4XSN:
reference: N,N,O and N,O,N Meridional cis Coordination of Two Guanines to Copper(II) by d(CGCGCG)2., Rohner M, Medina-Molner A, Spingler B, Inorg Chem. 2016 Jun 20;55(12):6130-40. doi: 10.1021/acs.inorgchem.6b00672. Epub , 2016 Jun 6. PMID: 27266259

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