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Related Concept Videos

NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

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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...
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¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

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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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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.3K
Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
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Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

1.1K
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...
1.1K
Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

3.0K
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...
3.0K
¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons00:58

¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons

2.9K
Replacing each alpha-hydrogen in chloroethane by bromine (or a different functional group) yields a pair of enantiomers. Such protons are called prochiral or enantiotopic and are related by a mirror plane. Enantiotopic protons are chemically equivalent in an achiral environment. Because most proton NMR spectra are recorded using achiral solvents, enantiotopic hydrogens yield a single signal.
In chiral compounds such as 2-butanol, replacing the methylene hydrogens at C3 produces a pair of...
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A Methionine Chemical Shift Based Order Parameter Characterizing Global Protein Dynamics.

Saeed Chashmniam1,2, João M C Teixeira1,3, Juan Carlos Paniagua4

  • 1Department of Inorganic and Organic Chemistry, University of Barcelona, Baldiri Reixac 10-12, 08028, Barcelona, Spain.

Chembiochem : a European Journal of Chemical Biology
|November 9, 2020
PubMed
Summary

Long-distance methionine side chain dynamics influence protein allostery. Their conformational averaging, measured by 13C NMR chemical shifts, correlates with flexibility, providing insights into protein dynamics.

Keywords:
DFTchemical-shift calculationsmethionine methyl NMRmethyl NMRorder parameters

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Area of Science:

  • Biophysics
  • Structural Biology
  • Protein Dynamics

Background:

  • Allostery involves long-distance coupling of side chain dynamics.
  • Methionine side chains exhibit high intrinsic flexibility.
  • Conformational averaging of methionine residues is influenced by neighboring residues.

Purpose of the Study:

  • To investigate the relationship between methionine side chain dynamics and protein structure.
  • To assess the utility of 13C NMR chemical shifts for studying long-range conformational averaging.
  • To establish a correlation between calculated and observed chemical shifts for methionine methyl groups.

Main Methods:

  • Utilized 13C Nuclear Magnetic Resonance (NMR) spectroscopy to measure chemical shifts.
  • Employed quantum mechanical methods to predict chemical shifts from static X-ray structures.
  • Analyzed the linear correlation between calculated and observed chemical shifts across different proteins.

Main Results:

  • Observed similar scaling of methionine methyl 13C NMR chemical shifts with predicted values.
  • Established a good linear correlation between calculated and observed chemical shifts.
  • Found that the slope of this correlation varies with protein flexibility, from 0 for calmodulin to 0.7 for calcineurin.

Conclusions:

  • The linear correlation indicates consistent side-chain conformational averaging over long distances.
  • The slope of the correlation serves as an order parameter for global side-chain flexibility.
  • This approach provides a quantitative measure of protein flexibility based on methionine dynamics.