The synthesis and characterization of the Fe(III) complex of a novel crown ether-porphyrin conjugate, 5(2)-N-(4-aza-18-crown-6)methyl-5(4),10(4),15(4),20(4)-tetra-tert-butyl-5(6)-methyl-5,10,15,20-tetraphenylporphyrin (H(2)Porph), as well as the corresponding hydroxo, dimeric, Fe(II), and peroxo species are reported. The crystal structure of [Fe-III(Porph)Cl]center dot H3O+center dot FeCl4-center dot C6H6 center dot EtOH is also reported. [Fe-III(Porph)(DMSO)(2)](+) and K[Fe-III(Porph)(O-2(2-))] are high-spin species (Mossbauer data: delta = 0.38 mm s(-1), Delta E-q = 0.83 mm s(-1) and delta = 0.41 mm s(-1), Delta E-q = 0.51 mm s(-1), respectively), whereas in a solution of reduced [Fe-III(Porph)(DMSO)(2)](+) complex the low-spin [Fe-II(Porph)(DMSO)(2)] (delta = 0.44 mm s(-1), Delta E-q = 1.32 mm s(-1)) and high-spin [Fe-II(Porph)(DMSO)] (delta = 1.27 mm s(-1), Delta E-q = 3.13 mm s(-1)) iron(II) species are observed. The reaction of [Fe-III(Porph)(DMSO)(2)](+) with KO2 in DMSO has been investigated. The first reaction step, involving reduction to [Fe-II(Porph)(DMSO)(2)], was not investigated in detail because of parallel formation of an Fe(III)-hydroxo species. The kinetics and thermodynamics of the second reaction step, reversible binding of superoxide to the Fe(II) complex and formation of an Fe(III)-peroxo species, were studied in detail (by stopped-flow time-resolved UV/vis measurements in DMSO at 25 degrees C), resulting in k(on) = 36 500 +/- 500 M-1 s(-1), k(off) = 0.21 +/-0.01 s(-1) (direct measurements using an acid as a superoxide scavenger), and K-O2- = (1.7 +/- 0.2) x 10(5) (superoxide binding constant kinetically obtained as k(on)/k(off)), (1.4 +/- 0.1) x 10(5), and (9.0 +/- 0.1) x 10(4) M-1 (thermodynamically obtained in the absence and in the presence of 0.1 M NBu4PF6, respectively). Temperature-dependent kinetic measurements for k(on) (-40 to 25 degrees C in 3:7 DMSO/CH3CN mixture) yielded the activation parameters Delta H = 61.2 +/- 0.9 kJ mol(-1) and Delta S = +48 +/- 3 J K-1 mol(-1). The observed reversible binding of superoxide to the metal center and the obtained kinetic and thermodynamic parameters are unique. The finding that fine-tuning of the proton concentration can cause the Fe(III)-peroxo species to release O-2(-) and form an Fe(II) species is of biological interest, since this process might occur under very specific physiological conditions.