Hong-In Lee

Robust and Efficient Amide-Based Nonheme Manganese(III) Hydrocarbon Oxidation Catalysts: Substrate and Solvent Effects on Involvement and Partition of Multiple Active Oxidants

Two new mononuclear nonheme manganese(III) complexes of tetradentate ligands containing two deprotonated amide moieties, [Mn(bpc)Cl(H(2)O)] (1) and [Mn(Me(2)bpb)Cl(H(2)O)]⋅CH(3)OH (2), were prepared and characterized. Complex 2 has also been characterized by X-ray crystallography. Magnetic measurements revealed that the complexes are high spin (S = 5/2) Mn(III) species with typical magnetic moments of 4.76 and 4.95 μ(B), respectively. These nonheme Mn(III) complexes efficiently catalyzed olefin epoxidation and alcohol oxidation upon treatment with MCPBA under mild experimental conditions. Olefin epoxidation by these catalysts is proposed to involve the multiple active oxidants Mn(V)=O, Mn(IV)=O, and Mn(III)-OO(O)CR. Evidence for this approach was derived from reactivity and Hammett studies, KIE (k(H)/k(D)) values, H(2)(18)O-exchange experiments, and the use of peroxyphenylacetic acid as a mechanistic probe. In addition, it has been proposed that the participation of Mn(V)=O, Mn(IV)=O, and Mn(III)-OOR could be controlled by changing the substrate concentration, and that partitioning between heterolysis and homolysis of the O-O bond of a Mn-acylperoxo intermediate (Mn-OOC(O)R) might be significantly affected by the nature of solvent, and that the O-O bond of the Mn-OOC(O)R might proceed predominantly by heterolytic cleavage in protic solvent. Therefore, a discrete Mn(V)=O intermediate appeared to be the dominant reactive species in protic solvents. Furthermore, we have observed close similarities between these nonheme Mn(III) complex systems and Mn(saloph) catalysts previously reported, suggesting that this simultaneous operation of the three active oxidants might prevail in all the manganese-catalyzed olefin epoxidations, including Mn(salen), Mn(nonheme), and even Mn(porphyrin) complexes. This mechanism provides the greatest congruity with related oxidation reactions by using certain Mn complexes as catalysts.

Investigation of exchange couplings in [Fe3S4]+ clusters by electron spin-lattice relaxation

We have studied four proteins containing oxidized 3Fe clusters ([Fe3S4]+, S=1/2, composed of three, antiferromagnetically coupled high-spin ferric ions) by continuous wave (CW) and pulsed EPR techniques: Azotobacter vinelandii ferredoxin I, Desulfovibrio gigas ferredoxin II, and the 3Fe forms of Pyrococcus furiosus ferredoxin and aconitase. The 35 GHz (Q-band) CW EPR signals are simulated to yield experimental g tensors, which either had not been reported, or had been reported only at X-band microwave frequency. Pulsed X- and Q-band EPR techniques are used to determine electron spin-lattice (T1, longitudinal) relaxation times at several positions on the samples’ EPR envelope over the temperature range 2–4.2 K. The T1 values vary sharply across the EPR envelope, a reflection of the fact that the envelope results from a distribution in cluster properties, as seen earlier as a distribution in g3 values and in 57 Fe hyperfine interactions, as detected by electron nuclear double resonance spectroscopy. The temperature dependence of 1/T1 is analyzed in terms of the Orbach mechanism, with relaxation dominated by resonant two-phonon transitions to a doublet excited state at ∼20 cm−1 above the doublet ground state for all four of these 3Fe proteins. The experimental EPR data are combined with previously reported 57Fe hyperfine data to determine electronic spin exchange-coupling within the clusters, following the model of Kent et al. Their model defines the coupling parameters as follows: J13 = J, J12 = J(1+ɛ′), J23 = J(1+ɛ), where Jij is the isotropic exchange coupling between ferric ions i and j, and ɛ and ɛ′9 are measures of coupling inequivalence. We have extended their theory to include the effects of ɛ′≠ 0 and thus derived an exact expression for the energy of the doublet excited state for any ɛ, ɛ′. This excited state energy corresponds roughly to ɛJ and is in the range 5–10 cm−1 for each of these four 3Fe proteins. This magnitude of the product ɛJ, determined by our time-domain relaxation studies in the temperature range 2–4 K, is the same as that obtained from three other distinct types of study: CW EPR studies of spin relaxation in the range 5.5–50 K, NMR studies in the range 293–303 K, and static susceptibility measurements in the range 1.8-200 K. We suggest that an apparent disagreement as to the individual values of J and ɛ be resolved in favor of the values obtained by susceptibility and NMR (JE ≳200 cm−1 and ɛ ≳200 cm−1), as opposed to a smaller J and larger ɛ as suggested in CW EPR studies. However, we note that this resolution casts doubt on the accepted theoretical model for describing the distribution in magnetic properties of 3Fe clusters.

Dopamine and Cu+/2+ can induce oligomerization of α-synuclein in the absence of oxygen: Two types of oligomerization mechanisms for α-synuclein and related cell toxicity studies.

