2I89:
reference: A histidine/tryptophan pi-stacking interaction stabilizes the heme-independent folding core of microsomal apocytochrome b5 relative to that of mitochondrial apocytochrome b5., Wang L, Sun N, Terzyan S, Zhang X, Benson DR, Biochemistry. 2006 Nov 21;45(46):13750-9. PMID: 17105194

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