Probing the dynamic properties of two sites simultaneously in a protein-protein interaction process: a SDSL-EPR study.

During molecular processes, protein flexibility is a fundamental property allowing protein-protein interaction. Following structural changes during these interactions is then of crucial interest. Site-Directed Spin Labeling (SDSL) combined to EPR spectroscopy is a powerful technique to follow structural modifications within proteins and during protein-protein interactions. Usual nitroxide labels target cysteine residues and afford a 3-line spectrum, whose shape is informative of the structural environment of the label. However, it is not possible to probe two regions of a protein or two partner proteins at the same time because of the overlapping of EPR signatures. Previously, we reported the design and the characterization of a spin label based on a ß-phosphorylated (PP) nitroxide yielding a 6-line spectrum. Here, we report the use of two labels with different EPR signatures, namely maleimido-proxyl (P) and PP, to follow structural changes during a protein-protein interaction process in one single experiment. As a model system, we chose a disordered protein that undergoes an induced α-helical folding upon binding to its partner. We show that the EPR spectrum of a mixture of labeled interacting proteins can be analyzed in terms of structural changes during the interaction. This study represents an important step forward in the extension of the panoply of SDSL-EPR approaches.

High binding yet accelerated guest rotation within a cucurbit[7]uril complex. Toward paramagnetic gyroscopes and rolling nanomachines.

The (15-oxo-3,7,11-triazadispiro[5.1.5.3]hexadec-7-yl)oxidanyl, a bis-spiropiperidinium nitroxide derived from TEMPONE, can be included in cucurbit[7]uril to form a strong (K(a)∼ 2 × 10(5) M(-1)) CB[7]@bPTO complex. EPR and MS spectra, DFT calculations, and unparalleled increased resistance (a factor of ∼10(3)) toward ascorbic acid reduction show evidence of deep inclusion of bPTO inside CB[7]. The unusual shape of the CB[7]@bPTO EPR spectrum can be explained by an anisotropic Brownian rotational diffusion, the global tumbling of the complex being slower than rotation of bPTO around its "long molecular axis" inside CB[7]. The CB[7] (stator) with the encapsulated bPTO (rotator) behaves as a supramolecular paramagnetic rotor with increased rotational speed of the rotator that has great potential for advanced nanoscale machines requiring wheels such as cucurbiturils with virtually no friction between the wheel and the axle for optimum wheel rotation (i.e. nanopulleys and nanocars).

Magnetic fullerenes inside single-wall carbon nanotubes.

C(59)N magnetic fullerenes were formed inside single-wall carbon nanotubes by vacuum annealing functionalized C(59)N molecules encapsulated inside the tubes. A hindered, anisotropic rotation of C(59)N was deduced from the temperature dependence of the electron spin resonance spectra near room temperature. Shortening of the spin-lattice relaxation time T(1) of C(59)N indicates a reversible charge transfer toward the host nanotubes above approximately 350 K. Bound C(59)N-C(60) heterodimers are formed at lower temperatures when C(60) is coencapsulated with the functionalized C(59)N. In the 10-300 K range, T(1) of the heterodimer shows a relaxation dominated by the conduction electrons on the nanotubes.

Solution chemical properties and catecholase-like activity of the copper(II)-Ac-His-His-Gly-His-OH system, a relevant functional model for copper containing oxidases.

The solution chemical properties, superoxide dismutase and catecholase activity of the copper(ii)-Ac-His-His-Gly-His-OH (hhgh) complexes were studied to identify functional and structural models of copper-containing oxidases. The solution speciation was determined in the pH range 3-11 by two independent methods (potentiometry and pH-dependent EPR measurements). The results obtained by the two methods agree very well with each other and show the formation of differently protonated CuH(x)L complexes (where x= 2 ,1, 0, -1, -2, -3) in aqueous solution. The spectroscopic (UV-Vis, CD, EPR) data indicate that the coordination of the imidazole rings is a determinant factor in all these complexes. Amide coordinated complexes are dominant only above pH 8. This offers excellent possibilities for structural/functional modelling of copper(ii) containing metalloenzymes. Indeed, the {3N(im)} coordinated CuL species (pH = 6-7) has efficient superoxide dismutase-like activity. The {3N(im),OH(-)} coordinated CuH(-1)L possesses outstanding activity to catalyze the oxidation of 3,5-di-tert-butylcatechol (H(2)dtbc) by dioxygen in 86 wt% methanol-water, providing the first example that copper(ii)-peptide complexes are able to mimic copper containing oxidases.

