| Abstract
| - The polyanionic water-soluble and non-μ-oxo-dimer-forming iron porphyrin iron(III) 54,104,154,204-tetra-tert-butyl-52,56,152,156-tetrakis[2,2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin, (P8-)FeIII (1), was synthesized asan octasodium salt by applying well-established porphyrin and organic chemistry procedures to bromomethylatedprecursor porphyrins and characterized by standard techniques such as UV−vis and 1H NMR spectroscopy. Asingle pKa1 value of 9.26 was determined for the deprotonation of coordinated water in (P8-)FeIII(H2O)2 (1-H2O)present in aqueous solution at pH <9. The porphyrin complex reversibly binds NO in aqueous solution to give themononitrosyl adduct, (P8-)FeII(NO+)(L), where L = H2O or OH-. The kinetics of the binding and release of NOwas studied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniques.The diaqua-ligated form of the porphyrin complex binds and releases NO according to a dissociative interchangemechanism based on the positive values of the activation parameters ΔS⧧ and ΔV⧧ for the “on” and “off” reactions.The rate constant kon = 6.2 × 104 M-1 s-1 (24 °C), determined for NO binding to the monohydroxo-ligated(P8-)FeIII(OH) (1-OH) present in solution at pH >9, is markedly lower than the corresponding value measured for1-H2O at lower pH (kon = 8.2 × 105 M-1 s-1, 24 °C, pH 7). The observed decrease in the reactivity is contradictoryto that expected for the diaqua- and monohydroxo-ligated forms of the iron(III) complex and is accounted for interms of a mechanistic changeover observed for 1-H2O and 1-OH in their reactions with NO. The mechanisticinterpretation offered is further substantiated by the results of water-exchange studies performed on the polyanionicporphyrin complex as a function of pH, temperature, and pressure.
- The polyanionic water-soluble (P8-)FeIII(H2O)2 complex reversibly binds NO in aqueous solution according to a dissociative interchange mechanism, whereas NO binding to the monohydroxo-ligated (P8-)FeIII(OH) complex, present in solution at pH >9, is markedly slower and proceeds according to an associative mechanism. The mechanistic interpretation is further substantiated by water-exchange studies performed as a function of pH, temperature, and pressure.
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