Electron spin resonance study of the copper(II) chelates of certain monothio-.beta.-diketones and diethyldithiocarbamate

Copper(II) Complexes with Carnosine Conjugates of Hyaluronic Acids at Different Dipeptide Loading Percentages Behave as Multiple SOD Mimics and Stimulate Nrf2 Translocation and Antioxidant Response in In Vitro Inflammatory Model

A series of copper(II) complexes with the formula [Cu2+Hy(x)Car%] varying the molecular weight (MW) of Hyaluronic acid (Hy, x = 200 or 700 kDa) conjugated with carnosine (Car) present at different loading were synthesized and characterized via different spectroscopic techniques. The metal complexes behaved as Cu, Zn-superoxide dismutase (SOD1) mimics and showed some of the most efficient reaction rate values produced using a synthetic and water-soluble copper(II)-based SOD mimic reported to date. The increase in the percentage of Car moieties parallels the enhancement of the I50 value determined via the indirect method of Fridovich. The presence of the non-functionalized Hy OH groups favors the scavenger activity of the copper(II) complexes with HyCar, recalling similar behavior previously found for the copper(II) complexes with Car conjugated using β-cyclodextrin or trehalose. In keeping with the new abilities of SOD1 to activate protective agents against oxidative stress in rheumatoid arthritis and osteoarthritis diseases, Cu2+ interaction with HyCar promotes the nuclear translocation of erythroid 2-related factor that regulates the expressions of target genes, including Heme-Oxigenase-1, thus stimulating an antioxidant response in osteoblasts subjected to an inflammatory/oxidative insult.

Mechanism and Efficacy of Cu2O-Treated Fabric

Pathogenic bacteria can remain viable on fabrics for several days and therefore are a source of infection. Antimicrobial fabrics are a potential method of reducing such infections, and advances in antimicrobial fabrics can be enhanced by knowledge of how the fabric kills bacteria. Metal oxides have been considered and used as antimicrobial ingredients in self-sanitizing surfaces, including in clinical settings. In this work, we examine how the addition of cuprous oxide (Cu2O) particles to polypropylene fibers kills bacteria. First, we show that the addition of the Cu2O particles reduces the viability of common hospital pathogens, Staphylococcus aureus, Pseudomonas aeruginosa, and Streptococcus pneumoniae, by 99.9% after 30 min of contact with the treated polypropylene. Then, we demonstrate that the main killing effect is due to the drying of the bacteria onto the cuprous oxide particles. There is also a weaker effect due to free Cu+ ions that dissolve into the liquid. Other dissolved species were unimportant. Chelation of these Cu+ ions in soluble form or precipitation removes their antimicrobial activity.

Electron Spin Relaxation and Biochemical Characterization of the Hydrogenase Maturase HydF: Insights into [2Fe-2S] and [4Fe-4S] Cluster Communication and Hydrogenase Activation

Nature utilizes [FeFe]-hydrogenase enzymes to catalyze the interconversion between H₂ and protons and electrons. Catalysis occurs at the H-cluster, a carbon monoxide-, cyanide-, and dithiomethylamine-coordinated 2Fe subcluster bridged via a cysteine to a [4Fe-4S] cluster. Biosynthesis of this unique metallocofactor is accomplished by three maturase enzymes denoted HydE, HydF, and HydG. HydE and HydG belong to the radical S-adenosylmethionine superfamily of enzymes and synthesize the nonprotein ligands of the H-cluster. These enzymes interact with HydF, a GTPase that acts as a scaffold or carrier protein during 2Fe subcluster assembly. Prior characterization of HydF demonstrated the protein exists in both dimeric and tetrameric states and coordinates both [4Fe-4S]^2+/+ and [2Fe-2S]^2+/+ clusters [Shepard, E. M., Byer, A. S., Betz, J. N., Peters, J. W., and Broderick, J. B. (2016) Biochemistry 55, 3514-3527]. Herein, electron paramagnetic resonance (EPR) is utilized to characterize the [2Fe-2S]⁺ and [4Fe-4S]⁺ clusters bound to HydF. Examination of spin relaxation times using pulsed EPR in HydF samples exhibiting both [4Fe-4S]⁺ and [2Fe-2S]⁺ cluster EPR signals supports a model in which the two cluster types either are bound to widely separated sites on HydF or are not simultaneously bound to a single HydF species. Gel filtration chromatographic analyses of HydF spectroscopic samples strongly suggest the [2Fe-2S]⁺ and [4Fe-4S]⁺ clusters are coordinated to the dimeric form of the protein. Lastly, we examined the 2Fe subcluster-loaded form of HydF and showed the dimeric state is responsible for [FeFe]-hydrogenase activation. Together, the results indicate a specific role for the HydF dimer in the H-cluster biosynthesis pathway.

