1X0I:
reference: Crystal structures of Acid blue and alkaline purple forms of bacteriorhodopsin., Okumura H, Murakami M, Kouyama T, J Mol Biol 2005 Aug 19;351(3):481-95. PMID: 16023672

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