Nitric oxide binding to cationic and anionic ferric porphyrins in aqueous solution: reversible formation of ferrous NO species of the cationic porphyrin

Two water-soluble ferric porphyrins, sodium 5α,10β,15α,20β-tetrakis(2-(sulfonatoacetamido)phenyl)porphyrinatoiron(III) (FeIIITanP) and 5α,10β,15α,20β-tetrakis(2-(N,N,N-trimethylammoniumacetamido)phenyl)porphyrinatoiron(III) chloride (FeIIITcatP), were synthesized. The pKa values of the coordinated H2O of FeIIITanP and FeIIITcatP were evaluated to be 8.0 and 4.1, respectively. Reactions of NO with the ferric porphyrins were examined spectrophotometrically in aqueous solution. Porphyrin FeIIITanP binds NO reversibly to give the corresponding ferric NO species at pH 1.3 and pH 3.0, and FeIIITcatP reacts similarly with NO at pH 1.3. The thermodynamic data for the NO binding were estimated from vanʹt Hoff plots. At pH 3.0, visible and ESR spectral data indicated that FeIIITcatP binds NO reversibly to produce ferrous NO species depending on NO partial pressures. These results were discussed based on through-space intramolecular interactions between the coordinated H2O or NO and the ionic substituents of the porphyrins.