An analysis of molecular dynamics in water-cellulose systems by pulsed NMR.

摘要

This thesis presents important advances in the application of Nuclear Magnetic Resonance (NMR) in three different areas: experimental apparatus, data analysis techniques in heterogeneous systems, and understanding of the determinants of the water relaxation in water-cellulose systems.;The common use of models based on a distribution of correlation times to analyze NMR relaxation time data in heterogeneous systems has been critically examined. Serious difficulties of principle in such analyses have been found.;An adaptive numerical integration strategy is necessary for accurate results. Nonlinear least squares analysis of the relaxation data requires the use of slower, but more stable techniques than those considered standard. A single fit requires a daunting amount of computing under these conditions.;These numerical techniques lead to a unique set of parameters in cases where the error on the data is very small. However, when data with reasonable experimental errors are analyzed, the fitting routines become sensitive to the starting parameters. The usefulness of a distribution of correlation times model for routine analysis of relaxation data in heterogeneous systems must be seriously questioned.;A microprocessor-based pulse programmer is described which can conveniently produce virtually any conceivable pulse sequence, is synchronized with the spectrometer rf source, and is suitable for external computer control and automated data acquisition. A special programming language was implemented for use in the pulse programmer to allow it to be controlled by simple strings of ASCII characters.;NMR relaxation in three systems is analyzed: (a) dry Sigmacell cellulose and samples hydrated with H;The first is an anisotropic motion model in which the water molecules are hydrogen bonded to the cellulose with their proton-proton vector at right angles to the hydrogen bond axis and rapidly reorient about the hydrogen bond axis (room temperature ;The second involves the majority of the water oriented as discussed above and undergoing a rapid reorientation about the hydrogen bond axis and a small fraction (about 2.4%), hydrogen bonded with their O-H bond colinear with the hydrogen bond and reorienting about the hydrogen bond with the slow correlation time parameters discussed above.;The system containing large amounts of water (less than 3% cellulose) shows two distinct water phases; one is water bound directly to the cellulose surface; the other is bulk water. Rapid exchange between phases results in the observed relaxation times being strongly influenced by the processes at the cellulose surface. To our knowledge, this is the first study of very high moisture content samples in which the dynamics at the biomolecular substrate were known in detail.;In all cases, the results were consistent with surface water relaxing as it would if there were no excess water and undergoing a rapid exchange with bulk water.