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NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

1.6K
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...
1.6K
Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

1.8K
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...
1.8K
Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

1.2K
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...
1.2K
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons01:03

¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons

2.5K
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...
2.5K
¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons00:58

¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons

1.9K
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...
1.9K
Carbon-13 (¹³C) NMR: Overview01:10

Carbon-13 (¹³C) NMR: Overview

6.0K
Carbon-13 is a naturally occurring NMR-active isotope of carbon with a low natural abundance of 1.1%. In contrast, carbon-12 is the most abundant isotope of carbon with zero nuclear spin. Therefore, it is NMR inactive. The gyromagnetic ratio of carbon-13 is smaller than that of protons. As a result, carbon-13 resonance is about 6000 times weaker than proton resonance. For a given magnetic field strength, the resonance frequency of carbon-13 is about one-fourth of the resonance frequency for...
6.0K

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Pure Shift Nuclear Magnetic Resonance: a New Tool for Plant Metabolomics
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Chemical shift variations in common metabolites.

Fatema Bhinderwala1, Heidi E Roth2, Hannah Noel2

  • 1Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588-0304, United States; Nebraska Center for Integrated Biomolecular Communication, University of Nebraska-Lincoln, Lincoln, NE 68588-0304, United States; University of Pittsburgh School of Medicine, Department of Structural Biology, Pittsburgh, PA 15260, United States(2).

Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|November 21, 2022
PubMed
Summary

Environmental factors like pH and temperature significantly impact nuclear magnetic resonance (NMR) metabolite assignments. Accurate automated assignments in complex mixtures require expert validation and improved reference databases.

Keywords:
(1)H NMRAutomated metabolite assignmentBiofluidsTemperaturepH

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Area of Science:

  • Metabolomics
  • Nuclear Magnetic Resonance (NMR) Spectroscopy
  • Analytical Chemistry

Background:

  • Metabolite assignments in 1H NMR are challenged by variations in temperature, pH, buffer, ionic strength, and mixture composition.
  • Peak overlap, spectral crowding, peak drift, and line broadening complicate accurate deconvolution and chemical assignment.
  • Fluctuating sample conditions hinder the reliability and robustness of metabolite identification in complex biological samples.

Purpose of the Study:

  • To quantify chemical shift variability in common metabolites under varied pH and temperature conditions.
  • To assess the challenges in automated metabolite assignments within complex mixtures.
  • To provide insights for improving the accuracy and reproducibility of NMR-based metabolomic analyses.

Main Methods:

  • Acquisition of 1D 1H NMR spectra for 54 common metabolites.
  • Systematic variation of pH (6.0–8.0) and temperature (290 K–308 K).
  • Analysis of chemical shift variability and false negative rates in various mixture compositions.

Main Results:

  • Phosphorylated metabolites exhibited greater chemical shift variation with pH changes, while amino acids showed higher variation with temperature.
  • Mixtures of phosphorous compounds demonstrated consistently poor assignment reliability.
  • False negative rates reached 40% for phosphorylated cholines, amino acids, and glycerols across multiple mixture conditions, and 57% for amino acids at pH 8 and 298 K.

Conclusions:

  • Automated metabolite assignments in complex biofluid mixtures necessitate expert intervention for accuracy confirmation.
  • Current reference databases require expansion to include spectra under diverse conditions, including mixtures and varying pH/temperature.
  • Improved databases are crucial for enhancing the accuracy and reproducibility of metabolite assignments in metabolomics research.