Figure 1. Chemical structures of the dark and light states in BLUF photoreceptors.

The arrows indicate the proposed photoactivation reactions of the conserved glutamine: 3 → 1 rotation¹⁶−¹⁸; 1 → 2 tautomerisation to the Z-Z imidic acid and rotation²⁰; 1 → 4 tautomerisation to the Z-Z imidic acid²¹; 3 → 2 → 1 rotation via transient tautomerisation¹⁹,²³. Structures 1 and 3 show glutamine orientations in the Met-in and Trp-in structures, respectively. Residue numbering is according to the BlrB sequence. Numbering of the flavin atoms is indicated in structure 1.