Speciation study of an imidazolate-bridged copper(II)-zinc(II) complex in aqueous solution.

The solution equilibrium and the binding mode of the species in the five-component system containing two metal ions (copper(II) and zinc(II)) and three ligands (A=diethylenetriamine, B=imidazole, C=tris(2-aminoethyl)amine) were investigated by pH-potentiometric titration, UV-visible spectrophotometry and EPR (electron paramagnetic resonance) spectroscopic titration in aqueous solution in the 2-11 pH range. An imidazolate-bridged heterobinuclear complex (ACuBH(-1)ZnC) was found to evolve above pH=7 and was stable between pH 7 and 11. The existence of the ACuBH(-1)ZnC complex (by determination of its molecular weight) was proved by mass spectrometry (ESI-MS (electrospray ionization mass spectrometry) and MALDI (matrix-assisted laser desorption/ionization) techniques). The electrochemical behaviour and the superoxide dismutase activity of this complex were also tested by cyclic voltammetry and the Riboflavin/NBT (nitro blue tetrazolium) assay, respectively.

Electron delocalization and dimerization in solid C59N doped C60 fullerene.

Electron spin resonance and ab initio electronic structure calculations show an intricate relation between molecular rotation and chemical bonding in the dilute solid solution. The unpaired electron of C59N is delocalized over several C60 molecules above 700 K, while at lower temperatures it remains localized within short range. The data suggest that below 350 K rigid C59N-C60 heterodimers are formed in thermodynamic equilibrium with dissociated rotating molecules. The structural fluctuations between heterodimers and dissociated molecules are accompanied by simultaneous electron spin transfer between C60 and C59N molecules. The calculation confirms that in the C59N-C60 heterodimer the spin density resides mostly on the C60 moiety, while it is almost entirely on C59N in the dissociated case.

Valence tautomerism and metal-mediated catechol oxidation for complexes of copper prepared with 9,10-phenanthrenequinone.

Bis(pyridine)(9,10-phenanthrenequinone)(9,10-phenanthrenediolato)copper(II), Cu(py)(2)(PhenCat)(PhenBQ), has been prepared by treating copper metal with 9,10-phenanthrenequinone in pyridine solution. In dilute solution, both Cu(py)(2)(PhenCat)(PhenBQ) and the related complex Cu(tmeda)(PhenCat)(PhenBQ) lose PhenBQ to form Cu(II)L(2)(PhenCat), where L(2)= tmeda, 2 py. EPR spectra recorded at temperatures between 300 and 77 K reveal the presence of species with radical and metal localized spins together at equilibrium. Equilibria between Cu(II)L(2)(PhenCat) and Cu(I)L(2)(PhenSQ) redox isomers are solvent dependent, with a shift to higher temperature for polar solvents. Both complexes are oxygen sensitive, reacting with dioxygen to give complexes of diphenic acid. Structural characterization on products obtained with tmeda show that dioxygen insertion across the C-C bond within the chelate ring leads to dimeric products with adjacent Cu(II) ions bridged by diphenate ligands. The addition of O(2) to Cu(tmeda)(PhenCat) in acetonitrile solution at 0 degrees C appears to form a peroxo complex, tentatively identified as Cu(tmeda)(O(2))(PhenQ) on the basis of iodometric titration, as the precursor to the diphenate complex.

A two-dimensional (magnetic field and concentration) electron paramagnetic resonance method for analysis of multispecies complex equilibrium systems. information content of EPR spectra.

A two-dimensional simulation method has been developed for the interpretation of electron paramagnetic resonance (EPR) spectra consisting of a multitude of strongly overlapping signal components. The set of EPR spectra for complex equilibrium systems is analyzed simultaneously as a function of metal and ligand concentrations and pH. The formation constants of the various species are adjusted together with the magnetic parameters of the component EPR spectra. At most 10 EPR-active and 5 EPR-silent species can be involved to simulate a maximum of 36 experimental spectra, while the number of adjusted parameters is at most 100. Statistical parameters are suggested to give the confidence intervals for parameter estimation and to distinguish alternative speciation models. The efficiency of the program is demonstrated for the copper(II)--L-asparagine system, in which 10 species, including 3 pairs of isomers, are characterized with magnetic parameters and formation constants. On the basis of the magnetic parameters, a structural assignment is made for the detected species. The two-dimensional approach can also supply the formation constant of the EPR-silent species, as demonstrated for the copper(II)--glycyl-L-serine system.

