1VWO: Streptavidin Complexed With Cyclo-Ac-[Chpqgppc]-Nh2 Monomer,
reference: In crystals of complexes of streptavidin with peptide ligands containing the HPQ sequence the pKa of the peptide histidine is less than 3.0., Katz BA, Cass RT, J Biol Chem 1997 May 16;272(20):13220-8. PMID: 9148939

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