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The Pople nomenclature system classifies spin systems based on the difference between their chemical shifts. Coupled spins are denoted by capital letters with subscripts indicating the number of equivalent nuclei. When the coupled nuclei have well-separated chemical shifts, they are assigned letters that are far apart in the alphabet, such as A and X. When the difference in chemical shifts is small, coupled nuclei are named using adjacent letters of the alphabet (AB, MN, or XY).
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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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Nuclear Overhauser Enhancement (NOE)01:07

Nuclear Overhauser Enhancement (NOE)

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Irradiation of a spin-active nucleus causes an increase or decrease in the signal intensity of neighboring nuclei that are not necessarily chemically bonded or involved in J-coupling.  This phenomenon, called the Nuclear Overhauser Enhancement (NOE), results from through-space interactions between the nuclear spins. The NOE effect decreases with increasing internuclear distance and is generally not observed beyond 4 angstroms. In NOE, dipole-dipole interactions between neighboring...
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Double Resonance Techniques: Overview01:12

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Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
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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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Using PELDOR to count spins on multi-nitroxides.

Matthias Bretschneider1, Burkhard Endeward1, Jörn Plackmeyer1

  • 1Institute of Physical and Theoretical Chemistry and Center of Biomolecular Magnetic Resonance, Goethe University Frankfurt am Main, Germany.

Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|April 30, 2025
PubMed
Summary
This summary is machine-generated.

Pulsed electron-electron double resonance (PELDOR) can count coupled nitroxide spins in molecules up to six. Accuracy is limited by pulse calibration, but dipolar defocusing effects are minimized with specific pulse types for distances over 2 nm.

Keywords:
DEERModulation depthMulti-spin complexesPELDORSpin counting

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Area of Science:

  • Magnetic Resonance Spectroscopy
  • Physical Chemistry
  • Supramolecular Chemistry

Background:

  • Pulsed electron-electron double resonance (PELDOR) is a powerful technique for measuring distances between electron spins.
  • Accurately determining the number of coupled spins in multi-spin systems is crucial for understanding molecular structure and dynamics.
  • Previous methods for spin counting using PELDOR modulation depth have limitations in complex systems.

Purpose of the Study:

  • To investigate the accuracy and limitations of using PELDOR modulation depth for counting coupled spins in multi-nitroxide molecules.
  • To identify the key factors affecting the precision of spin counting using this technique.
  • To assess the applicability of PELDOR for spin counting in systems with varying numbers of coupled spins.

Main Methods:

  • Synthesis of multi-nitroxide molecules with 2-6 coupled spins.
  • Application of pulsed electron-electron double resonance (PELDOR) experiments at Q-band frequencies.
  • Utilized broadband sech/tanh and short rectangular pump pulses to analyze modulation depth suppression effects.

Main Results:

  • The reproducibility of pump pulse excitation efficiency is the primary limitation for accurate spin counting of larger spin numbers.
  • Modulation depth suppression effects were avoided for intramolecular spin distances > 2 nm using specific pulse shapes.
  • Transverse relaxation times were independent of spin number, but primary Hahn echo signal intensity decreased significantly with increasing spin count due to dipolar defocusing.

Conclusions:

  • PELDOR modulation depth is a viable method for counting coupled nitroxide spins, potentially applicable up to hexameric complexes.
  • Careful calibration of pump pulse excitation efficiency is critical for accurate spin counting, especially in systems with many spins.
  • Dipolar defocusing effects reduce echo intensity and accuracy, necessitating optimized pulse sequences and experimental conditions.