Pentacoordinate iron(III) porphyrin carboxylates: Synthesis, physicochemical characteristics and x-ray crystal structure of acetato(5, 10, 15, 20-tetraparatolylporphyrinato) iron(III)

Preparation and spectroscopic properties of the (cyanato-N) and (oxalato) iron(III) “picket fence” porphyrins: Structure of the (cyanato-N)(α,α,α,α-tetrakis(o-pivalamidophenyl)porphinato)iron(III) complex

The reaction of the iron(III) aqua triflato picket fence porphyrin starting material with NaOCN and K 2 C 2 O 4 salts, solubilized by the crown ether 18- C -6 in organic solvents, yields respectively the iron(III) porphyrin species [ Fe III( TpivPP )( NCO )] and [ Fe III( TpivPP )( ox )]−. These compounds have been characterized by UV-vis, IR, and 1 H NMR spectroscopies. Proton NMR data for the isolated products are in accordance with high-spin ( S = 5/2) ferric porphyrin derivatives. The X-ray molecular structure of the (cyanato-N)(α,α,α,α-tertakis(o-pivalamidophenyl)porphinato)iron(III), has been determined. This structure, which is the first one reported of an iron-cyanato species, confirms the spin-state of the cyanato derivative. The iron atom is pentacoordinated by the four nitrogen atoms of the pyrrole rings and the nitrogen atom of the NCO − group. It lies at 0.540(1) Å out of the porphinato plane and 0.486(1) Å out of the four nitrogen plane of the porphyrin ring. The Fe – N ( NCO ) bond length is 1.970(2) Å and the Fe - N - C ( NCO ) bond angle is 169.9(3)°.

Speciation and structure of ferriprotoporphyrin IX in aqueous solution: spectroscopic and diffusion measurements demonstrate dimerization, but not μ-oxo dimer formation

Changes in epsilon (393) (the Soret band) of aqueous ferriprotoporphyrin IX [Fe(III)PPIX] with concentration indicate that it dimerizes, but does not form higher aggregates. Diffusion measurements support this observation. The diffusion coefficient of aqueous Fe(III)PPIX is half that of the hydrated monomeric dicyano complex. Much of the apparent instability of aqueous Fe(III)PPIX solutions could be attributed to adsorption onto glass and plastic surfaces. However, epsilon (347) was found to be independent of the aggregation state of the porphyrin and was used to correct for the effects of adsorption. The UV-vis spectrum of the aqueous dimer is not consistent with that expected for a mu-oxo dimer and the (1)H NMR spectrum is characteristic of five-coordinate, high-spin Fe(III)PPIX. Magnetic susceptibility measurements using the Evans method showed that there is no antiferromagnetic coupling in the dimer. By contrast, when the mu-oxo dimer is induced in 10% aqueous pyridine, characteristic UV-vis and (1)H NMR spectra of this species are observed and the magnetic moment is consistent with strong antiferromagnetic coupling. We propose a model in which the spontaneously formed aqueous Fe(III)PPIX dimer involves noncovalent interaction of the unligated faces of two five-coordinate H(2)O/HO-Fe(III)PPIX molecules, with the axial H(2)O/OH(-) ligands directed outwards. This arrangement is consistent with the crystal structures of related five-coordinate iron(III) porphyrins and accounts for the observed pH dependence of the dimerization constant and the spectra of the monomer and dimer. Structures for the aqueous dimer are proposed on the basis of molecular dynamics/simulated annealing calculations using a force field previously developed for modeling metalloporphyrins.

