Magnetic dipole-dipole interaction
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Magnetic dipole-dipole interaction, also called dipolar coupling, refers to the direct interaction between two magnetic dipoles. The energy of the interaction is as follows:
H = - (ħμ/4πrij3) (3(mj.ejk)(mk.ejk) - mj.mk)
where ejk is a unit vector parallel to the line joining the centers of the two dipoles. rij is the distance between two dipoles, mk and mj.
For two interacting nuclear spins:
H = -(μ/4π) (γjγk/rij3) (h/2π) (3(Ij.ejk)(Ik.ejk) - Ij.Ik)
γj, γk and rij are gyromagnetic ratios of two spins and spin-spin distance respectively.
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[edit] Dipolar coupling and NMR spectroscopy
The direct dipole-dipole coupling is very useful for molecular structural studies, since it depends only on known physical constants and the inverse cube of internuclear distance. Estimation of this coupling provides a direct spectroscopic route to the distance between nuclei and hence the geometrical form of the molecule. Although internuclear magnetic dipole couplings contain a great deal of structural information, in isotropic solution, they average to zero as a result of rotational diffusion. However, their effect on nuclear spin relaxation results in measurable nuclear Overhauser effects (NOEs).
The residual dipolar coupling (RDC) occur if the molecules in solution exhibit a partial alignment leading to an incomplete averaging of spatially anisotropic magnetic interactions i.e. dipolar couplings. RDC measurement provides information on the global folding of the protein -long distance structural information. It also provides information about "slow" dynamics in molecules.
[edit] Dipolar coupling in 1D NMR
In 1D NMR dipolar coupling acts merely as a relaxation phenomenon and give rise to line broadening (T2 process).
[edit] Dipolar coupling in 2D NMR
Mixing though dipolar coupling in 2D NMR occurs as follows:
Ix --t1--> Iz --NOE--> Sz --t2--> Sx
which is different (incoherent magnetization transfer) from mixing through J-coupling, where coupling does not play role during mixing period.
[edit] References
- Malcolm H. Levitt , Spin Dynamics: Basics of Nuclear Magnetic Resonance. ISBN 0-471-48922-0.
