4FDK:
reference: Porphyrin pi-stacking in a heme protein scaffold tunes gas ligand affinity., Weinert EE, Phillips-Piro CM, Marletta MA, J Inorg Biochem. 2013 Jun 15;127C:7-12. doi: 10.1016/j.jinorgbio.2013.06.004. PMID: 23831583

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