NMR relaxation of spin-(3)/(2) nuclei: Effects of structure, order, and dynamics in aqueous heterogeneous systems

The mathematical expressions for interpreting the results of spin-lattice r elaxation, transverse relaxation, spin-lattice relaxation in the rotating f rame, and Ostroff-Waugh multiple-pulse sequence relaxation experiments on s pin-(3)/(2) nuclei with electric quadrupole interaction in an isotropic env ironment are biexponential. A protocol for applying these expressions to in vestigate quadrupole interactions and ionic motions in complex aqueous hete rogeneous systems such as biopolymer gels and biological tissue is describe d. Experimental evidence for at least two different environments for hydrat ed sodium ions, including an isotropic liquid aqueous phase and an anisotro pic interfacial phase at the macromolecules, is reviewed. The use of the ba sic equations and experimental protocol are illustrated for systems of incr easing complexity, ranging from simple NaCl aqueous solutions, highly conce ntrated agarose gels, gelatin gels, and gellan gels to xanthan gets that co ntain ordered macromolecules. Many different experiments on various biopoly mer samples indicate a correlation time of approximately I ns for ions at t he interface that dominates in determining spin-lattice relaxation and also the long component of the transverse relaxation that arises from the centr al transitions. The transverse relaxation of the satellite transitions is d ominated by much longer correlation times that may be related to the geomet ry of the macromolecular matrix in which the hydrated aqueous sodium ions d iffuse. in the case of xanthan, the macromolecular ordering causes residual quadrupolar splitting and enables the measurement of several very long cor relation times. One of these appears to agree with the Kuhn length of xanth an that was determined by light scattering experiments on dilute solutions. (C) 2001 John Wiley & Sons, Inc.