To gain insight into the binding modes of the AzTG derivatives and to uncover the structural basis for the reversal in their photoinhibiton profiles, SERCA was co-crystallised with both AzTG-4 and AzTG-6. Subsequent X-ray analysis resulted in two structures: E2(AzTG-4) and E2(AzTG-6), determined at 2.85 Å and 3.0 Å resolution, respectively (Table ). For each structure, the positioning of the TG skeleton and AzTG azobenzene moiety were clearly revealed by unbiased Fo-Fc electron density maps, calculated before addition of the derivative into the model. The inhibitors were not subjected to UV illumination during the crystallisation process, and both derivatives were found to adopt the trans-configuration.