Zn(II), Ni(II), Cu(II), and Fe(III) complexes of potentially bimetalating tris(pyridine- and imidazole-appended) picket-fence naphthylporphyrins with benzyl ether spacers: implications for cytochrome c oxidase active-site modeling

Two new unsymmetrical picket-fence naphthylporphyrin ligands, 1 and 2, and several of their metalated porphyrinato complexes have been synthesized as precursor model compounds for the binuclear (Fe/Cu) cytochrome c oxidase (CcO) active site. 1 and 2 have a naphthylporphyrin superstructure that has been specifically incorporated to confer long-term configurational stability to the atropisomeric products. The two picket-fence porphyrin ligands also bear covalently linked, axially offset tris(heterocycle) coordination sites for a copper ion, much like that found in the native enzyme. Monometallic porphyrin complexes [M = Zn(II), Ni(II), Cu(II), and Fe(III)] of the pyridine-appended ligand 1 have been prepared and spectroscopically and magnetically characterized. An unusual monomeric iron(III) hydroxo porphyrin complex was isolated upon workup of the compound formed under ferrous sulfate/acetic acid reflux conditions. There is general difficulty in forming binuclear complexes of 1, which is attributed to the conformational flexibility of the benzyl ether type picket spacers. The potential of ligands such as 1 and 2 for future CcO active-site modeling studies is considered.

Molecular Assemblies Containing Unsupported [Fe(III)-(&mgr;(2):eta(2)-RCO(2))-Cu(II)] Bridges

Formate is an inhibitor of cytochrome oxidases and also effects conversion of the bovine heart enzyme from the "fast" to the "slow" cyanide-binding form. The molecular basis of these effects is unknown; one possibility is that formate inserts as a bridge into the binuclear heme a(3)-Cu(B) site, impeding the binding of dioxygen or cyanide. Consequently, Fe-Cu-carboxylate interactions are a matter of current interest. We have initiated an examination of such interactions by the synthesis of the first examples of [Fe(III)-(&mgr;(2):eta(2)-RCO(2))-Cu(II)] bridges, minimally represented by Fe(III)-L + Cu(II)-O(2)CR --> [Fe(III)-(RCO(2))-Cu(II)] + L. A series of Cu(II) precursor complexes and solvate forms have been prepared and their structures determined, including [Cu(Me(5)dien)(O(2)CH)](+) (3), [Cu(Me(5)dien)(O(2)CH)(MeOH)](+) (4), [Cu(Me(6)tren)(O(2)CH)](+) (5), and [Cu(Me(5)dien)(OAc)](+) (6). [4](ClO(4)) was obtained in monoclinic space group P2(1)/n with a = 8.166(3) Å, b = 15.119(5) Å, c = 15.070(4) Å, beta = 104.65(2) degrees, and Z = 4. [5](ClO(4))/[6](ClO(4)) crystallize in orthorhombic space groups Pnma/Pna2(1) with a = 16.788(2)/14.928(5) Å, b = 9.542(1)/9.341(4) Å, c = 12.911(1)/12.554(4) Å, and Z = 4/4. In all cases, the carboxylate ligand is terminal and is bound in a syn orientation. Also prepared for the purpose of structural comparison was [Fe(OEP)(O(2)CH)], which occurred in monoclinic space group P2(1)/c with a = 13.342(2) Å, b = 13.621(2) Å, c = 19.333(2) Å, beta = 106.12(2) degrees, and Z = 4. The desired bridges were stabilized in the assemblies [(OEP)Fe(O(2)CH)Cu(Me(5)dien)(OClO(3))](+) (9), [(OEP)Fe(OAc)Cu(Me(5)dien)](2+) (10), and {(OEP)Fe[(O(2)CH)Cu(Me(6)tren)](2)}(3+) (11), which were prepared by the reaction of 3, 6, and 5, respectively, with [Fe(OEP)(OClO(3))] in acetone or dichloromethane. [9](ClO(4))/[10](ClO(4))(2).CH(2)Cl(2) crystallize in triclinic space group P&onemacr; with a = 9.016(3)/13.777(3) Å, b = 15.377(5)/13.847(3) Å, c = 19.253(5)/17.608(4) Å, alpha = 78.12(3)/96.82(3) degrees, beta = 86.30(4)/108.06(3) degrees, gamma = 76.23(3)/114.32(3) degrees, and Z = 2/2. Each assembly contains a [Fe(III)-(RCO(2))-Cu(II)] bridge but with the differing orientations anti-anti (9) and syn-anti (10, 11). The compound [11](ClO(4))(2)(SbF(6)) occurs in orthorhombic space group Pbcn with a = 12.517(6) Å, b = 29.45(1) Å, c = 21.569(8) Å, and Z = 4. Complex 11 is trinuclear; the Fe(III) site has two axial formate ligands with bond distances indicative of a high-spin configuration. Structural features of 9-11 are discussed and are considered in relation to the possible insertion of formate into the binuclear sites of two oxidases whose structures were recently determined. The present results contribute to the series of molecular assemblies with the bridge groups [Fe(III)-X-Cu(II)], X = O(2)(-), OH(-), and RCO(2)(-), all with a common high-spin heme, thereby allowing an examination of electronic structure as dependent on the bridging atom or group and bridge structure. (Me(5)dien = 1,1,4,7,7-pentamethyldiethylenetriamine; Me(6)tren = tris(2-(dimethylamino)ethyl)amine; OEP = octaethylporphyrinate(2-).)

