Mark W. Renner

Structural characterization of horseradish peroxidase using EXAFS spectroscopy. Evidence for Fe = O ligation in compounds I and II

Extended X-ray absorption fine structure spectroscopy has been utilized to determine the structural environment of the heme iron sites in horseradish peroxidase compounds I and II. For comparison, analogous studies have been undertaken on putative ferryl (Fe/sup IV/=O) porphyrin model compounds and on crystallographically characterized Cr/sup IV/=O and Cr/sup V/ identical with N porphyrins. In a preliminary communication, they suggested that a short ca. 1.6 A Fe-O bond is present in the high valent forms of both the enzyme and the synthetic porphyrins. The present work demonstrates unambiguously that a short, ca. 1.64 A, Fe-O bond length is present both in HRP compounds I and II and in their synthetic analogues. This structure is consistent only with an oxo-ferryl (Fe=O) complex as the active oxygen species in horseradish peroxidase. The structural details, their implications for heme protein mediated oxygen activation, and the difference between their results and those recently published by other workers.

Use of X-ray absorption spectroscopy and esterification to investigate Cr(III) and Ni(II) ligands in alfalfa biomass

Previously performed studies have shown that alfalfa shoot biomass can bind an appreciable amount of nickel(II) and chromium(III) ions from aqueous solution. Direct and indirect approaches were applied to study the possible mechanis ms involved in metal binding by the alfalfa biomass. The direct approach involves investigations of the metal-bound alfal fa shoot biomass by X-ray absorption spectroscopic analysis (XANES and EXAFS). Results from these studies suggest that ni ckel(II) and chromium(III) binding mostly occurs through coordination with oxygen ligands. Indirect approaches consist of chemical modification of carboxylate groups that have been shown to play an important role in metal binding to the alfal fa biomass. An appreciable decrease in metal binding resulted after acidic methanol esterification of the biomass, indica ting that carboxyl groups are entailed in the metal binding by the alfalfa biomass. In addition, base hydrolysis of the a lfalfa biomass increased the binding of these metals, which further indicates that carboxyl groups play an important role in the binding of these metal ions from solution. Therefore, by combining two different techniques, our results indicate that carboxylate groups are the major ligands responsible for the binding of nickel(II) and chromium(III) by alfalfa bio mass.

Oxidative chemistry of nickel porphyrins

Abstract. The oxidative chemistry of nickel(II) porphyrins is reviewed. Whether electron abstraction occurs from the metal to yield Ni(III) or from the porphyrin to yield Ni(II) π cation radicals is discussed in terms of the relative energy levels of the metal and porphyrin orbitals. The effects of axial ligands in further modulating this ordering as well as the orbital occupancy of Ni(III) are also reviewed. Structural considerations, based on existing stereochemical data for Ni(I), high spin Ni(II) and related Ni(III) tetraaza complexes, are used to predict the metrics of Ni(III) porphyrins for which no structural data are available.

Conformational landscapes of nonplanar porphyrins: superstructure, ligation, binding pockets and oxidation effects in Cu(II) porphyrins

Abstract The crystal structure of the porphyrin π cation radical Cu(II)OETPP + pyl 3 − ·py is reported (OETPP = 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin; py = pyridine). The compound is readily prepared by oxidation of Cu(II)OETPP in CH 2 Cl 2 with excess I 2 in the presence of py. The structure exhibits the following features: (1) The porphyrin is oxidized, not the Cu(II). (2) The Cu(II) is pentacoordinated with one py as axial ligand. (3) The sterically crowded macrocycle is saddle shaped. This distortion and the peripheral substituents combine to form superstructural, orthogonal ‘trenches’ above and below the porphyrin plane. (4) The trenches impose a specific orientation on the ligated axial py along the NCuN axis of the porphyrin and also form a ‘distal’ binding pocket that traps a second molecule of py. (5) The Cu(II)OETPP + py radical is diamagnetic because of antiferromagnetic coupling between the unpaired electrons of the Cu d x 2 -v 2 and the porphyrin π orbitals which is favored by the nonplanarity of the macrocycle. (6) The radical is more distorted than its nonplanar parent and provides additional support for the thesis that nonplanar porphyrins can easily undergo additional structural excursions. Conformational changes of the type reported here may also influence the reactivities of the π cation radicals formed in the (photo)catalytic cycles of photosynthetic and hemoproteins also comprised of nonplanar porphyrins. Crystallographic data: Cc ( No. 9), a = 21.706(2) A , b = 16.077(2) A , c = 19.993(2) A , β = 103.34(1)°, V = 6789(1) A 3 , Z = 4 .

Molecular structure of (5,10,15,20-tetrabutyl-2,3,7,8,12,13,17,18- octaethylporphyrinato)nickel(II)—correlation of nonplanarity with frontier orbital shifts †

Highly substituted porphyrins exhibit significantly distorted, nonplanar conformations in the solid state. The crystallographically determined degree of nonplanarity correlates with a bathochromic shift of the absorption maxima in solution. In addition, nonplanar porphyrins with meso aryl groups show increasing in-plane rotation of the meso aryl groups, which potentially could account for the observed changes in spectroscopic and physicochemical properties of nonplanar porphyrins. A crystal structure analysis of the title compound NiTBuOEP reveals a highly nonplanar conformation with an average deviation from planarity for the 24 macrocycle atoms of 0.462 A and displacements of the meso carbon atoms from the 4N-plane of 1.044 A. The average Ni–N bond distance in the crystal (1.873(3) A) is in good agreement with the Ni–N bond distance in solution (1.87 A) that was determined by EXAFS. Compared to more planar reference compounds, NiTBuOEP exhibits significantly red-shifted absorption spectra in solution, correctly predicted by INDO/s calculations. As the shortness of the Ni–N bonds has been shown to be an excellent indicator for the degree of conformational distortion in porphyrins, this proves that the highly nonplanar conformation of sterically strained porphyrins is maintained in solution. Thus, the physical and chemical properties measured in solution do indeed reflect the stereochemistry of the single crystals. In addition, the use of only alkyl substituents in NiTBuOEP to cause nonplanarity obviously circumvents potential electronic effects due to aryl ring interactions.

