- Spin label studies of tropomyosin
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Studies are reported on nitroxide spin labeled tropomyosin. The labels attach to sulfhydryl groups and to amino groups. The amino spins are highly mobile, the sulfhydryl much less so. Spin count studies show an average of 0.5 labeled sulfhydryl/tropomyosin molecule and only 0.15 labeled amino group/molecule. The spectra ae used to study the denaturation of tropomyosin by guanidine hydrochloride. The information obtained reveals the course of denaturation at sites near the sulfhydryl group. It is found that these sites are more susceptible to guanidine than the bulk of the molecule; denaturation at the sulfhydryl sites is complete by 1.5 M guanidine, whereas optical studies indicate the molecule as a whole is not completely denatured until the concentration reaches 3.5 M. Spectra are also shown of tropomyosin fibers oriented variously with respect to the applied magnetic field. Strong orientation effects are seen and these indicate that the sulfhydryl attached spins (but not the amino attached spins) have a definite orientation in the fiber. Interpretation of the spectra reveals that the normal to the nitroxide plane is inclined to the fiber axis at an angle of 50°. Circular dichroism (CD) studies in the tyrosine region also reveal drastic changes wih guanidine denaturation, confirming the idea that denaturation produces pronounced increased in mobility at the β carbon (as in the sulfhydryl case). A strong negative band existing only in helical tropomyosin at pH's where the tyrosines are uncharged appears to be due to interaction of tyrosines with the helical backbone, whereas the appearance of a strong positive CD band at 250 nm at high pH (11) seems to be ascribable to interaction between the charged phenolic groups and the dissymmetric backbone α carbon atom.
- Chao,Holtzer
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Read Online
- Synthesis of Isoselenocyanates
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Isoselenocyanates were synthesized by two methods under phase-transfer conditions (50% aq NaOH, CH2Cl2, Aliquat 336); the first started from isocyanides and selenium and gave isoselenocyanates in 61-89% yields, while the second started from amines and used chloroform and selenium, by applying sequentially the Hofmann isonitrile synthesis and the addition of selenium, in 4-70% yields.
- Zakrzewski, Jerzy,Huras, Bogumi?a,Kie?czewska, Anna
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- Kinetics and thermodynamics of reversible disproportionation-comproportionation in redox triad oxoammonium cations - Nitroxyl radicals - Hydroxylamines
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Kinetics and equilibrium of the acid-catalyzed disproportionation of cyclic nitroxyl radicals R2NO· to oxoammonium cations R2NO+ and hydroxylamines R2NOH is defined by redox and acid-base properties of these compounds. In a recent work (J. Phys. Org. Chem. 2014, 27, 114-120), we showed that the kinetic stability of R2NO· in acidic media depends on the basicity of the nitroxyl group. Here, we examined the kinetics of the reverse comproportionation reaction of R2NO+ and R2NOH to R2NO· and found that increasing in -I-effects of substituents greatly reduces the overall equilibrium constant of the reaction K4. This occurs because of both the increase of acidity constants of hydroxyammonium cations K3H+ and the difference between the reduction potentials of oxoammonium cations ER2NO+/R2NO· and nitroxyl radicals ER2NO·/R2NOH. pH dependences of reduction potentials of nitroxyl radicals to hydroxylamines E1/3σ and bond dissociation energies D(O-H) for hydroxylamines R2NOH inwater were determined. For a wide variety of piperidine- and pyrrolidine-1-oxyls values of pK3H+ and ER2NO+/R2NO· correlate with each other, as well aswith the equilibriumconstants K4 and the inductive substituent constants ωI. The correlations obtained allowprediction of the acid-base and redox characteristics of redox triads R2NO·-R2NO+-R2NOH.
- Sen, Vasily D.,Tikhonov, Ivan V.,Borodin, Leonid I.,Pliss, Evgeny M.,Golubev, Valery A.,Syroeshkin, Mikhail A.,Rusakov, Alexander I.
