| The utility of solution state NMR for the study of large molecules such as proteins results from the averaging effects of molecular reorientation. Only isotropic magnetic interactions persist, producing the necessary high resolution and simplified spectra. However, as higher magnetic field strengths are achieved, the anisotropic nature of most magnetic interactions will become more apparent as small perturbations to normal solution state spectra. This dissertation focuses on the study of these anisotropic effects, from both a theoretical and experimental perspective.; The measurement of {dollar}sp{lcub}15{rcub}{dollar}N spin relaxation rates is a useful probe of the magnitude of the anisotropic interactions present and of the motions responsible for modulating them. At high magnetic fields, the {dollar}sp{lcub}15{rcub}{dollar}N CSA interaction becomes a significant relaxation mechanism for the backbone amide nitrogens in a protein. We will demonstrate that it is in principle possible to characterize an axially symmetric CSA tensor by making relaxation rate measurements which include quantitation of the effects arising due to dipolar-CSA cross-correlation. In addition, some methodology will be presented which can facilitate the measurement of these relaxation rates.; At high magnetic fields and for molecules with a large susceptibility anisotropy, a small degree of order can be achieved with respect to the magnetic field. This leads to the appearance of small residual dipolar couplings which arise due to incomplete averaging of the dipole-dipole interaction. We demonstrate that for the protein cyanometmyoglobin and at a field of 17.6 T, one-bond {dollar}sp{lcub}15{rcub}{dollar}N-{dollar}sp1{dollar}H dipolar couplings reach magnitudes of up to a couple of Hz. We have developed methodology which has enabled these couplings to be measured with high precision. Analysis of these couplings in terms of available solid state or solution structures is very suggestive of the presence of motions considerably more extensive in nature than can be probed via {dollar}sp{lcub}15{rcub}{dollar}N spin relaxation experiments. A dynamic model of myoglobin which includes the collective motions of rigid helices proves sufficient to explain our data within experimental precision. These studies have important implications for the study of slow, collective motions in proteins, which are likely to be important for function. |
