Nitric Oxide Binding to Ferric and Ferrous Porphyrins Encapsulated in the Cyclodextrin Nanocavities in Aqueous Solution

<i>meso</i>-Tetraphenylporphyrin Fe(II/III) complexes encapsulated in a cyclodextrin (CD) dimer with pyridine (P) or imidazole (I) ligands have been developed as synthetic heme models in water. Here, the binding of nitric oxide (NO) to the models (hemoCD-P and hemoCD-I) was investigated. The introduction of NO gas into the hemoCD complexes formed stable ferric and ferrous NO adducts. UV-vis, electron paramagnetic resonance, and resonance Raman measurements revealed the presence of six-coordinate ferric complexes and five- and six-coordinate ferrous complexes. The reductive nitrosylation from Fe(III)NO to Fe(II)NO proceeded at high pH, and the reaction rate was dependent on the Fe(II/III) redox potentials of hemoCD-P and -I. In acidic solution, the ferric NO complexes were stably formed due to slow autoreduction. The binding constants of NO to ferric and ferrous complexes were determined to be 10^5-6 and 10^11-12 M^-1, respectively. The binding data were highly comparable to those for natural heme systems, and the data for other gases (CO, O₂, HCN, and H₂S) were integrated and discussed. We conclude that hemoCD was an effective aqueous heme model system that reproduced the gas-binding properties of native heme proteins in both the ferric and ferrous states.