XAS studies of Ni(I), Ni(II), and Ni(III) complexes

Abstract XAS techniques for studying structural and electronic changes taking place after oxidation and reduction reactions of nickel complexes are described and applied to selected models for Factor 430, a nickel containing cofactor catalyzing an important step in the conversion of carbon dioxide to methane by methanogenic bacteria.

Correlation of macrocycle distortion with oxidation potentials of iron(III) porphyrins: molecule structure of the sterically crowded chloro-iron(III) 7,8,17,18-tetrabromo-5,10,15,20-tetraphenylporphyrin

Abstract The chloro-iron(III) complex of the tetraphenylporphyrin tetrabrominated at the antipodal β-pyrrole positions [(7,8,17,18-tetrabromo-5,10,15,20-tetraphenyl)porphyrin] has been synthesized and characterized by spectroscopy and X-ray crystallography. The iron atom is bonded to the chloride ion and the four pyrrole nitrogens. The Fe-Cl bond distance is 2.209(4) Å, and the mean value of the two opposite Fe-Np lengths at the brominated pyrrole rings is 2.079(8) Å, whereas the mean value of the two opposite Fe-Np′ bond distances at the non-brominated pyrrole rings is 2.041(8) Å. The X-ray structure determination and the analysis of the UV-Vis spectra obtained in solution and on thin films indicate that |FeCl(tpp-Br4)| (1) is principally saddle-shaped in the solid state and in solution. Variable-temperature (195–325 K) 1H NMR spectroscopy confirms the high-spin state (S=5/2) of the iron(III) center and indicates that the saddle-shaped conformation of 1 is maintained in solution. EPR spectra obtained in frozen CH2Cl2 solution and in the solid state show a rhombic symmetry with g values of 6.25, 5.70 and 1.99. Kadish et al. have shown that the one-electron oxidation potential of 1 increases only by 0.06 V relative to that of the non-brominated complex |FeCl(tpp)|. The present study indicates that the increase of the first oxidation potential of 1 is related to the non-planar distortion of the porphyrin. Relative to the unbrominated derivative |FeCl(tpp)|, this distortion destabilizes the π system of the macrocycle and thus compensates for the effects of the four electron-withdrawing bromine substituents.

A cell for extended x-ray absorption fine structure studies of oxygen sensitive products of redox reactions

We describe a cell suitable for extended x‐ray absorption fine structure (EXAFS) studies of oxygen and/or water sensitive products of redox reactions. The cell utilizes aluminized Mylar windows that are transparent to x rays, provide low gas permeability, and allow vacuum to be maintained in the cell. The windows are attached to the glassware with an epoxy that resists attack by common organic solvents. Additional side arms allow multiple spectroscopic probes of the same sample under anaerobic and anhydrous conditions.

Ni(II) and Ni(I) forms of pentaalkylamide derivatives of cofactor F430 of Methanobacterium thermoautotrophicum

A series of pentaalkylamide forms of F430 and of its 12,13-diepimer have been generated and characterized. Carbodiimide-assisted N-hydroxysulfosuccinimide activation of all five peripheral carboxylates of the F430 macrocycle allows nucleophilic attack by a number of primary amines (RNH2, R- = CH3-, CH3CH2-, CF3CH2-, CH3(CH2)3-) generating the pentaalkylamide derivatives. The identity of each derivative has been verified by fast-atom bombardment mass spectrometry (FAB-MS). The solubility of these derivatives in aprotic organic solvents varies as the amine alkyl substituent (R-) is changed. Electrochemical measurements have shown that the Ni(II/I) reduction potentials in N,N-dimethylformamide (DMF) are approximately -1 V (Ag/AgCl). Reduction by sodium amalgam in THF generates the Ni(I) form of the F430 diepimer pentabutylamide. The visible and EPR spectra of this Ni(I) species are very similar to the corresponding spectra of Ni(I) F430M (Jaun, B. and Pfaltz, A. (1986) J. Chem. Soc. Chem. Commun. 1327-1329.).

Molecular structure and spectroscopic properties of octaethylbenzochloracene, the porphycene analog of a benzochlorin

The X-ray structure, electrochemistry, and optical spectroscopy of the porphycene analog of a benzochlorin, octaethylbenzochloracene (OEBzC) are reported. The OEBzC macrocycle is easier to oxidize and harder to reduce than octaethylporphycene (OEPc). The optical spectra of OEBzC and of the diacid salt are also reported. The optical spectrum for the one-electron oxidation of OEBzC is indicative of π-cation radical formation. In porphyrins, saturation of one pyrrole ring and addition of an exocyclic benzene ring progressively red-shifts the first absorption band. For porphycenes, saturation of one pyrrole ring results in a 35 nm blue shift in the first absorption relative to OEPc. Compared to 2,3-dihydroporphycene (DHPc), the first absorption band in OEBzC is red-shifted 24 nm. These studies further illustrate how the fused exocyclic benzene ring influences the macrocycle conformation, chemical and physical properties of porphycenes.