1B2C:
reference: Crystallographic titration of cubic insulin crystals: pH affects GluB13 switching and sulfate binding., Diao J, Acta Crystallogr D Biol Crystallogr 2003 Apr;59(Pt 4):670-6. PMID: 12657786

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