Crystal structures of BI-D bound to WT and H171T CCD dimers. Panel A is the H171T CCD dimer and panel B is the WT CCD dimer. BI-D is colored green and individual IN subunits are colored yellow and cyan. Oxygen atoms are shown in red and nitrogen atoms are in blue. Black dash-lines indicate hydrogen bonding interactions, whereas the magenta dash-line shows the electrostatic interaction between the protonated Nδ- on His171 and the carboxylic acid of BI-D. The arrow indicates the hydrogen bond between the protonated Nδ- on His171 and the ether oxygen on the tert-butoxy (B), which is absent in the H171T IN CCD structure (A).