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 Parkinson's 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.

Effect of Electron-donating Group on NO Photolysis of {RuNO}6 Ruthenium Nitrosyl Complexes with N2 O2 Lgands Bearing π-Extended Rings.

In this study, we introduced the electron-donating group (-OH) to the aromatic rings of Ru(salophen)(NO)Cl (0) (salophenH2 =N,N'-(1,2-phenylene)bis(salicylideneimine)) to investigate the influence of the substitution on NO photolysis and NO-releasing dynamics. Three derivative complexes, Ru((o-OH)2 -salophen)(NO)Cl (1), Ru((m-OH)2 -salophen)(NO)Cl (2), and Ru((p-OH)2 -salophen)(NO)Cl (3) were developed and their NO photolysis was monitored by using UV/Vis, EPR, NMR, and IR spectroscopies under white room light. Spectroscopic results indicated that the complexes were diamagnetic Ru(II)-NO+ species which were converted to low-spin Ru(III) species (d5 , S=1/2) and released NO radicals by photons. The conversion was also confirmed by determining the single-crystal structure of the photoproduct of 1. The photochemical quantum yields (ΦNO s) of the photolysis were determined to be 0>1, 2, 3 at both the visible and UV excitations. Femtosecond (fs) time-resolved mid-IR spectroscopy was employed for studying NO-releasing dynamics. The geminate rebinding (GR) rates of the photoreleased NO to the photolyzed complexes were estimated to be 0≃1, 2, 3. DFT and TDDFT computations found that the introduction of the hydroxyl groups elevated the ligand π-bonding orbitals (π (salophen)), resulting in decrease of the HOMO-LUMO gaps in 1-3. The theoretical calculations suggested that the Ru-NNO bond dissociations of the complexes were mostly initiated by the ligand-to-ligand charge transfer (LLCT) of π(salophen)→π*(Ru-NO) with both the visible and UV excitations and the decreasing ΦNO s could be explained by the changes of the electronic structures in which the photoactivable bands of 1-3 have relatively less contribution of transitions related with Ru-NO bond than those of 0.

Dichlorido{2,6-diisopropyl-N-[(S)-pyrrolidin-2-ylmeth-yl]aniline-κ(2) N,N'}palladium(II).

In the title compound, [PdCl2(C17H28N2)], the Pd(II) atom displays a square-planar coordination involving two N atoms of a 2,6-diisopropyl-N-[(S)-pyrrolidin-2-ylmeth-yl]aniline ligand and two chloride ligands, with a deviation of 0.090 (1) Šfor the Pd(II) atom from the best plane. The absolute configuration of the chiral C atom of the pyrrolidine ring is S, which induces R configurations at the two N atoms of the aniline ligand. Optical isomerism arising from the chelate five-membered ring is configured as δ. The Pd-N bond lengths are 2.040 (3) and 2.072 (2) Å, and the Pd-Cl bond lengths are 2.3055 (8) and 2.3160 (8) Å. In the crystal, pairs of N-H⋯Cl hydrogen bonds link mol-ecules into discrete dimers.

Substituent Effects of Fluorescein on Photoredox Initiating Performance under Visible Light.

We demonstrated the effects of substituents in fluorescein on the photoredox catalytic performance under visible light. For the systematic investigation, the phenyl ring of fluorescein was substituted with six different functional groups (i.e., amine, amide, isothiocyanate, aminomethyl, bromo, or nitro group) at the 5- or 6-position. The fluorescein derivatives were carefully characterized through photophysical and electrochemical analyses. The substituent effects were estimated by comparing the photopolymerization of poly(ethylene glycol) diacrylate (PEGDA) and N-vinylpyrrolidone (VP) in the presence of triethanolamine (TEOA) under aerobic conditions to that of intact fluorescein. As a result, the amine and nitro groups exhibited the lowest performances, presumably due to intramolecular photoinduced electron transfer (PET) promoted by the strong electron push-pull effect. The others, representative moderate or weak deactivators and activators, exhibited inferior performances than intact fluorescein, presumably owing to the more negative ΔGPET values, resulting in a decreased rate of intermolecular PET. These results are crucial for understanding the structure-performance relationship and the development of visible-light photoredox catalysts with improved performance and functionality.

A new photoactivable NO-releasing {Ru-NO}6 ruthenium nitrosyl complex with a tetradentate ligand containing aniline and pyridine moieties.

