Tentative mechanisms of chemical reactions of the chromophore hydration/dehydration in Dreiklang are proposed in the subsequent works . To clarify these approaches, we show in Fig. molecular models of the reactants and the products in the ON → OFF reaction in Dreiklang, drawn by motifs of the computationally derived protein structures described in Ref. . The paper reports the protein models obtained in quantum mechanics/molecular mechanics (QM/MM) calculations, which are well consistent with the crystal structures deposited to the Protein Data Bank PDB ID 3ST4 (ON-state) and PDB ID 3ST3 (OFF-state). Ref. shows that these models allow one to compute the energy profile for the OFF → ON reaction of thermal recovery of the fluorescent state, which is also well consistent with the experimental data . The panels in Fig. illustrate fragments of the chromophore-containing pocket with the initial chromophore Chro and the reactive water molecule Wat (left side) and the hydrated chromophore Chro' (right side). In the simplest representation, the water molecule Wat splits to the hydroxyl ion and the proton binding to the C65 and N66 atoms of Chro, respectively. The detailed mechanism of this photochemical reaction should take into consideration environmental molecular groups; the panels in Fig. show the nearest to the chromophore amino acid residues, His145, Tyr203, Glu222, which are important in the hydration reaction, according to the proposals formulated in Refs. . Lacombat et al. described the results of femtosecond spectroscopy studies of Dreiklang and tentatively suggested the mechanism of the light-induced hydration reaction after the illumination of the protein in the ON-state. The authors presumed that the S0 → S1 local excitation of the cis-neutral chromophore leads to evolution of the system on the excited state potential energy surface (PES) followed by the switch to the plausible ground state reaction intermediate called X, and later to the OFF-state with the hydrated chromophore. The X intermediate, which by the author's suggestion is an adduct of the anionic chromophore with the water molecule, is characterized by the transient absorption band at 450 nm (2.76 eV) observed in the femtosecond experiments. The mechanistic scheme assumes the excited-state electron and proton transfer with participation of the nearby residues His145 and Glu222 including chromophore protonation at N66 by the Glu222 side chain followed by addition of the hydroxyl at C65 (see Fig. ). For the reader's convenience, we present an illustration of this proposal in Supporting materials for the present paper.