Andrzej Ozarowski

EPR Spectroscopic Characterization of the Manganese Center and a Free Radical in the Oxalate Decarboxylase Reaction IDENTIFICATION OF A TYROSYL RADICAL DURING TURNOVER

Several molecular mechanisms for cleavage of the oxalate carbon-carbon bond by manganese-dependent oxalate decarboxylase have recently been proposed involving high oxidation states of manganese. We have examined the oxalate decarboxylase from Bacillus subtilis by electron paramagnetic resonance in perpendicular and parallel polarization configurations to test for the presence of such species in the resting state and during enzymatic turnover. Simulation and the position of the half-field Mn(II) line suggest a nearly octahedral metal geometry in the resting state. No spectroscopic signature for Mn(III) or Mn(IV) is seen in parallel mode EPR for samples frozen during turnover, consistent either with a large zero-field splitting in the oxidized metal center or undetectable levels of these putative high-valent intermediates in the steady state. A narrow, featureless g = 2.0 species was also observed in perpendicular mode in the presence of substrate, enzyme, and dioxygen. Additional splittings in the signal envelope became apparent when spectra were taken at higher temperatures. Isotopic editing resulted in an altered line shape only when tyrosine residues of the enzyme were specifically deuterated. Spectral processing confirmed multiple splittings with isotopically neutral enzyme that collapsed to a single prominent splitting in the deuterated enzyme. These results are consistent with formation of an enzyme-based tyrosyl radical upon oxalate exposure. Modestly enhanced relaxation relative to abiological tyrosyl radicals was observed, but site-directed mutagenesis indicated that conserved tyrosine residues in the active site do not host the unpaired spin. Potential roles for manganese and a peripheral tyrosyl radical during steady-state turnover are discussed.

Influence of Pb(II) ions on the EPR properties of the semiquinone radicals of humic acids and model compounds: high field EPR and relativistic DFT studies.

X-band (9.76 GHz) and high field (416.00 GHz) electron paramagnetic resonance spectroscopy (EPR) was used to study the interactions between Pb(II) ions and semiquinone radicals of natural humic acids and their simple models. The EPR experiments were performed on powder samples. The formation of Pb(II) complexes with the radicals was accompanied by a significant decrease of g parameters as compared to those observed for parent radicals. Two types of complexes were identified depending on the initial concentration of Pb(II) ions. For one of them the anisotropic hyperfine coupling with the (207)Pb nucleus was observed. Systematic DFT calculations were carried out for complexes with different forms of radical ligands (L(2)(-*), HL(-*), and H(2)L*) derived from 3,4-dihydroxybenzoic acid representing different ligation schemes. The g parameters calculated for the structure characterized by a significant accumulation of spin density on the Pb atom are strongly deviated from the values observed experimentally. Moreover, a decrease of the spin population on all oxygen atoms as a result of complexation of Pb(II) via carboxyl oxygens and protonation of hydroxyl oxygens is required to reproduce the experimental g parameters.

Crystal structure, EPR and magnetic susceptibility studies of tetrakis [μ-(β-alanine)-O,O′]dichlorodicopper(II) dichloro monohydrate

Abstract Tetrakis[μ-(β-alanine)- O,O ′]dichlorodicopper(II) dichloro monohydrate, [Cu 2 (β-alanine) 4 Cl 2 ]Cl 2 ·H 2 O, crystallizes in the triclinic system, space group P I , a = 9.444(2), b = 9.498(2), c = 14.107(3) A , α = 104.03(3), β = 105.90(3) and γ = 97.15(3)° with one formula weight in a unit cell. Two different centrosymmetric carboxylato-bridged dimeric subunits are formed. The coordination geometry about each copper(II) center is square-pyramidal, with four oxygen atoms in the basal plane and the chloride ion at the apical position. The coordination polyhedra of two dimers are slightly different: for the dimer with a shorter Cu(1)Cu(1) 1 distance of 2.6331(10) A a Cu(1)Cl bond length of 2.4511(10) A is observed, in comparison with Cu(2)Cu(2) 11 of 2.6608(10) A and Cu(2)Cl bond length of 2.4387(10) A for the other dimer. Magnetic and EPR properties of the title complex are compared with those exhibited by [Cu 2 (β-alanine) 4 (H 2 O) 2 ] (NO 3 ) 4 ·4H 2 O that forms dimeric molecules of only one kind. The isotropic exchange interactions are strongly antiferromagnetic with a J value of 320 cm −1 (corresponding to the Hamiltonian H = J S 1 ·S 2 ) for the chloride dimers and 334 cm −1 for the nitrate complex. The complexes exhibit spin-triplet EPR spectra with very well-resolved hyperfine structure of two equivalent copper nuclei at low temperature. The chloride compound shows two overlapping EPR spectra with different spin Hamiltonian parameters. Only one EPR spectrm is observed in the nitrate complex. Structural differences seem to affect significantly the zero-field splitting parameter D .