A new type of photoactivable NO-releasing ruthenium nitrosyl complex, [Ru(EPBP)Cl(NO)], with a tetradentate ligand, N,N'-(ethane-1,2-diyldi-o-phenylene)-bis(pyridine-2-carboxamide) (= H2 EPBP) was synthesized. Single crystal X-ray crystallography revealed that the complex has a distorted octahedral coordination geometry and NO is positioned at cis to Cl- ion. NO-photolysis was observed under a white room light. The photodissociation of Ru-NO bond was identified by various techniques including X-ray crystallography, IR, UV/Vis absorption, electron paramagnetic resonance (EPR), and NMR spectroscopies. Quantum yields for the NO-photolysis of the complex in CH3 OH, CHCl3 , DMSO, CH3 CN, and CH3 NO2 were measured to be 0.19-0.36 with 400 (±5) nm excitation. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed to understand the details of the photodissociation of the complex. The calculations suggest that the NO photolysis is most likely initiated by the electronic transition from the aniline moiety π MOs (π (aniline)) of the EPBP2- chelating ligand to the π-antibonding MO of Ru-NO (π*(Ru-NO)). Experimental and theoretical investigations indicate that the EPBP2- ligand provides an effective platform forming ruthenium nitrosyl complexes useful for NO-photoreleasing.

Visible-light NO photolysis of ruthenium nitrosyl complexes with N2O2 ligands bearing π-extended rings and their photorelease dynamics.

NO photorelease and its dynamics for two {RuNO}6 complexes, Ru(salophen)(NO)Cl (1) and Ru(naphophen)(NO)Cl (2), with salen-type ligands bearing π-extended systems (salophenH2 = N,N'-(1,2-phenylene)-bis(salicylideneimine) and naphophenH2 = N,N'-1,2-phenylene-bis(2-hydroxy-1-naphthylmethyleneimine)) were investigated. NO photolysis was performed under white room light and monitored by UV/Vis, EPR, and NMR spectroscopies. NO photolysis was also performed under 459 and 489 nm irradiation for 1 and 2, respectively. The photochemical quantum yields of the NO photolysis (ΦNO) of both 1 and 2 were determined to be 9% at the irradiation wavelengths. The structural and spectroscopic characteristics of the complexes before and after the photolysis confirmed the conversion of diamagnetic Ru(II)(L)(Cl)-NO+ to paramagnetic S = ½ Ru(III)(L)(Cl)-solvent by photons (L = salophen2- and naphophen2-). The photoreleased NO radicals were detected by spin-trapping EPR. DFT and TDDFT calculations found that the photoactive bands are configured as mostly the ligand-to-ligand charge transfer (LLCT) of π(L) → π*(Ru-NO), suggesting that the NO photorelease was initiated by the LLCT. Dynamics of NO photorelease from the complexes in DMSO under 320 nm excitation were investigated by femtosecond (fs) time-resolved mid-IR spectroscopy. The primary photorelease of NO occurred for less than 0.32 ps after the excitation. The rate constants (k-1) of the geminate rebinding of NO to the photolyzed 1 and 2 were determined to be (15 ps)-1 and (13 ps)-1, respectively. The photochemical quantum yields of NO photolysis (ΦNO, λ = 320 nm) were estimated to be no higher than 14% for 1 and 11% for 2, based on the analysis of the fs time-resolved IR data. The results of fs time-resolved IR spectroscopy and theoretical calculations provided some insight into the overall kinetic reaction pathway, localized electron pathway or resonance pathway, of the NO photolysis of 1 and 2. Overall, our study found that the investigated {RuNO}6 complexes, 1 and 2, with planar N2O2 ligands bearing π-extended rings effectively released NO under visible light.

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.

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

The title mol-ecule, C(26)H(22)N(4)O(2), is centrosymmetric and adopts an anti conformation. Two intra-molecular hydrogen bonds, viz. amide-pyridine N-H⋯N and phen-yl-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 inter-molecular C-H⋯O hydrogen bonds results in the formation of a three-dimensional network.

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(C(2)O(4))(C(3)H(7)NO)(2)](n), the Zn(II) ion is situated on a twofold rotation axis and has a distorted octa-hedral coordination geometry defined by the O atoms of two dimethyl-formamide mol-ecules 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].

(2R)-N-(2-Benzoyl-phen-yl)-1-benzyl-pyrrolidine-2-carboxamide.

