No abstract

The deuterated hydration shells of bovine serum (BSA) albumin, and purple membrane sheets have been studied by the aid of deuteron field-cycling relaxation spectroscopy. The deuteron Larmor frequency range was 10(3) to 10(8) Hz. The temperature and the water content has been varied. The data distinguish translational diffusion on the protein surface from macromolecular tumbling or exchange with free water. A theory well describing all dependences has been developed on this basis. All parameters have successfully been tested concerning consistency with other sources of information. The concept is considered as a major relaxation scheme determining, apart from cross-relaxation effects, the water proton relaxation in tissue.

Diverse proteins and polypeptides have been studied by proton field-cycling relaxation spectroscopy and-after deuterium exchange-by solid-state deuteron NMR. These techniques cover very different and partially complementary time scales of molecular motions spread over many orders of magnitude. On the other hand, deuteron NMR is predominantly sensitive to rotational fluctuations, while proton NMR can be affected by translational displacements in addition. The latter type of motion is considered to be connected with the multiple trapping diffusion mechanism of dilating defects in the protein structure. In particular the mean-square displacement of the diffusing defects is expected to obey a nonlinear time dependence ("anomalous diffusion"). This concept enables us to explain simultaneously (a) the small amplitudes of rotational backbone fluctuations as far as can be judged by the diverse deuteron experiments, (b) the existence of translational backbone fluctuations concluded from the proton 7) dispersion measurements, and (c) the peculiar v1 23/4 frequency dependence evident for the proton spin-lattice relaxation time in the vicinity of room temperature. In the limit far below room temperature, indications of a crossover to a fractal time process appear.

Proton and deuteron field-cycling relaxation spectroscopy has been employed for the characterization of fluctuations in proteins and in their hydration shells. The nature of the fluctuations is shown to be different. Deuteron relaxation dispersion of water in particular does not reflect the dynamics specific for protein backbones. Protein backbone fluctuations are characterized by simple power laws describing the overall frequency dependence of the spin-lattice relaxation time over several decades. The exponent changes at about 200 K from a constant value above this temperature to values decreasing with decreasing temperatures. This may be interpreted by a transition from ergodic to nonergodic behavior in the time scale of the experiments.

No abstract