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

meso-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 105-6 and 1011-12 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.