5DTL:
reference: Arginine 66 controls dark-state formation in green-to-red photoconvertible fluorescent proteins., Berardozzi R, Adam V, Martins A, Bourgeois D, J Am Chem Soc. 2015 Dec 17. PMID: 26675944

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).