Electron spin relaxation of a boron-containing heterocyclic radical

Preparation of the stable boron-containing heterocyclic phenanthrenedione radical, (C₆F₅)₂B(O₂C₁₄H₈), by frustrated Lewis pair chemistry has been reported recently. Electron paramagnetic resonance measurements of this radical were made at X-band in toluene:dichloromethane (9:1) from 10 to 293K, in toluene from 180 to 293K and at Q-band at 80K. In well-deoxygenated 0.1mM toluene solution at room temperature hyperfine splittings from ¹¹B, four pairs of ¹H, and 5 pairs of ¹⁹F contribute to an EPR spectrum with many resolved lines. Observed hyperfine couplings were assigned based on DFT calculations and account for all of the fluorines and protons in the molecule. Rigid lattice g values are g_x=2.0053, g_y=2.0044, and g_z=2.0028. Near the melting point of the solvent 1/T_m is enhanced due to motional averaging of g and A anisotropy. Increasing motion above the melting point enhances 1/T₁ due to contributions from tumbling-dependent processes. The overall temperature dependence of 1/T₁ from 10 to 293K was modeled with the sum of contributions of a process that is linear in T, a Raman process, spin rotation, and modulation of g anisotropy by molecular tumbling. The EPR measurements are consistent with the description of this compound as a substituted aromatic radical, with relatively small spin density on the boron.

The world as viewed by and with unpaired electrons

Recent advances in electron paramagnetic resonance (EPR) include capabilities for applications to areas as diverse as archeology, beer shelf life, biological structure, dosimetry, in vivo imaging, molecular magnets, and quantum computing. Enabling technologies include multifrequency continuous wave, pulsed, and rapid scan EPR. Interpretation is enhanced by increasingly powerful computational models.

Electron transfer flavoprotein domain II orientation monitored using double electron-electron resonance between an enzymatically reduced, native FAD cofactor, and spin labels

Human electron transfer flavoprotein (ETF) is a soluble mitochondrial heterodimeric flavoprotein that links fatty acid β-oxidation to the main respiratory chain. The crystal structure of human ETF bound to medium chain acyl-CoA dehydrogenase indicates that the flavin adenine dinucleotide (FAD) domain (αII) is mobile, which permits more rapid electron transfer with donors and acceptors by providing closer access to the flavin and allows ETF to accept electrons from at least 10 different flavoprotein dehydrogenases. Sequence homology is high and low-angle X-ray scattering is identical for Paracoccus denitrificans (P. denitrificans) and human ETF. To characterize the orientations of the αII domain of P. denitrificans ETF, distances between enzymatically reduced FAD and spin labels in the three structural domains were measured by double electron-electron resonance (DEER) at X- and Q-bands. An FAD to spin label distance of 2.8 ± 0.15 nm for the label in the FAD-containing αII domain (A210C) agreed with estimates from the crystal structure (3.0 nm), molecular dynamics simulations (2.7 nm), and rotamer library analysis (2.8 nm). Distances between the reduced FAD and labels in αI (A43C) were between 4.0 and 4.5 ± 0.35 nm and for βIII (A111C) the distance was 4.3 ± 0.15 nm. These values were intermediate between estimates from the crystal structure of P. denitrificans ETF and a homology model based on substrate-bound human ETF. These distances suggest that the αII domain adopts orientations in solution that are intermediate between those which are observed in the crystal structures of free ETF (closed) and ETF bound to a dehydrogenase (open).

