3QBK:
reference: Crystal structures of an O-like blue form and an anion-free yellow form of pharaonis halorhodopsin., Kanada S, Takeguchi Y, Murakami M, Ihara K, Kouyama T, J Mol Biol. 2011 Oct 14;413(1):162-76. Epub 2011 Aug 16. PMID: 21871461

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