A comparative mechanistic study of the reversible binding of NO to a water-soluble octa-cationic Fe(III) porphyrin complex.
The water-soluble, non-mu-oxo dimer-forming porphyrin, [5,10,15,20-tetrakis-4'-t-butylphenyl-2',6'-bis-(N-methylene-(4''-t-butylpyridinium))porphyrinato]iron(III) octabromide, (P(8+))Fe(III), with eight positively charged substituents in the ortho positions of the phenyl rings, was characterized by UV-vis and 1H NMR spectroscopy and 17O NMR water-exchange studies in aqueous solution. Spectrophotometric titrations of (P(8+))Fe(III) indicated a pKa1 value of 5.0 for coordinated water in (P(8+))Fe(III)(H2O)2. The monohydroxo-ligated (P(8+))Fe(III)(OH)(H2O) formed at 5 < pH < 12 has a weakly bound water molecule that undergoes an exchange reaction, k(ex) = 2.4 x 10(6) s(-1), significantly faster than water exchange on (P(8+))Fe(III)(H2O)2, viz. k(ex) = 5.5 x 10(4) s(-1) at 25 degrees C. The porphyrin complex reacts with nitric oxide to yield the nitrosyl adduct, (P(8+))Fe(II)(NO+)(L) (L = H2O or OH-). The diaqua-ligated (P(8+))Fe(III)(H2O)2 binds and releases NO according to a dissociatively activated mechanism, analogous to that reported earlier for other (P)Fe(III)(H2O)2 complexes. Coordination of NO to (P(8+))Fe(III)(OH)(H2O) at high pH follows an associative mode, as evidenced by negative deltaS(double dagger)(on) and deltaV(double dagger)(on) values measured for this reaction. The observed ca. 10-fold decrease in the NO binding rate on going from six-coordinate (P(8+))Fe(III)(H2O)2 (k(on) = 15.1 x 10(3) M(-1) s(-1)) to (P(8+))Fe(III)(OH)(H2O) (k(on) = 1.56 x 10(3) M(-1) s(-1) at 25 degrees C) is ascribed to the different nature of the rate-limiting step for NO binding at low and high pH, respectively. The results are compared with data reported for other water-soluble iron(III) porphyrins with positively and negatively charged meso substituents. Influence of the porphyrin periphery on the dynamics of reversible NO binding to these (P)Fe(III) complexes as a function of pH is discussed on the basis of available experimental data.
Jee
Joo-Eun
JE
Institute for Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany.
Wolak
Maria
M
Balbinot
Domenico
D
Jux
Norbert
N
Zahl
Achim
A
van Eldik
Rudi
R
D001665:Binding Sites; D002412:Cations / Q000737:chemistry; D005290:Ferric Compounds / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008968:Molecular Conformation; D009569:Nitric Oxide / Q000737:chemistry*; D011166:Porphyrins / Q000737:chemistry*; D012680:Sensitivity and Specificity; D012995:Solubility; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D014867:Water / Q000737:chemistry
D002412:Cations; D005290:Ferric Compounds; D008024:Ligands; D011166:Porphyrins; D014867:Water; D009569:Nitric Oxide; D007501:Iron