Fig. 1. The proposed prFMN^im → prFMN^ket photoisomerization mechanisms obtained based on high-resolution crystal structures, mass spectrometric and kinetic experiments.⁵ All the symbols are explained in the text. Type (1) = mechanism involving intramolecular proton transfer in the H-bond in prFMN^im with four consecutive elementary reactions; Type (2) = mechanism involving intermolecular proton transfer in the H-bond between prFMN^im and Glu282 with three consecutive elementary reactions. Int = internal base; Ext = external base.