2GRH:
reference: Allosteric action in real time: Time-resolved crystallographic studies of a cooperative dimeric hemoglobin., Knapp JE, Pahl R, Srajer V, Royer WE Jr, Proc Natl Acad Sci U S A. 2006 May 16;103(20):7649-54. Epub 2006 May 9. PMID: 16684887

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