FIGURE 1.

Fluorescence mechanism of microbial rhodopsins. (A) Crystal structure of Arch (PDB code: 6GUZ). Rhodopsin is a membrane protein with a seven-fold transmembrane alpha-helix structure and consists of a protein moiety called opsin and a retinal chromophore that is covalently bound to the apoprotein via a Schiff base. (B) Photoreaction scheme of microbial rhodopsins. The spontaneous emission (left) occurs from the nonreactive S₁ state. The photointermediate fluorescence (right) is from the Q-intermediate state produced by a photon absorption of the N-intermediate in its photocycle. Photointermediate fluorescence arises through the sequential action of three photons (① - ③). The dashed line represents the non-radiative relaxation process.