Fluorescent proteins, which are able to switch between bright and dark states for multiple photocycles, are important markers for the bioimaging. The engineered protein Dreiklang , a member of the green fluorescent proteins (GFP) family with a chromophore formed from the Gly65-Tyr66-Gly67 tripeptide is a notable moiety, because its optical properties are modified in the chemical reaction of its chromophore. Mostly, the photoswitching in fluorescent proteins is due to the conformational changes of the chromophore, or to the tuning of the hydrogen-bond network near the chromophore . As proposed in the pioneering work of Brakemann et al. , a structural water molecule in Dreiklang attaches to its chromophore at the imidazolinone ring upon the light illumination at 405 nm (3.06 eV). This process converts the protein from the fluorescent (ON) state to the non-fluorescent (OFF) state. The ON-state can be restored either by the light illumination of the OFF-state at 365 nm (3.40 eV), or the recovery reaction can take place spontaneously in the dark. Ref. describes the synthesis, steady-state absorption spectra, electrospray ionization mass spectra and crystal structures of the protein in the ON-and OFFstates.