Fig. 1. Model for chromophore isomerization in FPs.

(A) Rotation can occur about either the P or I bond, leading to a P-ring flip or cis-trans isomerization, respectively. R₁ and R₂ represent residues Gly⁶⁴ and Cys⁶², respectively, which covalently link the chromophore to the rest of the FP (Fig. 2A). (B) General potential energy diagram along the isomerization reaction coordinate for a photoisomerizable chromophore. 0–0 TE represents the TE between the lowest vibrational state of the ground and excited electronic states. Three features studied in this work are emphasized: fluorescence (1, green), ES barrier crossing (2, purple), and GS barrier crossing (3, yellow).