α‐Synuclein oligomers can induce neurotoxicity and are implicated in Parkinsons disease etiology and disease progression. Many studies have reported α‐synuclein oligomerization by dopamine (DA) and transition metal ions, but few studies provide insight into joint influences of DA and Cu2+. In this study, DA and Cu2+ were coadministered aerobically to measure α‐synuclein oligomerization under these conditions. In the presence of oxygen, DA induced α‐synuclein oligomerization in a dose‐dependent manner. Cu+/2+ did not effect oligomerization in such a manner in the presence of DA. By electrophoresis, Cu2+ was found easily to induce oligomerization with DA. This implies that oligomerization invoked by DA is reversible in the presence of Cu2+, which appears to be mediated by noncovalent bond interactions. In the absence of oxygen, DA induced less oligomerization of α‐synuclein, whereas DA/Cu2+ induced aerobic‐level amounts of oligomers, suggesting that DA/Cu2+ induces oligomerization independent of oxygen concentration. Radical species were detected through electron paramagnetic resonance (EPR) spectroscopic analysis arising from coincubation of DA/Cu2+ with α‐synuclein. Redox reactions induced by DA/Cu2+ were observed in multimer regions of α‐synuclein oligomers through NBT assay. Cellular toxicity results confirm that, for normal and hypoxic conditions, copper or DA/Cu2+ can induce cell death, which may arise from copper redox chemistry. From these results, we propose that DA and DA/Cu2+ induce different mechanisms of α‐synuclein oligomerization, cross‐linking with noncovalent (or reversible covalent) bonding vs. likely radical‐mediated covalent modification.

ENDOR and ESEEM investigation of the Ni-containing superoxide dismutase

Superoxide dismutases (SODs) protect cells against oxidative stress by disproportionating O2− to H2O2 and O2. The recent finding of a nickel-containing SOD (Ni-SOD) has widened the diversity of SODs in terms of metal contents and SOD catalytic mechanisms. The coordination and geometrical structure of the metal site and the related electronic structure are the keys to understanding the dismutase mechanism of the enzyme. We performed Q-band 14N,1/2H continuous wave (CW) and pulsed electron–nuclear double resonance (ENDOR) and X-band 14N electron spin echo envelope modulation (ESEEM) on the resting-state Ni-SOD extracted from Streptomyces seoulensis. In-depth analysis of the data obtained from the multifrequency advanced electron paramagnetic resonance techniques detailed the electronic structure of the active site of Ni-SOD. The analysis of the field-dependent Q-band 14N CW ENDOR yielded the nuclear hyperfine and quadrupole coupling tensors of the axial Nδ of the His-1 imidazole ligand. The tensors are coaxial with the g-tensor frame, implying the g-tensor direction is modulated by the imidazole plane. X-band 14N ESEEM characterized the hyperfine coupling of Nε of His-1 imidazole. The nuclear quadrupole coupling constant of the nitrogen suggests that the hydrogen-bonding between Nε–H and OGlu-17 present for the reduced-state Ni-SOD is weakened or broken upon oxidizing the enzyme. Q-band 1H CW ENDOR and pulsed 2H Mims ENDOR showed a strong hyperfine coupling to the protons(s) of the equatorially coordinated His-1 amine and a weak hyperfine coupling to either the proton(s) of a water in the pocket at the side opposite the axial Nδ or the proton of a water hydrogen-bonded to the equatorial thiolate ligand.

Effects of substrates (methyl isocyanide, C2H2) and inhibitor (CO) on resting-state wild-type and NifV(-)Klebsiella pneumoniae MoFe proteins.

We report the use of electron nuclear double resonance (ENDOR) spectroscopy to examine how the metal sites in the FeMo-cofactor cluster of the resting nitrogenase MoFe protein respond to addition of the substrates acetylene and methyl isocyanide and the inhibitor carbon monoxide. 1H, 57Fe and 95Mo ENDOR measurements were performed on the wild-type and the NifV(-)proteins from Klebsiella pneumoniae. Among the molecules tested, only the addition of acetylene to either protein induced widespread changes in the 57Fe ENDOR spectra. Acetylene also induced increases in intensity from unresolved protons in the proton ENDOR spectra. Thus we conclude that acetylene may bind to the resting-state MoFe protein to perturb the FeMo-cofactor environment. On the other hand, the present results show that methyl isocyanide and carbon monoxide do not substantially alter the FeMo cofactors geometric and electronic structures. We interpret this as lack of interaction between those two molecules and the FeMo cofactor in the resting state MoFe protein. Thus, although it is generally accepted that substrates or inhibitors bind to the FeMo-cofactor only under turnover condition, this work provides evidence that at least one substrate can perturb the active site of nitrogenase under non-catalytic conditions.