2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole-1-oxide: evaluation of the spin trapping properties.

The 2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole-l-oxide (EMPO), an easily prepared pyrroline-N-oxide has been tested as a free radical scavenger. Spin adducts of superoxide, hydroxyl radical, and other free radicals were characterized in phosphate buffer at pH 7.0 and 5.6. At pH 7 in phosphate buffer, the EMPO/O(2)(-*) spin adduct was estimated to be about five times more persistent than its DMPO analogue. Furthermore, its decay does not produce the EMPO/HO&z.rad; adduct.

Metal complexes of taurine. The first reported solution equilibrium studies for complex formation by taurine at physiological pH; the copper(II)-glycylglycinate-taurine and the copper(II)-glycylaspartate-taurine systems.

The first solution studies at physiological pH for the formation of metal complexes of taurine, +NH3CH2CH2S03-, one of the most abundant low molecular weight organic compounds in the animal kingdom, are reported. The complexes Cu(Gly-GlyH-1) (1) and [Cu(Gly-AspH-1)] (2) react with taurine to give the ternary complexes [Cu(Gly-GlyH-1)taurine]- (3) (log K=2.95+/-0.03, I=0.2M, T=25.0 degrees C) and [Cu(Gly-AspH-1)taurine]2- (4) (log K=2.68+/-0.02) in which taurine acts as an N-donor ligand, most likely monodentate, without involvement of the sulphonate group in coordination. The results of the pH-metric studies are confirmed by visible and EPR spectrophotometric studies. The taurine complexes are less stable than the analogous complexes of beta-alanine due to the decreased basicity of the amino group in the former ligand, and in the case of the Cu(Gly-GlyH-1) complexes due to involvement of the carboxylate group of beta-alanine in axial coordination.

An electron spin resonance (ESR) study of free radicals formed from hematoxylin on tissue sections after blueing (alkalization).

A free radical signal of 12 G width and g = 2.0045 can be observed in hematoxylin stained tissue blocks and sections. The amount of paramagnetic centres in stained specimens is significantly larger than in unstained ones. After alkalization simultaneously with the colour change the former free radical is detectable in hemalum precipitate and on stained paper strips. After solution of the stain in dioxane and alkalization, a well resolved hyperfine structure could be seen which could be assigned to three different radicals with the same g value as observed in the rigid matrix (tissue and paper). Quantitative evaluation of free radical concentration is also carried out for tissue sections.

A study of free radicals in paraffin embedded and deparaffinized human heart muscle tissue using electron spin resonance (ESR).

Free radicals (spectroscopic splitting factor; g factor = 2.003-2.005) were investigated in formol-fixed, paraffin embedded heart-muscle tissue sections using electron spin resonance (ESR) spectra. Changes in signal amplitude, g factor and line width were registered during deparaffinization, chloroform-methanol extraction, vapour treatment and bromination. An attempt was made to identify the source of the ESR signals by a correlation between the signal amplitude and number of fluorescent and/or Sudan-black-positive granules counted in the tissue sections. An increase in signal amplitude, g value and line narrowing were characteristic of the ascorbyl radical after deparafinization in air. Vapour treatment revelated that the broader signal has lower g factor, a characteristic that is tentatively assigned to oxidized lipids. The bromination resistant minor fraction of free radical centres with small g factor might be associated with the pigment content of the samples.

Free radical signal of bile pigment in paraffin embedded liver tissue.

A linear correlation have been found between the amplitude of the free radical signal of the electron spin resonance (ESR) spectrum of paraffin embedded liver blocks and the number of bile casts in the histological sections made from these blocks. It has been suggested that the major part of the ESR free radical signal arises from bile pigment, but the contribution of "age pigment" cannot be excluded. On the basis of model experiments the majority of these radical centers was assigned to protein bound bilirubin. In the course of histological processing more than 80% of various free radical centers arising in air dried liver tissue is extracted.

The free radical signal of pigment gallstones.

Bile stones have been studied by Electron Spin Resonance (ESR) spectroscopy. Pigment containing stones show a 8.76 +/- 0.02 gauss wide free radical singlet at g approximately 2.003 region of the spectrum. Amplitude of this free radical signal linearly correlates with pigment content of the stone, which can be applied in the estimation of pigment content. The possible role of free radicals is suggested in the formation of pigment bile stones.

