Figure 1.

(A) RFP chromophore in the anionic form. The chromophore in KillerRed is linked to the protein via Glu68 (R1) and Ile64 (R3), and R2 is a side chain of Gln forming the chromophore. (B) Photoinduced processes that can lead to the formation of reactive oxygen species (ROS), ¹O₂ (singlet oxygen), and O₂^•– (superoxide). Relevant chromophore states are the ground and excited singlet states (¹Chro^– and ¹Chro^∗–), a triplet state (³Chro^–), and electron-attached and electron-detached doublet states (²Chro^2– and ²Chro). (C) Electronic configurations of the ground-state singlet, lowest excited singlet and triplet states, and two doublet radicals derived by electron attachment and detachment from ¹Chro^–. Solid arrows denote pathways supported by the previous and present studies, whereas dotted lines represent merely speculative (at this moment) pathways of ROS formation.