Magnetic relaxation dispersion studies of biomolecular solutions

Although the MRD method has a long record in biomolecular systems, it has undergone a renaissance in the past few years as methodological developments have provided access to new types of information. In particular, MRD studies of quadrupolar nuclei such as 17O and 23Na have yielded valuable insights about the interactions of proteins and oligonucleotides with their solvent environment. The biomolecular MRD literature is still dominated by hydration studies, but the method has also been used to study the interaction of organic cosolvents and inorganic counterions with biomolecules. The MRD method can potentially make important contributions to the understanding of the mechanisms whereby protein conformational stability is affected by nonaqueous solvent components, such as denaturants, stabilizers, and helix promoters. Residence times of water molecules and other low molecular weight species in association with biomolecules can be determined by MRD. Such residence times are of general interest for understanding the kinetics of biomolecule-ligand interactions and, when exchange is gated by the biomolecule, can be used to characterize large-scale conformational fluctuations on nanosecond-millisecond time scales. By monitoring the integrity and specific internal hydration sites as well as the global solvent exposure, the MRD method can also shed light on the structure and dynamics of biomolecules in fluctuating nonnative states. Because it does not rely on high resolution, the MRD method is also applicable to very large biomolecules and complexes and has even been used to investigate protein crystals, gels, and biological tissues. In fact, dynamic studies of solids and liquid crystals were among the earliest applications of the MRD method. In many of its diverse applications, the MRD method provides unique information, complementing that available from high-resolution NMR.