Nitric oxide (NO) binds to hemes even more strongly than CO (and hence O2),10 so strongly, in fact, that the Fe-N1m bond is very weak and easily ruptured. II ,III,133 Attachment to the metal is via the nitrogen atom; however, the geometry of attachment is sensitive to the 7T basicity of the metalloporphyrin, and ranges from linear to strongly bent. In binding to COlI the NO ligand is effectively reduced to NO -, with concomitant oxidation of COIl to Co III:
In much the same way that cobalt-dioxygen systems are paramagnetic (5 = t) and amenable to EPR studies, iron-nitric oxide (also called iron nitrosyl) species are also paramagnetic and isoelectronic with cobalt-dioxygen species. The unpaired spin is localized mostly on the NO group.
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