6FTD:
reference: Coordination of the biliverdin D-ring in bacteriophytochromes., Lenngren N, Edlund P, Takala H, Stucki-Buchli B, Rumfeldt J, Peshev I, Hakkanen H, Westenhoff S, Ihalainen JA, Phys Chem Chem Phys. 2018 Jul 11;20(27):18216-18225. doi: 10.1039/c8cp01696h. PMID: 29938729

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