1QXX:
reference: Dimerization of TonB is not essential for its binding to the outer membrane siderophore receptor FhuA of Escherichia coli., Koedding J, Howard P, Kaufmann L, Polzer P, Lustig A, Welte W, J Biol Chem 2004 Mar 12;279(11):9978-86. Epub 2003 Dec 8. PMID: 14665631

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