2PIS:
reference: Efforts toward expansion of the genetic alphabet: structure and replication of unnatural base pairs., Matsuda S, Fillo JD, Henry AA, Rai P, Wilkens SJ, Dwyer TJ, Geierstanger BH, Wemmer DE, Schultz PG, Spraggon G, Romesberg FE, J Am Chem Soc. 2007 Aug 29;129(34):10466-73. Epub 2007 Aug 8. PMID: 17685517

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