A later report from the same group added a third bond to the model , from Mg 2+ to Tyr148 3x33 (using GPCRdb numbering ) (Figure ). However, this would require an O-Mg bond length of 3.1 Å; compared with high-resolution structures (2.10 ± 0.04 Å), this is untenable (Z = 25) . It is instead suggestive of a hydrogen bond to another element. Note also the large gap in the electron density along this proposed bond, unlike the strong and uninterrupted density for the bonds to BU72 and His54 (Figure ). Additionally, note the highly asymmetrical geometry required, with a bond angle of 105°, compared to 90° for the N atoms; magnesium complexes are symmetrical . Other evidence against Mg 2+ was revealed by CheckMyMetal . The values of five of the eight parameters evaluated were classed as dubious, including three that strongly suggest a misidentified element: