3BIR:
reference: Dissecting histidine interactions of ribonuclease T1 with asparagine and glutamine replacements: analysis of double mutant cycles at one position., De Vos S, Doumen J, Langhorst U, Steyaert J, J Mol Biol. 1998 Jan 30;275(4):651-61. PMID: 9466938

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