4S2H:
reference: Direct determination of protonation states and visualization of hydrogen bonding in a glycoside hydrolase with neutron crystallography., Wan Q, Parks JM, Hanson BL, Fisher SZ, Ostermann A, Schrader TE, Graham DE, Coates L, Langan P, Kovalevsky A, Proc Natl Acad Sci U S A. 2015 Sep 21. pii: 201504986. PMID: 26392527

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