5GX9:
reference: Neutron crystallography of photoactive yellow protein reveals unusual protonation state of Arg52 in the crystal., Yonezawa K, Shimizu N, Kurihara K, Yamazaki Y, Kamikubo H, Kataoka M, Sci Rep. 2017 Aug 24;7(1):9361. doi: 10.1038/s41598-017-09718-9. PMID: 28839266

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