Concept

Residual dipolar coupling

Résumé
The residual dipolar coupling between two spins in a molecule occurs if the molecules in solution exhibit a partial alignment leading to an incomplete averaging of spatially anisotropic dipolar couplings. Partial molecular alignment leads to an incomplete averaging of anisotropic magnetic interactions such as the magnetic dipole-dipole interaction (also called dipolar coupling), the chemical shift anisotropy, or the electric quadrupole interaction. The resulting so-called residual anisotropic magnetic interactions are useful in biomolecular NMR spectroscopy. NMR spectroscopy in partially oriented media was reported by Alfred Saupe. After this initiation, a several NMR spectra in various liquid crystalline phases were reported (see e.g. ). A second technique for partial alignment that is not limited by a minimum anisotropy is strain-induced alignment in a gel (SAG). The technique was extensively used to study the properties of polymer gels by means of high-resolution deuterium NMR, but only lately gel alignment was used to induce RDCs in molecules dissolved into the gel. SAG allows the unrestricted scaling of alignment over a wide range and can be used for aqueous as well as organic solvents, depending on the polymer used. As a first example in organic solvents, RDC measurements in stretched polystyrene (PS) gels swollen in CDCl3 were reported as a promising alignment method. In 1995, NMR spectra were reported for cyanometmyoglobin, which has a very highly anisotropic paramagnetic susceptibility. When taken at very high field, these spectra may contain data that can usefully complement NOEs in determining a tertiary fold. In 1996 and 1997, the RDCs of a diamagnetic protein ubiquitin were reported. The results were in good agreement with the crystal structures. The secular dipolar coupling Hamiltonian of two spins, and is given by: where is the reduced Planck constant. and are the gyromagnetic ratios of spin and spin respectively. is the inter-spin distance. is the angle between the inter-spin vector and the external magnetic field.
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