What is the structure of GFP chromophore?

The chromophore is formed from three amino acids within the protein sequence of Green Fluorescent Protein: serine, tyrosine, and glycine. Specifically, the sequence is Ser65-Tyr66-Gly67. These residues undergo a cyclization and oxidation process that creates the imidazolidone ring, which is responsible for the fluorescence of GFP in the green region of the visible spectrum.