Fig. 6.
Summary of structural changes observed for H75 during the transition from the dark state to the trapped M state. After M state trapping of GPRE108Q, a proton is transferred from the protonated Schiff base to the primary proton acceptor D97 (step 1). The protonated D97 influences the neighboring H75 to change its tautomeric form from the (τ)-state to the (π)-state (step 2). Additionally, the cross-protomer contact with W34 forces (π)-state H75 to perform a rotation, presumably around the Cβ–Cγ bond (step 3). This leads to close proximity of Nδ1(π) and N(π) and W34 Cδ1 and Nε2(π) in the illuminated M state (steps 4 and 5). Data from PDB ID: 4JQ6 (7).