Abstract
The mathematical expressions for interpreting the results of spin‐lattice relaxation, transverse relaxation, spin‐lattice relaxation in the rotating frame, and Ostroff–Waugh multiple‐pulse sequence relaxation experiments on spin‐3/2 nuclei with electric quadrupole interaction in an isotropic environment are biexponential. A protocol for applying these expressions to investigate quadrupole interactions and ionic motions in complex aqueous heterogeneous systems such as biopolymer gels and biological tissue is described. Experimental evidence for at least two different environments for hydrated sodium ions, including an isotropic liquid aqueous phase and an anisotropic interfacial phase at the macromolecules, is reviewed. The use of the basic equations and experimental protocol are illustrated for systems of increasing complexity, ranging from simple NaCl aqueous solutions, highly concentrated agarose gels, gelatin gels, and gellan gels to xanthan gels that contain ordered macromolecules. Many different experiments on various biopolymer samples indicate a correlation time of approximately 1 ns for ions at the interface that dominates in determining spin‐lattice relaxation and also the long component of the transverse relaxation that arises from the central transitions. The transverse relaxation of the satellite transitions is dominated by much longer correlation times that may be related to the geometry of the macromolecular matrix in which the hydrated aqueous sodium ions diffuse. In the case of xanthan, the macromolecular ordering causes residual quadrupolar splitting and enables the measurement of several very long correlation times. One of these appears to agree with the Kuhn length of xanthan that was determined by light scattering experiments on dilute solutions. © 2001 John Wiley & Sons, Inc. Concepts Magn Reson 13: 294–325, 2001