2WJK:
reference: Directional Proton Transfer in Membrane Proteins Achieved through Protonated Protein-Bound Water Molecules: A Proton Diode., Wolf S, Freier E, Potschies M, Hofmann E, Gerwert K, Angew Chem Int Ed Engl. 2010 Aug 2. PMID: 20680951

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