Investigation by EPR and ENDOR spectroscopy of the novel 4Fe ferredoxin fromPyrococcus furiosus

The hyperthermophilic archaeonPyrococcus furiosus contains a four-Fe ferredoxin (Pf- Fd) that differs from most other 4Fe-Fd’s in that its [Fe4S4] cluster is anchored to protein by only three cysteinyl residues.Pf- Fd also is of interest because in its reduced form, [Fe4S4]+, the cluster exhibits bothS = 1/2 andS = 3/2 spin states. Addition of excess cyanide ion converts the cluster exclusively to anS = 1/2 state (g1 = 2.09, g2 = 1.95, g3 = 1.92), however dialysis restores the EPR signal of native reduced protein indicating that the cluster is not irreversibly altered by cyanide. Both the native protein and protein in the presence of excess cyanide ion (Pf- Fd 4Fe-CN) were investigated here using the techniques of electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. In particular,Pf- Fd 4Fe-CN was investigated using13CN− and C15N− ligands.13C and15N ENDOR indicated that a single cyanide ion bound directly, with the cluster showing an unusually small contact interaction (aiso(13C)∼ −3 MHz, aiso(15N) ∼ 0). This is in contrast to cyanide bound to monomeric low-spin Fe(III)-containing proteins such as transferrin and myoglobin, for which the13C hyperfine coupling has a large isotropic component (aiso(13C) ≈ −30 MHz). This small contact interaction is not due to low spin density of Fe, as57Fe ENDOR of the singly and triply labeledPf- Fd 4FeCN isotopologs, [57FeFe3S4]+ and [Fe57Fe3S4]+, show hyperfine coupling characteristic for [Fe4S4]+ clusters, particularly for the Fe to which cyanide binds. Thus, the low spin density on13C is not due to low spin density on the Fe ion to which it binds. Further theoretical work is needed to explain the contrast between the strong electronic effect of cyanide ion binding with the low spin density on the ligand.

N,N'-(Ethane-1,2-diyldi-o-phenyl-ene)bis-(pyridine-2-carboxamide).

The title molecule, C26H22N4O2, is centrosymmetric and adopts an anti conformation. Two intramolecular hydrogen bonds, viz. amide–pyridine N—H⋯N and phenyl–amide C—H⋯O, stabilize the trans conformation of the (pyridine-2-carboxamido)phenyl group about the amide plane. In the crystal, the presence of weak intermolecular C—H⋯O hydrogen bonds results in the formation of a three-dimensional network.

A linear trinuclear mixed valence vanadium(V/IV/V) complex: synthesis, characterization, and solution behavior

The reaction between vanadium(III) acetylacetonate and N-hexanoylsalicylhydrazide (H3hshz) yields a linear trinuclear mixed valence vanadium(V/IV/V) complex, V3O3(hshz)2(OEt)2, 1 (where hshz3- is a triply deprotonated trianionic N-hexanoyl salicylichydrazidate), with a pseudo C2 symmetry. A V(IV)O2+ group is at the center of complex 1 and is spanned by two terminal vanadium(V) ions with a square pyramidal geometry bridged via hydrazido ligands. In the crystalline form, the oxo group of the central vanadium(IV) ion is weakly coordinated to one of the terminal square pyramidal vanadium(V) ions of the neighboring trinuclear complex to form a dimeric structure. These dimers are linked via bis mu-alkoxo bridges to form a one-dimensional zigzag chain structure. In chloroform or methylene dichloride, the weak linkages between the trinuclear complexes present in the crystalline form are broken, and only the mixed valence trinuclear complex can be identified. In dimethyl sulfoxide or dimethylformamide, the trinuclear complex partially dissociates, and the unligated ligands remain in equilibrium with the trinuclear complex.

catena-Poly[[bis-(N,N-dimethyl-formamide-κO)zinc]-μ(2)-oxalato-κO,O:O,O].

In the crystal structure of the title compound, [Zn(C2O4)(C3H7NO)2]n, the ZnII ion is situated on a twofold rotation axis and has a distorted octahedral coordination geometry defined by the O atoms of two dimethylformamide molecules and four O atoms of two bidentate oxalate ligands. The oxalate anion is located on an inversion centre and bridges two metal ions, resulting in a polymeric structure with infinite zigzag chains extending parallel to [010].

Electronic Structure of (NiS4) - Investigated by Single-Crystal EPR and Density Functional Theory

To understand the electronic structure of (NiS4) - complex ions, two complexes with such (NiS4) - core, FcCH〓CHPymCH3(Ni(dmit)2) (Pym = pyridinium, dmit 2- = 2-thioxo-1,3-dithiole-4,5-dithiolate) and FcCH〓CHPymCH3(Ni(dddt)2)•½H2O (dddt 2- = 5,6-dihydro-1,4-dithiin-2,3- dithiolato), were synthesized to be characterized by X-ray crystallography, single crystal electron paramagnetic resonance (EPR) and density functional theory (DFT) calculation. Powder EPR spectra show narrow g-anisotropy but the anisotropy is bigger in (Ni(dmit)2) - than in (Ni(dddt)2) - , indicating bigger spin density in Ni(III) d- orbital of (Ni(dmit)2) - than in (Ni(dddt)2) - , which is consistent to DFT results. EPR studies of the crystals of the complexes surprisingly suggest that the gy-axis of (Ni(dddt)2) - is approximately on or perpendicular to the (NiS4) - plane while the gy- axis of (Ni(dmit)2) - is on the plane, though DFT study of the complexes of this study and previously reported (NiS4) - complexes indicate that the gy-axis is on the (NiS4) -