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Related Concept Videos

NMR Spectrometers: Resolution and Error Correction01:14

NMR Spectrometers: Resolution and Error Correction

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When magnetic nuclei in a sample achieve resonance and undergo relaxation, the signal detected in NMR is an approximately exponential free induction decay. Fourier transform of an exponential decay yields a Lorentzian peak in the frequency domain. Lorentzian peaks in an NMR spectrum are defined by their amplitude, full width at half maximum, and position, where the peak width is governed by the spin-spin relaxation time alone. In real experiments, however, the applied magnetic field is rendered...
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¹³C NMR: ¹H–¹³C Decoupling01:04

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The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...
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Carbon-13 (¹³C) NMR: Overview01:10

Carbon-13 (¹³C) NMR: Overview

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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...
9.3K
Two-Dimensional (2D) NMR: Overview01:12

Two-Dimensional (2D) NMR: Overview

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The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily interpreted using 2-dimensional (2D) NMR. Unlike 1D NMR, 2D NMR has two frequency axes that provide the coupling information between the nucleus A and nucleus B in a molecule. The process from which 2D spectra are obtained has four steps.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse....
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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

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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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NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

3.6K
The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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Spectral density mapping at multiple magnetic fields suitable for (13)C NMR relaxation studies.

Pavel Kadeřávek1, Vojtěch Zapletal2, Radovan Fiala2

  • 1National Centre for Biomolecular Research, Faculty of Science, Masaryk University, Kamenice 5, CZ-625 00 Brno, Czech Republic; Central European Institute of Technology, Masaryk University, Kamenice 5, CZ-625 00 Brno, Czech Republic; Institute of Biophysics of Academy of Sciences of the Czech Republic, Královopolská 135, CZ-612 65 Brno, Czech Republic.

Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|March 23, 2016
PubMed
Summary

Standard spectral density mapping protocols introduce errors for carbon-13 relaxation data. New protocols using cross-correlated rates and multiple magnetic fields improve accuracy for small molecules and dynamic residues.

Keywords:
CarbohydratesMagnetic fieldNuclear magnetic resonanceNucleic acidsRelaxationSpectral density function

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Area of Science:

  • Nuclear Magnetic Resonance (NMR) Spectroscopy
  • Biophysical Chemistry
  • Computational Chemistry

Background:

  • Standard spectral density mapping protocols are effective for nitrogen-15 relaxation data but introduce systematic errors for carbon-13 relaxation data, particularly for small molecules or dynamic regions of macromolecules.
  • These errors arise from motions with short correlation times, complicating the accurate analysis of molecular dynamics.

Purpose of the Study:

  • To investigate the improvement in accuracy for carbon-13 relaxation data analysis by utilizing cross-correlated relaxation rates and measurements at multiple magnetic fields.
  • To develop and test a suite of protocols for analyzing such multi-field, cross-correlated relaxation data.
  • To demonstrate the applicability of these protocols through case studies on RNA hairpin and disaccharide systems.

Main Methods:

  • Development and testing of novel protocols for spectral density mapping using both auto- and cross-correlated relaxation rates.
  • Acquisition of relaxation data at multiple magnetic fields (three and five fields in case studies).
  • Application of these protocols to uniformly labeled RNA hairpin and selectively labeled disaccharide systems with varying degrees of motional anisotropy.

Main Results:

  • Accurate values for auto- and cross-correlated spectral density functions at zero and carbon-13 frequencies can be obtained from data acquired at three magnetic fields for uniformly carbon-13 labeled molecules with moderate rotational anisotropy.
  • Analysis of auto-correlated relaxation rates at five magnetic fields provides a viable alternative for molecules exhibiting highly anisotropic motions.
  • The developed protocols successfully separated effects of fast motions from conformational or chemical exchange in the RNA hairpin study.

Conclusions:

  • The proposed protocols offer a significant improvement in the accuracy of spectral density mapping for carbon-13 relaxation data, overcoming limitations of standard methods.
  • Multi-field NMR relaxation measurements combined with cross-correlated rates are crucial for detailed molecular dynamics studies, especially for systems with complex motions.
  • These findings provide enhanced tools for characterizing the dynamics of small molecules and flexible regions in macromolecules.