In the title compound, C(25)H(24)N(2)O(2), the dihedral angle between the two benzene rings of the benzophenone moiety is 59.10 (6)°. An intra-molecular, bifurcated N-H⋯(O,N) hydrogen bond, which generates S(6) and S(5) rings, respectively, helps to establish the overall conformation of the mol-ecule.

Photoinitiated Free-Radical Polymerization of 4,5,6,7-Tetrahalogenated Fluoresceins.

We demonstrated the photoredox catalytic performances of fluorescein derivatives, bearing heavy halogen atoms (Br or I) on a benzoic acid group, using photoinitiated free-radical polymerization. 4,5,6,7-Tetrabromofluorescein and 4,5,6,7-tetraiodofluorescein were used as visible-light-photoredox catalysts to initiate polymerization of poly(ethylene glycol) diacrylate and N-vinylpyrrolidone in the presence of triethanolamine under aerobic conditions. Their photocatalytic performances were evaluated by the hydrogelation of photopolymerization both on the surface of an agarose film and in a liquid solution. The polymerization degree increased considerably in the following order: tetraiodofluorescein

ENDOR and ESEEM investigation of the Ni-containing superoxide dismutase.

Superoxide dismutases (SODs) protect cells against oxidative stress by disproportionating O2(-) to H(2)O(2) and O(2). 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 (14)N,(1/2)H continuous wave (CW) and pulsed electron-nuclear double resonance (ENDOR) and X-band (14)N 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 (14)N CW ENDOR yielded the nuclear hyperfine and quadrupole coupling tensors of the axial N(delta) 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 (14)N ESEEM characterized the hyperfine coupling of N(epsilon) of His-1 imidazole. The nuclear quadrupole coupling constant of the nitrogen suggests that the hydrogen-bonding between N(epsilon)-H and O(Glu-17) present for the reduced-state Ni-SOD is weakened or broken upon oxidizing the enzyme. Q-band (1)H CW ENDOR and pulsed (2)H 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 N(delta) or the proton of a water hydrogen-bonded to the equatorial thiolate ligand.

Acetato-(N-[(E)-1-(6-methyl-2-pyrid-yl)methyl-idene]-2-{2-[(E)-1-(6-methyl-2-pyrid-yl)methyl-idene-amino]-pheneth-yl}aniline)nickel(II) perchlorate.

In the title complex, [Ni(CH(3)COO)(C(28)H(26)N(4))]ClO(4), the Ni(II) atom is coordinated by two imine N atoms and two pyridine N atoms of the N-[(E)-1-(6-methyl-2-pyrid-yl)methyl-idene]-2-(2-[(E)-1-(6-methyl-2-pyrid-yl)methyl-idene-amino]-pheneth-yl)aniline donor ligand and two O atoms of the acetate ion in a distorted octa-hedral coordination. The average Ni-N and Ni-O bond lengths are 2.131 (13) and 2.098 (11) Å, respectively. An intramolecular N-H⋯O inter-action occurs. Relatively weak inter-molecular C-H⋯O inter-actions between the ligands and the ClO(4) (-) ions result in a chain extending along the b axis.

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 cofactor's 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.

Breaking the N2 triple bond: insights into the nitrogenase mechanism.

Nitrogenase is the metalloenzyme that performs biological nitrogen fixation by catalyzing the reduction of N2 to ammonia. Understanding how the nitrogenase active site metal cofactor (FeMo-cofactor) catalyzes the cleavage of the N2 triple bond has been the focus of intense study for more than 50 years. Goals have included the determination of where and how substrates interact with the FeMo-cofactor, and the nature of reaction intermediates along the reduction pathway. Progress has included the trapping of intermediates formed during turnover of non-physiological substrates (e.g., alkynes, CS2) providing insights into how these molecules interact with the nitrogenase FeMo-cofactor active site. More recently, substrate-derived species have been trapped at high concentrations during the reduction of N2, a diazene, and hydrazine, providing the first insights into binding modes and possible mechanisms for N2 reduction. A comparison of the current state of knowledge of the trapped species arising from non-physiological substrates and nitrogenous substrates is beginning to reveal some of the intricacies of how nitrogenase breaks the N2 triple bond.

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.

Substrate interactions with the nitrogenase active site.

