2DXM:
reference: Protonation states of buried histidine residues in human deoxyhemoglobin revealed by neutron crystallography., Chatake T, Shibayama N, Park SY, Kurihara K, Tamada T, Tanaka I, Niimura N, Kuroki R, Morimoto Y, J Am Chem Soc. 2007 Dec 5;129(48):14840-1. Epub 2007 Nov 9. PMID: 17990881

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