1GCN:
reference: X-ray analysis of glucagon and its relationship to receptor binding., Sasaki K, Dockerill S, Adamiak DA, Tickle IJ, Blundell T, Nature 1975 Oct 30;257(5529):751-7. PMID: 171582

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