Water and Protein Dynamics in Biological Systems Studied by Magnetic Relaxation Dispersion

Sammanfattning: The results presented in this thesis demonstrate that the magnetic relaxation dispersion (MRD) technique can provide information of relevance to protein biophysics, magnetic resonance imaging and cell biology. By immobilizing proteins with covalent cross-links, intermittent protein dynamics on the previously inaccessible ns-µs time scale could be probed with MRD via the exchange of water molecules between internal cavities and the surrounding bulk solvent phase. Persistent side-chain dynamics on the same time scale could also be probed via labile hydrogens that exchange with bulk water. A critical test of two physically distinct mechanisms that are used to interpret 1H MRD profiles from immobilized proteins was performed. A quantitative analysis of the 1H profiles from protonated and partially deuterated ubiquitin resolved the long-standing controversy over the molecular basis of water-1H relaxation in systems containing immobilized macromolecules, including biological tissue. The molecular mechanisms behind the 1H-14N magnetization transfer, the so called quadrupolar peaks, observed in immobilized systems was also investigated. The state of water in living cells is of fundamental biological importance, but previous attempts to characterize cell water have been inconclusive. MRD experiments on deuterated cell water are reported that, for the first time, quantify the slowing down of all intracellular water and rule out the idea of a highly perturbed cytoplasm. Water 2H and 17O MRD was also used to address a related problem of both fundamental and practical importance: the molecular mechanism behind the exceptional heat resistance and dormancy of bacterial spores. The dehydrated core region is thought to play a key role but little is known about the physical state of water in the different spore compartments or about the rate of water transport among them. These questions could be answered by monitoring the water dynamics in the different compartments of B. subtilis spores.