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Related Concept Videos

Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

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Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. It has a significant energy separation between its nuclear spin states due to its large gyromagnetic ratio. As per Boltzmann's distribution, an increase in the energy separation implies a greater excess population of nuclei available for excitation, resulting in a strong NMR absorption signal.
Absorption signals of all the protium nuclei...
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NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

3.4K
The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
3.4K
Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

2.2K
The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...
2.2K
Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

1.4K
In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
1.4K
π Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

1.7K
An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0,...
1.7K
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons01:03

¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons

4.2K
Protons in identical electronic environments within a molecule are chemically equivalent and have the same chemical shift. The replacement test is a useful tool to identify chemical equivalence and predict NMR spectra. A substituent replaces each of the protons being examined and the resulting molecules are compared. If the same molecule is obtained, the protons are equivalent or homotopic. Replacement of any hydrogens in ethane by chlorine yields chloroethane because all six protons are...
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Pure Shift Nuclear Magnetic Resonance: a New Tool for Plant Metabolomics
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Novel projected 4D triple resonance experiments for polypeptide backbone chemical shift assignment.

Youlin Xia1, Cheryl H Arrowsmith, Thomas Szyperski

  • 1Ontario Cancer Institute and Department of Medical Biophysics, The University of Toronto, Canada.

Journal of Biomolecular NMR
|November 27, 2002
PubMed
Summary

We developed new 4D triple-resonance Nuclear Magnetic Resonance (NMR) experiments for faster protein backbone assignment. These novel methods improve efficiency in structural genomics by reducing spectral overlap and optimizing measurement times.

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Area of Science:

  • Biochemistry
  • Structural Biology
  • Nuclear Magnetic Resonance (NMR) Spectroscopy

Background:

  • Sequential assignment of polypeptide backbone chemical shifts is crucial for protein structure determination.
  • Conventional NMR experiments can face challenges with peak overlap and extended measurement times, especially in high-throughput structural genomics.

Purpose of the Study:

  • To introduce a novel suite of projected 4D triple-resonance NMR experiments.
  • To enhance the efficiency and accuracy of sequential assignment of chemical shifts in labeled proteins.
  • To facilitate automated, high-throughput NMR structure determination.

Main Methods:

  • Development and application of 3D HNN[CAHA] and 3D HNN(CO)[CAHA] experiments utilizing a common dimension for 13C(alpha) and 1H(alpha) chemical shift evolution.
  • Simultaneous quadrature detection of chemical shifts to minimize spectral overlap.
  • Complementary use of 3D reduced-dimensionality (RD) HNN COCA and NNCACO experiments where 13C(alpha) and 13C' chemical shifts evolve in a common dimension.
  • Utilizing in-phase splitting for encoding 13C' chemical shifts.

Main Results:

  • The new experiments enable efficient sequential assignment of polypeptide backbone chemical shifts in 13C/15N doubly labeled proteins.
  • The methods are particularly effective for assigning glycine-rich polypeptide segments.
  • Separation of spectral regions for different chemical shifts prevents increased peak overlap compared to conventional methods.
  • The suite of four experiments provides a valuable tool for automated structure determination.

Conclusions:

  • The presented suite of projected 4D triple-resonance NMR experiments offers a significant advancement for protein backbone assignment.
  • These methods address limitations of conventional NMR by improving efficiency and reducing spectral overlap.
  • The experiments are well-suited for high-throughput structural genomics, aiding in faster protein structure determination.