The inhibitors were dissolved in distilled water to a final concentration of 40 mM, and the pH of the solution was adjusted to 8.0 with 1 M NaOH. The rhGCPII stock solution (8 mg/mL) was mixed with 1/10 v/v of the individual inhibitor, and the complexes were crystallized using the hanging drop vapor diffusion setup at 293 K. Crystallization droplets were made by combining 1 μL of the rhGCPII–inhibitor mixture and 1 μL of the reservoir solution containing 33% pentaerythriol propoxylate (PO/OH 5/4; Hampton Research), 1% PEG3350, and 100 mM Tris-HCl, pH 8.0. Crystals belonging to the space group I222 typically appeared within two days and reached their final size with approximate dimensions of 0.3 mm × 0.4 mm × 0.1 mm within a week. For data collection crystals were flash-frozen in liquid nitrogen directly from the reservoir solution, and diffraction intensities were collected at 100 K using synchrotron radiation at the SER-CAT sector 22 beamlines of the Advanced Photon Source (Argonne, IL) at the X-ray wavelength of 1.0 Å. In all cases, the diffraction data were collected from a single crystal, recorded on a CCD detector, and processed using the HKL2000 software package.38