1NEJ:
reference: COHbC and COHbS crystallize in the R2 quaternary state at neutral pH in the presence of PEG 4000., Patskovska LN, Patskovsky YV, Almo SC, Hirsch RE, Acta Crystallogr D Biol Crystallogr. 2005 May;61(Pt 5):566-73. Epub 2005, Apr 20. PMID: 15858266

This OCA Structure page
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