3D6C:
reference: The pK(a) values of acidic and basic residues buried at the same internal location in a protein are governed by different factors., Harms MJ, Castaneda CA, Schlessman JL, Sue GR, Isom DG, Cannon BR, Garcia-Moreno E B, J Mol Biol. 2009 May 29;389(1):34-47. Epub 2009 Mar 24. PMID: 19324049

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