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- Synthesis and spectral properties of polymethine-cyanine dye-nitroxide radical hybrid compounds for use as fluorescence probes to monitor reducing species and radicals
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Various hybrid compounds comprised of two types of nitroxide radicals and either a pentamethine (Cy5) or trimethine cyanine (Cy3) were synthesized. The nitroxide radicals were linked either via an ester-bond to one or two N-alkyl carboxyl-terminated groups of Cy5, or via two amido-bonds (aminocarbonyl or carbonylamino group) to the 5-position of the indolenine moieties of Cy5 and Cy3. Changes in fluorescence and ESR intensities of the hybrid compounds were measured before and after addition of Na ascorbate in PBS (pH 7.0) to reduce the radicals. Among the hybrid compounds synthesized, those that linked the nitroxide radicals via an aminocarbonyl residue at the 5-position of the indolenine moieties on Cy5 and Cy3 exhibited a 1.8- and 5.1-fold increase in fluorescence intensity with the reduction of the nitroxide segment by the addition of Na ascorbate, respectively. In contrast, fluorescence intensity was not enhanced in the other hybrid compounds. Thus, the hybrid compounds which exhibited an increase in fluorescent intensity with radical reduction can be used in the quantitative measurement of reducing species such as Fe2+ and ascorbic acid, and hydroxyl radicals. Because these hybrid compounds have the advantage of fluorescing at longer wavelengths-661 (Cy5) or 568 (Cy3) nm, respectively, they can be used to measure radical-reducing species or radicals either in solution or in vivo.
- Sato, Shingo,Tsunoda, Minoru,Suzuki, Minoru,Kutsuna, Masahiro,Takido-uchi, Kiyomi,Shindo, Mitsuru,Mizuguchi, Hitoshi,Obara, Heitaro,Ohya, Hiroaki
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experimental part
p. 2030 - 2039
(2009/03/12)
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- Reaction of Nitroxides with Sulfur Containing Compounds II [1]. Preparation of Nitroxides Bearing an Isothiocyanate Substituent in View of the Nitroxyl Group Reduction with Thiophosgene
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The reaction of thiophosgene with 2,2,6,6-tetramethylpiperidine-1-oxyl (used as a model nitroxyl radical) was examined. 2,2,6,6-Tetramethylpiperidine and 2,2,6,6-tetramethyl-1-hydroxypiperidine were identified as products. The reaction is not competitive with the reaction of thiophosgene with an amino group. Thus, three nitroxides with an isothiocyanate group were synthesized from thiophosgene and the nitroxides containing the amino substituent.
- Zakrzewski, Jerzy,Hupko, Jarosllaw,Kryczka, Krzysztof
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p. 843 - 850
(2007/10/03)
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- The role of oxoammonium cation in the SOD-mimic activity of cyclic nitroxides
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Cyclic nitroxides (RNO·) mimic the activity of superoxide dismutase (SOD) and demonstrate antioxidant properties in numerous in vitro and in vivo models. Their broad antioxidant activity may involve the participation of their reduced and oxidized forms, that is, hydroxylamine (RNO-H) and oxoammonium cation (RNO+). To examine this possibility we studied the reactions of RNO· and RNO+ with HO2·/O2·- and with several reductants by pulse radiolysis and rapid-mixing stopped-flow techniques. The oxoammonium cations were generated by electrochemical and radiolytic oxidation of 2,2,6,6-tetramethylpiperidinoxyl (TPO) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidinoxyl (3-CP). The rate constant for the reaction of RNO· with HO2· to form RNO+ was determined to be (1.2 ± 0.1) × 108 for TPO and (1.3 ± 0.1) × 106 M-1 s-1 for 3-CP. The kinetics results demonstrate that the reaction of RNO· with HO2· proceeds via an inner-sphere electron-transfer mechanism. The rate constant for the reaction of RNO· with O2·- is lower than 1 x 103 M-1 s-1. The rate constant for the reaction of RNO+ with O2·- was determined to be (3.4 ± 0.2) × 109 for TPO+ and (5.0 ± 0.2) × 109 M-1 s-1 for 3-CP+. Hence, both nitroxides catalyze the dismutation of superoxide through the RNO·/RNO+ redox couple, and the dependence of the catalytic rate constant, kcat, on pH displayed a bell-shaped curve having a maximum around pH 4. The oxoammonium cation oxidized ferrocyanide and HO2- by a one-electron transfer, whereas the oxidation of methanol, formate, and NADH proceeded through a two-electron-transfer reaction. The redox potential of RNO·/RNO+ couple was calculated to be 0.75 and 0.89 V for 3-CP and TPO, respectively. The elucidated mechanism provides a clearer insight into the biological antioxidant properties of cyclic nitroxides that should permit design of even more effective antioxidants.