Understanding natural semiquinone radicals – Multifrequency EPR and relativistic DFT studies of the structure of Hg(II) complexes

Multifrequency EPR spectroscopy and DFT calculations were used to investigate Hg(II) complexes with semiquinone radical ligands formed in a direct reaction between the metal ions and tannic acid (a polyphenol closely related to tannins). Because of the intricate structure of tannic acid a vast array of substituted phenolic compounds were tested to find a structural model mimicking its ability to react with Hg(II) ions. The components of the g matrix (the g tensor) determined from the high field (208 GHz) EPR spectra of the Hg(II) complexes with the radical ligands derived from tannic acid and from the model compounds were analogous, indicating a similar coordination mode in all the studied Hg(II) complexes. Since catechol (1,2-dihydroxybenzene) was the simplest compound undergoing the reaction with Hg(II) it was selected for DFT studies which were aimed at providing an insight into the structural properties of the investigated complexes. Various coordination numbers and different conformations and protonation states of the ligands were included in the theoretical analyses. g Matrices were computed for all the DFT optimized geometries. A good agreement between the theoretical and experimental values was observed only for the model with the Hg(II) ion tetracoordinated by two ligands, one of the ligands being monoprotonated with the unpaired electron mainly localized on it.

EPR studies of spin–spin interactions between Cu(II) centers in dimeric, hexameric and homo- and heteronuclear tetrameric complexes

Applications of conventional (X-band), High-Field and High-Frequency EPR to study the nature of exchange interactions between Cu(II) centres in different homo- and hetronuclear systems are reviewed. The fine structure EPR spectra have been observed and analyzed in terms of spin-Hamiltonian parameters corresponding to S ≥ 1 for different [Cu2(β-alanine)4Cl2]Cl2·H2O dimeric complexes present in one unit cell, Cu(II) dimers possessing different conformations, dimeric species formed between Cu(II) ions and polyfunctional ligand in solution, Cu-Zn-Cu-Zn and Cu-Co-Cu-Co tetrameric heterometallomacrocycles, Cu4O4 cubane-like tetramers and for the chain Cu(II) hexamer.

Study of complexes of a tryptophan-free mutant of E. coli trp aporepressor with tryptophan analogues using optically detected magnetic resonance (ODMR)

Phosphorescence and optically detected magnetic resonance (ODMR) spectra of tryptophan (W) and several of its analogues (4‐, 5‐, 6‐methyltryptophan (MeW); 4‐, 5‐, 6‐fluorotryptophan (FW); 5‐bromotryptophan) are compared with those of complexes formed with the W‐free trp aporepressor from Escherichia coli (W19,99F). W19,99F binds W and each analogue except 4‐FW with an estimated K D≤30 μM; triplet state spectroscopic and kinetic effects that accompany binding at the corepressor site are reported. ODMR data for the MeW isomers are presented for the first time. No binding of 7‐azaW is observed, in agreement with the low affinity found by previous workers.

Iron(III) bis(pyrazol-1-yl)acetate based decanuclear metallacycles: synthesis, structure, magnetic properties and DFT calculations

The synthesis, structural aspects, magnetic interpretation and theoretical rationalizations for a new member of the ferric wheel family, a decanuclear iron(iii) complex with the formula [Fe10(bdtbpza)10(μ2-OCH3)20] (1), featuring the N,N,O tridentate bis(3,5-di-tert-butylpyrazol-1-yl)acetate ligand, are reported. The influence of the steric effect on both the core geometry and coordination mode is observed. Temperature dependent (2.0-300 K range) magnetic susceptibility studies carried out on complexes 1 established unequivocally antiferromagnetic (AF) interactions between high-spin iron(iii) centers (S = 5/2), leading to a ground state S = 0. The mechanism of AF intramolecular coupling was proved using a broken-symmetry approach within the density functional method at the B3LYP/def2-TZVP(-f)/def2-SVP level of theory.

ODMR of the photoexcited triplet state of donor-acceptor complexes of naphthalene donor with pyromellitic dianhydride, 1,2,4,5-tetracyanobenzene, and 1,3,5-trinitrobenzene acceptors

Abstract Odmr measurements of photoexcited triplet state traps were carried out at 1.2 K on polycrystalline donor–acceptor complexes of naphthalene (N) with the acceptors pyromellitic dianhydride (PMDA), 1,2,4,5-tetracyanobenzene (TCNB), and 1,3,5-trinitrobenzene (TNB). The zero field splittings (zfs) are consistent with a locally excited N triplet state with considerable admixture of charge transfer character that increases in the order N/TNB

Electron spin–lattice relaxation of the triplet state in disordered solids exhibits dispersion

Abstract Global analysis of microwave-induced delayed phosphorescence (MIDP) of photoexcited triplet states at 1.2 K is employed to obtain their spin–lattice relaxation (SLR) rate constants, and other relevant parameters. The phosphorescence decay kinetics of quinoxaline in crystalline durene and n-pentane are predicted accurately, but not in disordered solids nor of tryptophan in aqueous glass. For the latter, the decay fits a bimodal SLR distribution; a subset decays with slow SLR and as predicted, while the remainder displays the expected decay properties of fast SLR. Although not following this bimodal pattern, SLR of quinoxaline in disordered solids also exhibits dispersion.