N-Mg bonds were unrealistically short (1.9 and 1.7 Å). Compare the N-Mg bond lengths in structures of subatomic resolution: 2.19 ± 0.06 Å . These bonds are thus extreme outliers, with Z scores of -5 and -9, respectively. The high resolution of the structure (2.1 Å) allows strong conclusions about bond lengths, with a diffraction precision index (DPI) of 0.22 Å for the Mg 2+ ion . Note also that the ion is not centered in the density even with these unrealistically short distances, suggesting that the actual bonds must be shorter still (Figure ). This resulted in a poor real-space R value (RSR) of 0.32 for the Mg 2+ ion, despite good values for His54 (0.11) and BU72 (0.08).