19-07-2012, 01:27 PM
NUCLEAR MAGNETIC RESONANCE
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INTRODUCTION
Nuclear magnetic resonance (NMR) spectroscopy uses radiofrequency radiation to induce transitions between different nuclear spin states of samples in a magnetic field.
NMR spectroscopy can be used for quantitative measurements, but it is most useful for determining the structure of molecules.
The utility of NMR spectroscopy for structural characterization arises because different atoms in a molecule experience slightly different magnetic fields and therefore transitions at slightly different resonance frequencies in an NMR spectrum.
Furthermore, splittings of the spectra lines arise due to interactions between different nuclei, which provides information about the proximity of different atoms in a molecule.
CHEMICAL SHIFT
The signal frequency that is detected in NMR spectroscopy is proportional to the magnetic field applied to the nucleus. This would be a precisely determined frequency if the only magnetic field acting on the nucleus was the externally applied field. But the response of the atomic electrons to that externally applied magnetic field is such that their motions produce a small magnetic field at the nucleus which usually acts in opposition to the externally applied field. This change in the effective field on the nuclear spin causes the NMR signal frequency to shift. The magnitude of the shift depends upon the type of nucleus and the details of the electron motion in the nearby atoms and molecules. It is called a "chemical shift".
In practice the chemical shift is usually indicated by a symbol δ which is defined in terms of a standard reference.
SPIN-SPIN COUPLING
Protons have a magnetic field associated with them (since they have a nuclear spin), and when they are placed in a magnetic field approximately half of the protons become aligned with the field and half become aligned against the field. It is the transition between these two states that we observe in NMR.
DECOUPLING
To understand decoupling, consider the familiar hydrogen NMR spectrum of HC-(CH2CH3)3. The HC hydrogen peaks are difficult to see in the spectrum due to the splitting from the 6 -CH2- hydrogens.
If the effect of the 6 -CH2- hydrogens could be removed, we would lose the 1:6:15:20:15:6:1 splitting for the HC hydrogen and get one peak.
We would also lose the 1:3:1 splitting for the CH3 hydrogens and get one peak. The process of removing the spin-spin splitting between spins is called decoupling.
Decoupling is achieved with the aid of a saturation pulse. If the affect of the HC hydrogen is removed, we see the following spectrum. Similarly, if the affect of the -CH3 hydrogens is removed, we see this spectrum.
APPLICATIONS
Solution structure The only method for atomic-resolution structure determination of biomacromolecules in aqueous solutions under near physiological conditions or membrane mimeric environments.
Molecular dynamics The most powerful technique for quantifying motional properties of biomacromolecules.
Protein folding The most powerful tool for determining the residual structures of unfolded proteins and the structures of folding intermediates.
Ionization state The most powerful tool for determining the chemical properties of functional groups in biomacromolecules, such as the ionization states of ionizable groups at the active sites of enzymes.
[attachment=28066]
INTRODUCTION
Nuclear magnetic resonance (NMR) spectroscopy uses radiofrequency radiation to induce transitions between different nuclear spin states of samples in a magnetic field.
NMR spectroscopy can be used for quantitative measurements, but it is most useful for determining the structure of molecules.
The utility of NMR spectroscopy for structural characterization arises because different atoms in a molecule experience slightly different magnetic fields and therefore transitions at slightly different resonance frequencies in an NMR spectrum.
Furthermore, splittings of the spectra lines arise due to interactions between different nuclei, which provides information about the proximity of different atoms in a molecule.
CHEMICAL SHIFT
The signal frequency that is detected in NMR spectroscopy is proportional to the magnetic field applied to the nucleus. This would be a precisely determined frequency if the only magnetic field acting on the nucleus was the externally applied field. But the response of the atomic electrons to that externally applied magnetic field is such that their motions produce a small magnetic field at the nucleus which usually acts in opposition to the externally applied field. This change in the effective field on the nuclear spin causes the NMR signal frequency to shift. The magnitude of the shift depends upon the type of nucleus and the details of the electron motion in the nearby atoms and molecules. It is called a "chemical shift".
In practice the chemical shift is usually indicated by a symbol δ which is defined in terms of a standard reference.
SPIN-SPIN COUPLING
Protons have a magnetic field associated with them (since they have a nuclear spin), and when they are placed in a magnetic field approximately half of the protons become aligned with the field and half become aligned against the field. It is the transition between these two states that we observe in NMR.
DECOUPLING
To understand decoupling, consider the familiar hydrogen NMR spectrum of HC-(CH2CH3)3. The HC hydrogen peaks are difficult to see in the spectrum due to the splitting from the 6 -CH2- hydrogens.
If the effect of the 6 -CH2- hydrogens could be removed, we would lose the 1:6:15:20:15:6:1 splitting for the HC hydrogen and get one peak.
We would also lose the 1:3:1 splitting for the CH3 hydrogens and get one peak. The process of removing the spin-spin splitting between spins is called decoupling.
Decoupling is achieved with the aid of a saturation pulse. If the affect of the HC hydrogen is removed, we see the following spectrum. Similarly, if the affect of the -CH3 hydrogens is removed, we see this spectrum.
APPLICATIONS
Solution structure The only method for atomic-resolution structure determination of biomacromolecules in aqueous solutions under near physiological conditions or membrane mimeric environments.
Molecular dynamics The most powerful technique for quantifying motional properties of biomacromolecules.
Protein folding The most powerful tool for determining the residual structures of unfolded proteins and the structures of folding intermediates.
Ionization state The most powerful tool for determining the chemical properties of functional groups in biomacromolecules, such as the ionization states of ionizable groups at the active sites of enzymes.