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A covalently linked porphyrin-quinone dyad crystallizes with two orientations of the quinone, extended away from (off) and cofacial with the porphyrin macrocycle (on), which induce different conformations of the macrocycle and model the recently proposed structural effect of a nearby residue on the heme prosthetic group of a nitric oxide synthase.
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Nurco et al. (2002) studied this question.
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