2M6Z:
reference: Solution structure and dynamics of human hemoglobin in the carbonmonoxy form., Fan JS, Zheng Y, Choy WY, Simplaceanu V, Ho NT, Ho C, Yang D, Biochemistry. 2013 Aug 27;52(34):5809-20. doi: 10.1021/bi4005683. Epub 2013 Aug, 15. PMID: 23901897

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