Electron paramagnetic resonance studies of ferric cytochrome c' from photosynthetic bacteria

Electronic ground nature of ferric cytochromes c' isolated from five photosynthetic bacteria. Chromatium vinosum ATCC 17899, Rhodobacter capsulatus ATCC 11166, Rhodopseudomonas palustris ATCC 17001, Rhodospirillum molischianum ATCC 14031, and Rhodospirillum rubrum ATCC 11170 has been investigated by electron paramagnetic resonance (EPR) spectroscopy. EPR spectra indicate that the electronic ground state of five ferric cytochromes c' is a quantum mechanical admixed-spin state of a high spin (S = 5/2) and an intermediate spin (S = 3/2) at pH 7.2 and is high-spin state at pH 11.0. At physiological pH, however, the content of an intermediate spin state differs with the bacterial source of the protein: approximately 50%, Chromatium vinosum; approximately 40%, Rhodobacter capsulatus and Rhodopseudomonas palustris; approximately 10%, Rhodospirillum molischianum and Rhodospirillum rubrum. Computer simulation of the spectra supports this diversity of the contribution of an intermediate spin state. Model studies of the ferric porphyrin complexes suggest that the correlation between content of an intermediate spin state and heme iron displacement from the mean heme plane. Therefore, the variation of the content of an intermediate spin state observed in the present study reflects the subtle difference in the degree of heme iron displacement among the proteins.

An EXAFS study of the interaction of substrate with the ferric active site of protocatechuate 3,4-dioxygenase

X-ray crystallographic studies of the intradiol cleaving protocatechuate 3,4-dioxygenase from Pseudomonas aeruginosa have shown that the enzyme has a trigonal bipyramidal ferric active site with two histidines, two tyrosines, and a solvent molecule as ligands [Ohlendorf, D.H., Lipscomb, J.D., & Weber, P.C. (1988) Nature 336, 403-405]. Fe K-edge EXAFS studies of the spectroscopically similar protocatechuate 3,4-dioxygenase from Brevibacterium fuscum are consistent with a pentacoordinate geometry of the iron active site with 3 O/N ligands at 1.90 A and 2 O/N ligands at 2.08 A. The 2.08-A bonds are assigned to the two histidines, while the 1.90-A bonds are associated with the two tyrosines and the coordinated solvent. The short Fe-O distance for the solvent suggests that it coordinates as hydroxide rather than water. When the inhibitor terephthalate is bound to the enzyme, the XANES data indicate that the ferric site becomes 6-coordinate and the EXAFS data show a beat pattern which can only be simulated with an additional Fe-O/N interaction at 2.46 A. Together, the data suggest that the oxygens of the carboxylate group in terephthalate displace the hydroxide and chelate to the ferric site but in an asymmetric fashion. In contrast, protocatechuate 3,4-dioxygenase remains 5-coordinate upon the addition of the slow substrate homoprotocatechuic acid (HPCA). Previous EPR data have indicated that HPCA forms an iron chelate via the two hydroxyl functions.(ABSTRACT TRUNCATED AT 250 WORDS)