1KGK:
reference: Direct observation of a cytosine analogue that forms five hydrogen bonds to guanosine: guanidino G-clamp., Wilds CJ, Maier MA, Tereshko V, Manoharan M, Egli M, Angew Chem Int Ed Engl 2002 Jan 4;41(1):115-7. PMID: 12491456

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