Sen et al. suggested another mechanism of chromophore hydration following a systematical computational study of the nature of the excited states in Dreiklang. The authors applied several quantum chemical and QM/MM approaches to estimate absorption spectra in Dreiklang and showed that in the ON-state model systems with the neutral form of the chromophore and the neutral forms of His145 and Glu222, the levels of the locally excited S1 and the charge-transfer (CT) state are nearly isoenergetic. The light-driven chromophore in a locally excited state S1 tends to relax into the CT state due to electron transfer from the neighboring residue Tyr203 to the chromophore (see Fig. ). To investigate possible excited-state pathways, QM/MM geometry optimization and short ab initio type molecular dynamics (AIMD) simulations based on the time dependent density functional theory (TD DFT) approach were carried out. The results allowed the authors to propose a series of alternative steps in the mechanism as compared to Ref. , assuming involvement of the Tyr203 and Glu222 side chains, as illustrated in Supporting materials for the present paper. In particular, few candidates for the critical reaction intermediate X were suggested in Ref. , promising a more elaborate characterization "in future studies".