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Related Concept Videos

Atomic Nuclei: Nuclear Spin State Population Distribution01:14

Atomic Nuclei: Nuclear Spin State Population Distribution

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Near absolute zero temperatures, in the presence of a magnetic field, the majority of nuclei prefer the lower energy spin-up state to the higher energy spin-down state. As temperatures increase, the energy from thermal collisions distributes the spins more equally between the two states. The Boltzmann distribution equation gives the ratio of the number of spins predicted in the spin −½ (N−) and spin +½ (N+) states.
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Double Resonance Techniques: Overview01:12

Double Resonance Techniques: Overview

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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.
Spin decoupling is usually achieved by...
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Insensitive Nuclei Enhanced by Polarization Transfer (INEPT)01:15

Insensitive Nuclei Enhanced by Polarization Transfer (INEPT)

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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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Atomic Nuclei: Nuclear Relaxation Processes01:23

Atomic Nuclei: Nuclear Relaxation Processes

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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
683
NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences01:17

NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences

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A pulse is a short burst of radio waves distributed over a range of frequencies that simultaneously excites all the nuclei in the sample. Upon passing a radio frequency pulse along the x-axis, the nuclei absorb energy corresponding to their Larmor frequencies and achieve resonance. This shifts the net magnetization vector from the z-axis toward the transverse plane. This angle of rotation of the magnetization vector, or the flip angle, is proportional to the duration and intensity of the pulse.
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Other Nuclides: 31P, 19F, 15N NMR01:16

Other Nuclides: 31P, 19F, 15N NMR

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Many organic, inorganic, and biological molecules contain spin-half nuclei such as nitrogen-15, fluorine-19, and phosphorus-31. As a result, NMR studies of these nuclei have found extensive applications in chemical and biological research.
While fluorine-19 and phosphorous-31 have high natural abundances (100%) and positive gyromagnetic ratios, nitrogen-15 has a low natural abundance and a negative gyromagnetic ratio. However, nitrogen-15 is still preferred over nitrogen-14 (which has a...
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Dissolution Dynamic Nuclear Polarization Instrumentation for Real-time Enzymatic Reaction Rate Measurements by NMR
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Dissolution Dynamic Nuclear Polarization Instrumentation for Real-time Enzymatic Reaction Rate Measurements by NMR

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Frequency-swept dynamic nuclear polarization.

Michael Mardini1, Ravi Shankar Palani1, Iram M Ahmad2

  • 1Department of Chemistry and Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, United States.

Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|June 29, 2023
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Summary
This summary is machine-generated.

A new microwave source enables frequency, amplitude, and phase modulation for dynamic nuclear polarization (DNP) NMR. This advance allows for enhanced sensitivity in aqueous samples and opens doors for new time-domain experiments.

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Area of Science:

  • Nuclear Magnetic Resonance Spectroscopy
  • Electron Paramagnetic Resonance
  • Microwave Engineering

Background:

  • Dynamic nuclear polarization (DNP) significantly enhances NMR spectroscopy sensitivity by transferring electron polarization to nuclei.
  • Conventional DNP methods rely on continuous-wave (CW) microwave sources with fixed frequency and power, limiting explored mechanisms.
  • High magnetic fields (>5 T) necessitate microwave sources above 140 GHz, historically favoring gyrotrons or fixed-frequency oscillators.

Purpose of the Study:

  • To introduce a novel microwave source capable of facile frequency, amplitude, and phase modulation for DNP-NMR experiments at 9 T (250 GHz).
  • To investigate the impact of modulated microwave irradiation on DNP mechanisms and explore new experimental possibilities.
  • To demonstrate the potential of affordable and compact microwave sources for achieving substantial sensitivity enhancements in aqueous samples.

Main Methods:

  • Integration of a frequency-, amplitude-, and phase-agile microwave source into a 9 T (250 GHz) magic-angle spinning (MAS) NMR setup.
  • Experimental investigation of continuous-wave (CW) DNP mechanisms under modulated irradiation.
  • Application of frequency-chirped irradiation and demonstration of Overhauser effect enhancements using a water-soluble BDPA radical.

Main Results:

  • Successful implementation of a versatile microwave source for DNP-MAS NMR experiments.
  • Demonstration of the advantages of frequency-chirped irradiation for DNP.
  • Achieved a significant Overhauser enhancement of approximately 25 in aqueous solution using a BDPA radical.

Conclusions:

  • The developed microwave source offers unprecedented control for DNP-NMR, enabling exploration of new mechanisms.
  • Affordable and compact microwave sources can achieve substantial sensitivity gains in aqueous samples, including biological macromolecules.
  • Future development of microwave amplifiers will facilitate advanced time-domain DNP experiments.