- Goldstein, Sara,Merenyi, Gabor,Russo, Angelo,Samuni, Amram
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p. 789 - 795
(2007/10/03)
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- The effect of the nature of the substituent at the nitrogen atom on transformations of 3-bromo-2,2,6,6-tetramethyl-4-piperidinone and its 1-hydroxy and 1-oxyl derivatives under conditions of the Favorsky rearrangement
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3-Bromo-2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl reacts with NH4OH to give 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl, a product of the Favorsky rearrangement. 3-Bromo-2,2,6,6-tetramethyl-4-piperidinone is transformed under these conditions into a bicyclic amino ketone, while its 1-hydroxy derivative affords acyclic nitrosoenone.
- Krinitskaya
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p. 1140 - 1142
(2007/10/03)
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- A Study of the Favorskii Rearrangement with 3-Bromo-4-oxo-2,2,6,6-tetramethylpiperidin-1-oxyl
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Favorskii rearrangement reactions of the title compound with sodium hydroxide, sodium ethoxide, ammonia, several aliphatic amines, and ethyl sodiomalonate resulting in pyrrolidine nitroxyl radicals are described.
- Sosnovsky, George,Cai, Zhen-wei
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p. 3414 - 3418
(2007/10/02)
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- Oxidation of Secondary Amines to Nitroxides with Oxone in Aqueous Buffered Solution
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Under biphasic conditions (CH2Cl2/H2O) using acetone, oxone, buffer and phase-transfer catalyst Bu4NHSO4-, some secondary amines without α hydrogens are oxidized to their corresponding nitroxides in rapid and good yields.
- Brik, M. E.
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p. 5519 - 5522
(2007/10/02)
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- Influence of chemical structure on nitroxyl spin magnetic relaxation characteristics
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An attampt was made to develop guidelines for the design of new contrast agents using nitroxyl spin labels (NSL).The structural parameters of ring size and of substituents were correlated with the stability towards reduction and the relaxation effectiveness using 5 piperidine and 5 pyrrolidine nitroxyls containing the same substituents.The susceptibility of NSL to reduction was assessed by EPR spectroscopy.The relaxation effectiveness of NSL on proton in buffer and plasma solution was measured on a NMR spectrometer.The ring size and substituents has a decisive effect on the stability of NSL, whereby the ring size effect was dominant.In the case of spin-lattice relaxivities (R1) and spin-spin relaxivities (R2), the ring size and the substition effect were marginal in buffer solution, while in plasma these effects were more pronounced.A number of guidelines were proposed for the design of suitable NSL contrast agents for MRI.
- Brasch, Robert C.,McNamara, Michael T.,Ehman, Richard L.,Couet, Williams R.,Tozer, Thomas N.,et.al.
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p. 335 - 340
(2007/10/02)
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- A CONVENIENT HIGH YIELD SYNTHESIS OF NITROXIDES
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Appropriate secondary amines are readily converted to nitroxides by dimethyldioxirane in a rapid, convenient, and essentially quantitative process.
- Murray, Robert W.,Singh, Megh
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p. 4677 - 4680
(2007/10/02)
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- INFLUENCE OF CHEMICAL STRUCTURE OF NITROXYL SPIN LABELS ON THEIR REDUCTION BY ASCORBIC ACID
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The influence of structure on the reduction of nitroxyl spin labels by ascorbic acid was examined using both piperidine and pyrrolidine nitroxyls.A five-fold molar excess of ascorbic acid and pH of 7.4 were used.The nitroxyl concentration was measured by electron spin resonance spectrometry.The five-membered (pyrrolidine) nitroxyls were more stable than the six-membered derivatives.Ring substituents also influenced the reaction.The anionic derivatives were more stable than the unionized compounds which, in turn, were more stable than the amines (cations at pH 7.4).
- Couet, W. R.,Brasch, R. C.,Sosnovsky, G.,Lukszo, J.,Prakash, I.,et. al.
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p. 1165 - 1172
(2007/10/02)
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