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Ideally, an unpaired electron shows a single peak in the EPR spectrum due to the transition between the two spin energy states. However, coupling interactions can occur between the spins of the unpaired electron and any neighboring spin-active nuclei. This hyperfine coupling results in hyperfine splitting, where the EPR signal is split into multiplets. The signals split into 2nI + 1 peaks, where n is the number of equivalent nuclei and I is the nuclear spin. These splitting patterns provide...
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Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) is an advanced Nuclear Magnetic Resonance (NMR) technique specifically designed to detect and enhance the signals of low-abundance nuclei, such as carbon-13 and nitrogen-15, in small molecules. The fundamental principle behind INEPT is the transfer of polarization from a more abundant and highly polarizable nucleus, typically hydrogen-1, to the low-abundance nucleus of interest. This process effectively boosts the NMR signal of the...
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ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
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When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...
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At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
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Sterically Shielded 3,3-Bis(hydroxymethyl) Pyrrolidine Nitroxides: Synthesis, EPR Spectra, Spin Relaxation and

Sophia Yu Trakhinina1, Andrey I Taratayko1, Nargiz B Asanbaeva1

  • 1N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, Ac. Lavrentiev Ave. 9, Novosibirsk 630090, Russia.

The Journal of Organic Chemistry
|March 17, 2026
PubMed
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Sterically shielded nitroxides show high resistance to bioreduction, making them valuable as spin labels for cellular and in vivo studies. New pyrrolidine and azaspiro decane nitroxides were synthesized and characterized for EPR and NMR applications.

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Area of Science:

  • Organic Chemistry
  • Biophysical Chemistry
  • Spectroscopy

Background:

  • Sterically shielded nitroxides are crucial as spin labels and probes in Electron Paramagnetic Resonance (EPR) and Nuclear Magnetic Resonance (NMR) applications.
  • Their resistance to bioreduction is a key property for in-cell and in-vivo studies.
  • Developing novel nitroxide structures with enhanced stability and specific properties is an ongoing area of research.

Purpose of the Study:

  • To synthesize and characterize a new series of sterically shielded nitroxides based on pyrrolidine and azaspiro[4.5]decane scaffolds.
  • To investigate the impact of structural modifications on the stability, spectral properties, and bioreduction resistance of these nitroxides.
  • To explore their potential as spin labels and probes in biophysical applications.

Main Methods:

  • Synthesis of pyrrolidine and azaspiro[4.5]decane nitroxides via [3 + 2] cycloaddition reactions.
  • Chemical modifications including N-oxidation, Grignard additions, hydrogenation, and oxidation.
  • Electron Paramagnetic Resonance (EPR) spectroscopy for characterization, including analysis of hyperfine coupling constants.
  • Assessment of bioreduction resistance using ascorbate assays.

Main Results:

  • A series of novel sterically shielded nitroxides were successfully synthesized.
  • EPR spectra revealed significant hyperfine coupling (hfc) with gamma-hydrogen in the side chain.
  • One spirocyclohexane-containing nitroxide exhibited remarkable resistance to ascorbate reduction (k2 = 4.5 ± 0.1 × 10^-3 M^-1s^-1).
  • The ethynyl derivatives showed EPR spectral sensitivity to media polarity.
  • Some compounds displayed minimal changes in phase memory time (Tm) across a temperature range.

Conclusions:

  • The synthesized nitroxides possess structural features contributing to their stability and resistance to bioreduction.
  • These compounds are promising candidates for advanced spin labeling and probing in biological systems.
  • The observed structure-property relationships provide valuable insights for designing future nitroxide-based probes.