[Electron spin resonance study of melanin in paraffin-imbedded ocular melanoma specimens]. / Melanin elektron spin rezonanciás (ESR) vizsgálata paraffinba ágyazott okuláris melanomákban.

The Electron Spin Resonance (ESP) spectrum of ocular melanomas embedded in paraffin was studied. In the g approximately 2.003 range of the spectrum a mark of a free radical was detected. The correlation between the amplitude of the latter and the number of melanin granules in sections was studied. Considering the positive correlation revealed, authors believe, that ESR spectroscopy is a suitable tool for detecting melanin in paraffin embedded specimens. As an effect of oxidation or reduction melanin turns pale, however this phenomenon is not in tight connection with the decrease of the amplitude of the free-radical-mark. Very seldom different types of spectrum (peroxide, cupric ion) may appear, so it is reasonable to study large number of specimens.

ESR investigation of paraffin-embedded ocular melanomas.

Ocular melanomas embedded in paraffin wax for histological examination have been studied by electron spin resonance (ESR) spectroscopy. A free-radical signal was detected at the g = 2.003 section of the spectrum. The amplitude of this signal was correlated with the number of melanin granules in microscopical slides made from the tumour. This positive correlation can make ESR spectroscopy suitable for estimating the melanin content in the embedded melanoma blocks. Additional paramagnetic signals can also be detected. The clarification of their significance needs, however, further ESR measurements.

[Determination of the copper content of liver tissue from paraffin-imbedded samples by electron spin resonance spectroscopy]. / Májszövet réztartalmának becslése parafiffinba ágyazott minták elektron spin rezonancia (ESR) spektruma alapján.

The Electron Spin Resonance spectrum of human liver specimens embedded in paraffin was investigated. An easily registrable mark could be seen in the g 2.05 range of the spectrum. A correlation was detected between the amplitude of this mark of the embedded specimens and the copper content of them measured by a flame ionic-absorptive technique. Authors assume that ESR-spectroscopy is a useful tool for the estimation of the copper content of a specimen, it does interfere with the following histological processing of it. Sources of errors are shortly discussed.

Electron spin resonance (ESR) spectrum of paraffin embedded human liver tissue. A possible method for the estimation of copper content.

Human liver tissues embedded in paraffin wax for histological examination have been studied by Electron Spin Resonance (ESR) spectroscopy. A signal was detected at g approximately 2.05 section of the spectrum. The amplitude of this signal was correlated with the copper content of the embedded specimens measured by flame atomic absorption technique. The positive correlation which has been found can make ESR spectroscopy suitable for estimating the copper content of tissues without damaging the sample. The limits and errors of this method have also been analysed.

Analysis of the electron spin resonance spectrum of human liver tissue embedded in paraffin.

The ESR spectra of the liver samples embedded in paraffin of newborn, adult, cirrhotic and control individuals always display the g approximately 2.05 signal and occasionally also the g = 4.3 line and the signal of free radicals. In adults, the amplitude of the g approximately 2.05 signal is negatively correlated with that of the g approximately 4.3 signal; in infants aged from three months to one year this correlation is positive. The copper content of the liver is in linear correlation with the amplitude of the g approximately 2.05 signal. The hyperfine structure of the part of the g approximately 2.05 signal recorded at low magnetic field may be of three types: it may correspond to Cu2+ centers of axial symmetry of type I, II or I + II. The parameters of the signal are the following: type I center, g = 2.35 +/- 0.05, g = 2.05 +/- 0.01, A < 100 gauss; type II center, g = 2.25 +/- 0.03, g = 2.05 +/- 0.01, A = 160 +/- 20 gauss. On the basis of the amplitude of the g approximately 2.05 signal, the copper content of the liver tissue can be estimated from paraffin blocks with an error of 30%, without losing the tissue embedded for histological examination.

Electron spin resonance spectra of chicken liver and hepatoma tissue embedded in paraffin.

ESR spectra of chicken liver and hepatoma tissue embedded in paraffin display signals characteristic of free radicals and paramagnetic complexes. Owing to embedding the spectrum is altered as compared to that of the native tissue but remains characteristic of the respective tissues. The spectra of tissues embedded in paraffin allow no more than qualitative conclusions to be drawn concerning the spectra of the native tissues, even if appropriate controls are applied.