The chemical mechanism for biological cleavage of the N(2) triple bond at ambient pressure and temperature has been the subject of intense study for many years. The site of substrate activation and reduction has been localized to a complex cofactor, called FeMo cofactor, yet until now the complexity of the system has denied information concerning exactly where and how substrates interact with the metal-sulfur framework of the active site. In this Account, we describe a combined genetic, biophysical, and biochemical approach that was used to provide direct and detailed information concerning where alternative alkyne substrates interact with FeMo cofactor during catalysis. The relevance and limitations of this work with respect to N(2) binding and reduction also are discussed.

Trapping H- bound to the nitrogenase FeMo-cofactor active site during H2 evolution: characterization by ENDOR spectroscopy.

We here show that the iron-molybdenum (FeMo)-cofactor of the nitrogenase alpha-70(Ile) molybdenum-iron (MoFe) protein variant accumulates a novel S = (1)/(2) state that can be trapped during the reduction of protons to H(2). (1,2)H-ENDOR measurements disclose the presence of two protons/hydrides (H(+/)(-)) whose hyperfine tensors have been determined from two-dimensional field-frequency (1)H ENDOR plots. The two H(+/)(-) have large isotropic hyperfine couplings, A(iso)( )() approximately 23 MHz, which shows they are bound to the cofactor. The favored analysis for these plots indicates that the two H(+/)(-) have the same principal values, which indicates that they are chemically equivalent. The tensors are further related to each other by a permutation of the tensor components, which indicates an underlying symmetry of binding relative to the cofactor. At present, no model for the structure of the iron-molybdenum (FeMo)-cofactor in the S = (1)/(2) state trapped during the reduction of H(+) can be shown unequivocally to satisfy all of the constraints generated by the ENDOR analysis. The data disfavors any model that involves protonation of sulfides, and thus suggests that the intermediate instead contains two chemically equivalent bound hydrides; it appears unlikely that these are terminal monohydrides.

The interstitial atom of the nitrogenase FeMo-cofactor: ENDOR and ESEEM evidence that it is not a nitrogen.

X-ray crystallographic study of the nitrogenase MoFe protein revealed electron density from an atom (denoted X) inside the active-site metal cluster, the [MoFe7S9:homocitrate] FeMo-cofactor. The electron density associated with X is consistent with a single N, O, or C atom. We now have tested whether X is an N or not by comparing the Q-band ENDOR and ESEEM signals from resting-state (S = 3/2) MoFe protein and NMF-extracted FeMo-co from bacteria grown with either 14N or 15N as the exclusive N source. All of the 14N or 15N signals associated with the protein are lost upon extraction of the FeMo-co. We interpret this as strong evidence that X is not an N.

Electron inventory, kinetic assignment (E(n)), structure, and bonding of nitrogenase turnover intermediates with C2H2 and CO.

Improved 1H ENDOR data from the S(EPR1) intermediate formed during turnover of the nitrogenase alpha-195Gln MoFe protein with C2(1,2)H2 in (1,2)H2O buffers, taken in context with the recent study of the intermediate formed from propargyl alcohol, indicate that S(EPR1) is a product complex, likely with C2H4 bound as a ferracycle to a single Fe of the FeMo-cofactor active site. 35 GHz CW and Mims pulsed 57Fe ENDOR of 57Fe-enriched S(EPR1) cofactor indicates that it exhibits the same valencies as those of the CO-bound cofactor of the lo-CO intermediate formed during turnover with CO, [Mo4+, Fe3+, Fe6(2+), S9(2-)(d43)](+1), reduced by m = 2 electrons relative to the resting-state cofactor. Consideration of 57Fe hyperfine coupling in S(EPR1) and lo-CO leads to a picture in which CO bridges two Fe of lo-CO, while the C2H4 of S(EPR1) binds to one of these. To correlate these and other intermediates with Lowe-Thorneley (LT) kinetic schemes for substrate reduction, we introduce the concept of an "electron inventory". It partitions the number of electrons a MoFe protein intermediate has accepted from the Fe protein (n) into the number transmitted to the substrate (s), the number that remain on the intermediate cofactor (m), and the additional number delivered to the cofactor from the P clusters (p): n = m + s - p (with p = 0 here). The cofactors of lo-CO and S(EPR1) both are reduced by m = 2 electrons, but the intermediates are not at the same LT reduction stage (E(n)): (n = 2; m = 2, s = 0) for lo-CO; (n = 4; s = 2, m = 2) for S(EPR1). This is the first proposed correlation of an LT E(n) kinetic state with a well-defined chemical state of the enzyme.