Electron spin relaxation enhancement measurements of interspin distances in human, porcine, and Rhodobacter electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO)

Electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO) is a membrane-bound electron transfer protein that links primary flavoprotein dehydrogenases with the main respiratory chain. Human, porcine, and Rhodobacter sphaeroides ETF-QO each contain a single [4Fe-4S](2+,1+) cluster and one equivalent of FAD, which are diamagnetic in the isolated enzyme and become paramagnetic on reduction with the enzymatic electron donor or with dithionite. The anionic flavin semiquinone can be reduced further to diamagnetic hydroquinone. The redox potentials for the three redox couples are so similar that it is not possible to poise the proteins in a state where both the [4Fe-4S](+) cluster and the flavoquinone are fully in the paramagnetic form. Inversion recovery was used to measure the electron spin-lattice relaxation rates for the [4Fe-4S](+) between 8 and 18K and for semiquinone between 25 and 65K. At higher temperatures the spin-lattice relaxation rates for the [4Fe-4S](+) were calculated from the temperature-dependent contributions to the continuous wave linewidths. Although mixtures of the redox states are present, it was possible to analyze the enhancement of the electron spin relaxation of the FAD semiquinone signal due to dipolar interaction with the more rapidly relaxing [4Fe-4S](+) and obtain point-dipole interspin distances of 18.6+/-1A for the three proteins. The point-dipole distances are within experimental uncertainty of the value calculated based on the crystal structure of porcine ETF-QO when spin delocalization is taken into account. The results demonstrate that electron spin relaxation enhancement can be used to measure distances in redox poised proteins even when several redox states are present.

Phosphorylation-dependent changes in structure and dynamics in ERK2 detected by SDSL and EPR

Mitogen-activated protein kinases are regulated by occupancy at two phosphorylation sites near the active site cleft. Previous studies using hydrogen exchange to investigate the canonical mitogen-activated protein kinase, extracellular signal-regulated protein kinase-2, have shown that phosphorylation alters backbone conformational mobility >10 A distal to the site of phosphorylation, including decreased mobility within amino acids 102-105 and increased mobility within 108-109. To further describe changes after enzyme activation, site-directed spin labeling at amino acids 101, 105-109, 111, 112 and electron paramagnetic resonance spectroscopy were used to investigate this region. The anisotropic hyperfine splitting of the spin labels in glassy samples was unchanged by phosphorylation, consistent with previous crystallographic studies that indicate no structural change in this region. At positions 101, 111, and 112, the mobility of the spin label was unchanged by diphosphorylation, consistent with little or no conformational change. However, diphosphorylation caused small but significant changes in rotational diffusion rates at positions 105-108 and altered proportions of probe in a motionally constrained state at positions 105, 107, and 109. Thus, electron paramagnetic resonance indicates reproducible changes in nanosecond side-chain mobilities at specific residues within the interdomain region, far from the site of phosphorylation and conformational change.

Comparison of electron paramagnetic resonance methods to determine distances between spin labels on human carbonic anhydrase II

Four doubly spin-labeled variants of human carbonic anhydrase II and corresponding singly labeled variants were prepared by site-directed spin labeling. The distances between the spin labels were obtained from continuous-wave electron paramagnetic resonance spectra by analysis of the relative intensity of the half-field transition, Fourier deconvolution of line-shape broadening, and computer simulation of line-shape changes. Distances also were determined by four-pulse double electron-electron resonance. For each variant, at least two methods were applicable and reasonable agreement between methods was obtained. Distances ranged from 7 to 24 A. The doubly spin-labeled samples contained some singly labeled protein due to incomplete labeling. The sensitivity of each of the distance determination methods to the non-interacting component was compared.

