 1.66 Å for enantiomer 1/R and 1.51 Å for enantiomer 2/S. For enantiomer 1/R the results reproduced the same results as for lower concentrations (Fig. ). Interestingly, for enantiomer 2/S, we obtained a positive Fo-Fc density that allowed the modeling of the enantiomer, as shown in Fig. . Comparing the crystal structures of the R and S enantiomers, it was evident how the only functional group occupying the same position is the pyridine ring located in the S1 pocket (Fig. ). The S enantiomer is mainly anchored there to the binding site, moreover the 2Fo-Fc density is less clear for this enantiomer, and its refined Bfactors are higher, overall confirming the biochemical data obtained.