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14. The Structure and Internal Dynamics of R6-p-C6H4-R6 Biradical: EPR, X-ray Crystallography and DFT Calculations.

Author

Kokorin, A. I., Gromov, O. I., Dorovatovskii, P. V., Lazarenko, V. A., Khrustalev, V. N., Hideg, K., and Kálai, T.

Abstract

A purposefully synthesized nitroxide biradical R6-p-C6H4-R6 (B1), where R6 is the 1-oxyl-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine group with a relatively short distance between the two radical sites, has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy. Hyperfine splitting (hfs) constants on the 14N atoms, electron spin exchange integral |J|, and the distance between the two N-O fragments rNO-NO were experimentally measured. Density functional theory, DFT, calculations were performed using the ORCA 4.0.1.2 program package. The optimized geometry was compared with X-ray crystallographic data and theoretical hfs constants were compared with the respective experimental EPR values. It is concluded that the current quantum chemical approaches provide good results in calculating hfs constants as well as some other EPR parameters. It is confirmed that the intramolecular electron spin exchange in biradicals analogous to B1 is realized by the indirect mechanism rather than direct collision of the N-O· groups. It is also shown that one can calculate and predict values of |J| in other similar biradicals based on the principle of "attenuation coefficients. [ABSTRACT FROM AUTHOR]

Published
2019
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15. Tailored Nitroxide Radicals and Biradical Containing C Enriched Acetylene Groups: ENDOR and DFT Investigation.

Author

Kokorin, A., Zaripov, R., Gromov, O., Hideg, K., and Kálai, T.

Abstract

A specially synthesized nitroxide biradical R-C≡C-( p-CH)-C≡C-R ( B4) and two radicals, R-C≡CH ( RCC) and R-C≡C-CH ( RCCPh), where R is 1-oxyl-2,2,5,5-tetramethyl-pyrroline group, have been studied by X- and W-band electron paramagnetic resonance (EPR) spectroscopy, and by W-band electron-nuclear double resonance (ENDOR). Spin density distribution and hyperfine splitting (hfs) constants on C atoms were experimentally determined and also calculated using ORCA 3.0.3 program package. The biradical and radicals geometries were optimized on UKS/B3LYP/cc-pVDZ level. Hfs constants were calculated using density functional theory (DFT) with PBE0 functional and N07D, and were compared with the experimental value of the hfs constant on C atoms, measured from ENDOR spectra. It is concluded that at small values of the exchange integral as J ≤ a/2 ≈ 7-8 G, the current quantum chemical approaches do not allow determining precise values of the hfs constants on the C atoms in the bridge connecting two paramagnetic nitroxide rings of the biradical. [ABSTRACT FROM AUTHOR]

Published
2018
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19. Spin Density Distribution in a Nitroxide Biradical Containing C-Enriched Acetylene Groups in the Bridge: DFT Calculations and EPR Investigation.

Author

Kokorin, A., Zaripov, R., Gromov, O., Sukhanov, A., Kálai, T., Lamperth, É., and Hideg, K.

Abstract

A specially synthesized nitroxide biradical R-C≡C- p-CH-C≡C-R, B3, where R = 1-oxyl-2,2,6,6-tetramethyl-1,2,3,6-tetrahydropyridine nitroxide, has been studied by electron paramagnetic resonance spectroscopy, and electron-nuclear double resonance (ENDOR). Spin density distribution and hyperfine splitting (hfs) constant on C atoms calculations for biradical B3 were carried out using B3LYP and PBE0 functionals and several different basis sets including N07 family and were compared with the experimental value of the hfs constant on C atoms, measured from ENDOR spectra of B3. The mechanism of the intramolecular electron spin exchange in B3 biradical is discussed. [ABSTRACT FROM AUTHOR]

Published
2016
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20. Structure-activity relationship of cyclic nitroxides as SOD mimics and scavengers of nitrogen dioxide and carbonate radicals

Author

Goldstein, S., Samuni, A., Hideg, K., Merenyi, Gabor, Goldstein, S., Samuni, A., Hideg, K., and Merenyi, Gabor

Abstract

Synthetic nitroxide antioxidants attenuate oxidative damage in various experimental models. Their protective effect reportedly depends on ring size and ring substituents and is greater for nitroxides having lower oxidation potential. The present study focuses on the kinetics and mechanisms of the reactions of piperidine, pyrrolidine and oxazolidine nitroxides with HO2 center dot/O-2(center dot-), (NO2)-N-center dot and CO3 center dot- radicals, which are key intermediates in many inflammatory and degenerative diseases. It is demonstrated that nitroxides are the most efficient scavengers of (NO2)-N-center dot at physiological pH (k = (3-9) x 10(8) M-1 s(-1)) and among the most effective metal-independent scavengers Of CO3 center dot- radicals (k = (2 - 6) x 10(8) M-1 s(-1)). Their reactivity toward HO2 center dot, though not toward center dot NO2 and CO3 center dot-, depends on the nature of the ring side-chain and particularly on the ring-size. All nitroxide derivatives react slowly with O-2(center dot-) and are relatively inefficient SOD mimics at physiological pH. Even piperidine nitroxides, having the highest SOD-like activity, demonstrate a catalytic activity of about 1000-fold lower than that of native SOD at pH 7.4. The present results do not indicate any correlation between the kinetics of HO2 center dot/O-2(center dot-), (NO2)-N-center dot, and CO3 center dot- removal by nitroxides and their protective activity against biological oxidative stress and emphasize the importance of target-oriented nitroxides, i.e., interaction between the biological target and specific nitroxides., QC 20100525

Published
2006
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48. Synthesis and reactions of new nitronyl nitroxides

Author

Kálai, T., Jeko, J., Szabó, Zoltan, Párkányi, L., Hideg, K., Kálai, T., Jeko, J., Szabó, Zoltan, Párkányi, L., and Hideg, K.

Abstract

Various new 2-substituted 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-yloxyl 3-oxide mono- 1b, 7, 13a,b, 14, 15, 16, 17, 22 and biradicals 8, 9 were prepared. The nitronyl nitroxide functions were selectively reduced by Fe/HOAc to 1-hydroxyimidazoline derivatives 3b, 11, 21. The reductions by Zn/HCl led to imidazolines 4, 12. The nitronyl nitroxide functions remained inert when the 2-phenyl compound was nitrated by H2SO4/HNO3 to give nitrophenyl nitronyl nitroxide derivatives 13a,b, 14., QC 20220915

Published
1997
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