Determination of high-spin iron(III)-nitroxyl distances in spin-labeled porphyrins by time-domain EPR

Continuous wave EPR spectra of the nitroxyl signals for four spin-labeled high-spin (h.s.) Fe(III) porphyrins showed partially resolved splittings at temperatures near 4 K. Axial ligands were fluoride, chloride, or bromide. As temperature was increased to 20 to 30 K the iron-nitroxyl splitting collapsed due to increasing rates of iron relaxation. Electron spin-echo (ESE) spectroscopy showed that above about 6 K collapse of the iron-nitroxyl spin-spin splitting caused a dramatic increase in the nitroxyl phase memory relaxation rates. Electron spin relaxation rates were determined for Fe(tetratolylporphyrin)X, X = F, Cl, Br, in toluene solution by ESE or inversion recovery at 4.5 to 6 K and by analysis of the temperature-dependent contributions to the continuous wave EPR linewidths between about 10 and 120 K. Above about 10 K iron relaxation rates increase in the order X = F < Cl < Br, which is the order of increasing zero-field splitting. Saturation recovery data for two spin-labeled h.s. iron(III) porphyrins between about 15 and 120 K and for two additional spin-labeled h.s. iron(III) porphyrins between about 85 and 120 K demonstrated that interaction with the h. s. iron enhanced the electron spin relaxation rate of the spin label. The saturation recovery curves for the nitroxyl were analyzed to determine interspin distances using a modified version of the Bloembergen equation and independently determined iron relaxation rates. Interspin distances were between 11.6 and 15.0 A, were independent of axial ligand, and were in good agreement with values obtained previously for low-spin Fe(III) and Cu(II) analogs.

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.).

Temperature and anation studies of the type 2 site in Rhus vernicifera laccase

Temperature-dependent structural changes involving the type 2 site in laccase are probed by EPR studies of a derivative of laccase in which the type 1 Cu has been replaced by Hg(II) [Morie-Bebel, M. M., Morris, M. C., Menzie, J. L., & McMillin, D. R. (1984) J. Am. Chem. Soc. 106, 3677-3678]. At the temperature extremes (123 and 299 K), single well-defined species are present, but at intermediate temperatures (between 213 and 253 K), the presence of multiple structures is indicated. For the first time, the room temperature EPR spectrum of the type 2 copper has been resolved. Azide binding and fluoride binding have also been studied as a function of temperature. The results suggest that each anion preferentially interacts with the type 3 site in fluid solution and that these adducts can be trapped by rapidly cooling the sample to 123 K. Annealing the adducts at 253 K permits rearrangement and binding at an equatorial position of the type 2 Cu. This pathway to anation at the type 2 site contrasts sharply with previous studies which required a large excess of anions, and it reveals important insight into the flexibility of the type 2/type 3 cluster in laccase.

EPR of Cu+2 binding to apo-yeast enolase

We have studied the electron paramagnetic resonance (epr) spectra of complexes of apo-yeast enolase with 65Cu+2 in the presence and absence of substrate and magnesium ion. An unusual epr spectrum with large g parallel, large g and A rhombicity and very narrow line-widths (10 G) is seen for the first two 65Cu+2 bound in the presence of substrate 2-phosphoglycerate (2PGA). the epr parameters, consistent with rhombic and tetragonal distortion of an octahedral geometry of the coordination sphere of the Cu+2 are g = (2.123, 2.042, 2.405) and A = (2.58, 4.19, 12.0) mK. The high g parallel and absence of super-hyperfine splitting are strong evidence for absence of nitrogen ligands. In the presence of Mg+2 and 2PGA, the Cu+2-enolase solutions exhibit a complex epr spectrum reflecting exchange and dipolar interaction between the first two Cu+2 ions bound. The spectra of Cu+2 plus enolase in the presence and absence of Mg+2 without 2PGA are distinct but not unambiguous, each reflecting at least two inequivalent binding sites. In addition to providing information on the geometry and location of the divalent cation binding sites, the data show unequivocally that imidazole residues, previously found to have a role in catalysis, do not participate in Cu+2 binding. Although Cu+2 does not activate the enzyme, direct binding measurements show that Cu+2 competes stoichiometrically with the activating ion, Mg+2. A reinterpretation of earlier Mn+2 enolase studies is proposed to reconcile the Cu+2 and Mn+2 data.