Water and protein solutions studied by field-dependent magnetic relaxation

Sammanfattning: In the work presented, nuclear magnetic resonance (NMR) relaxation is used to study wide range of systems. The thesis concerns solvent interactions studied with relaxation techniques that involve measurements at many fields, which allows the separation of individual relaxation mechanisms. The approach also makes it possible to characterize the involved dynamic properties in much greater detail. The structure of liquid water is studied by investigating a hitherto unexploited relaxation mechanism for water protons, induced by the anisotropy of the chemical shielding tensor. By comparing the experimental results to theoretical calculations on the relation between water structure and the shielding tensor, it was possible to determine the hydrogen bond geometry over nearly the full temperature range of liquid water. The magnetic relaxation dispersion (MRD) technique carries the unique potential to directly monitor the solvent interactions with macromolecules. Here, the MRD of water was used to investigate the hydration of the large cavity found in the intra-cellular lipid-binding proteins. The about 20 water molecules within the cavity were found to exchange positions on the nanosecond time-scale, while exchanging with bulk water at least an order of magnitude slower. Upon ligand binding, the proteins expand their cavity volumes. Focusing on the solvent, MRD is very useful in studies of protein stability. Thus, equilibrium urea denaturation of intestinal fatty acid-binding protein (I-FABP) was followed by the MRD of both water and the denaturing agent. At least one water molecule binds to the protein in the presence of 7.5 M urea, where I-FABP appears denatured by conventional methods. The MRD data also suggest that the denatured state is much more compact than a fully solvated polypeptide. Similarly, the beta to alpha transition of beta-lactoglobulin (BLG) induced by trifluoroethanol (TFE) was investigated by the MRD of both water and TFE. The data indicate a preferential binding of TFE to the protein surface and demonstrate that BLG binds several long-lived (5-10 ns) TFE molecules in both states. During the transition, the protein expands and the TFE induced state consists of alpha-helical segments. Our data encourage the speculation that these segments are loosely tied together by TFE molecules (via dispersion forces) and water (via hydrogen bonds). Finally, the MRD method is compared to the use of intermolecular NOEs (nuclear Overhauser effect) between water and protein protons in studies of protein hydration. To focus on the surface hydration, we obtained the water MRD in deeply supercooled solutions of the peptide oxytocin and the globular protein BPTI. A large majority of the surface waters are dynamically retarded by only a factor of 2 as compared to bulk water. The NOE method frequently yields a retardation that is at least an order of magnitude longer. This inconsistency is removed by invoking a new model for the interpretation of NOE data, which shows that the NOEs of surface protein protons to water are dominated by long-range dipolar couplings.