Residual dipolar coupling

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. 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 becoming increasingly important in biomolecular NMR spectroscopy. NMR spectroscopy in partially oriented media was first discovered in 1963, and in a very fundamental paper Alfred Saupe was also able to present the essential theory to describe and understand the observable phenomena only one year later. After this initiation a flood of NMR spectra in various liquid crystalline phases was 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), based on the pioneering work of Deloche and Samulski. 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, James H. Prestegard and coworkers demonstrated that NMR spectra of certain proteins (in this case cyanometmyoglobin, which has a very highly anisotropic paramagnetic susceptibility), taken at very high field, may contain data that can usefully complement NOEs in determining a tertiary fold. In 1996 and 1997, Adriaan Bax and coworkers measured RDCs in a diamagnetic protein (ubiquitin). The results were in good agreement with the crystal structures. The secular dipolar coupling Hamiltonian of two spins, I {displaystyle I} and S , {displaystyle S,} is given by: