With a view to obtaining information on the nature and extent of molecular motion of enclathrated "guests" and their interaction with the "host" lattices, nuclear magnetic resonance absorption of ten guest species included in the clathration voids of fully deuterated hydrates has been studied from a temperature of 77ºK upwards. The F¹⁹ resonance line shapes for CF₄ and SF₆ and the H¹ resonance line shape for ethylene oxide (C₂H₄0) in their respective clathrate hydrates indicate that these molecules are but little restricted by the walls of the clathrate cavities, and reorient freely about chosen axes of symmetry at low temperatures and at random at higher temperatures. Above 150ºK, a limited isotropic translation, or "rattling", of an SF₆ guest molecule up to a distance of [formula omitted] from the centre of the clathration volume has been demonstrated. Proton resonance has been studied for propane in two specimens of the clathrate hydrate, one of which was richer in guest content than the other. For these two specimens it has been suggested that, below 160ºK, propane assumes a staggered C₂ configuration inside the clathrate cavity. Molecular C₂ - axis reorientations superposed on methyl reorientations have been proposed, and a thermal activation energy barrier of 1.70 ± 0.08 kcal/ mole has been calculated for the above motion from spin-lattice relaxation time measurements in the 77ºK - 110°K range. Closer to the melting point of the two specimens, diffusion of propane through the host lattice has been indicated, and diffusional activation energies of 1.40 ± 0.02 kcal/mole and 0.75 ± 0.05 kcal/mole have been obtained for the guest - rich and guest - poor specimens, respectively. In sharp contrast to the above results, the low temperature proton resonance of three halomethanes, CH₃X(X = CI, Br, I), inside hydrate host cavities has revealed definite constraints to reorientational and translational motion, the second moment data indicating only low-amplitude oscillatory motions of the CH₃ groups in these clathrates. A triplet line shape has been observed for the CH₃Br clathrate at 77°K. At higher temperatures, expansion of the hydrate lattices has been proposed, which permits free C₃- reorientations of the CH₃ groups of the three guest molecules. From the associated linewidth transitions, activation energies of 2.48 ± 0.32, 9.30 ± 0.25, and 6.80 ± 0.50 kcal/mole have been calculated for the potential barrier hindering methyl reorientation in the CH₃Cl-, CH₃Br-, and [formula omitted] hydrates, respectively. The motional model proposed for this temperature range is adequately supported by measurements of H¹ spin - lattice relaxation times. For the dichloromethane clathrate hydrate, a clearly resolved doublet characteristic of rigid proton pairs has been obtained at 77°K. The possible existence at low temperatures of an aligned guest molecule in a suitably-sized cavity is thereby indicated. A line shape analysis of this doublet, performed on an IBM 7040 computer, yielded an accurate H-H interatomic distance of 1.73Å for the 'guest' dichloromethane molecule. This value has been discussed in the light of results from earlier microwave studies of dichloromethane. The proton resonance linewidth and second moment results between 77°K and 286°K for i-amyl groups included as guests in the clathrate hydrate of (i-C₅H₁₁)₄NF have been interpreted in terms of simple motional models of these guest moieties. The results complement the reported crystal structure of this clathrate. For the analogous hydrate containing 'guest' n-butyl groups, proton second moments in the same temperature range have supported the disordered guest structure reported from a previous x-ray diffraction study. In addition, hysteresis has been demonstrated in the second moment curve of this clathrate beyond 248°K, and this has been ascribed to a phase transition. / Science, Faculty of / Chemistry, Department of / Graduate

An N.M.R. study of the clathrate hydrates of SF₆, C₃H₆ and (CH₃)₂CO has been carried out to examine the type of motion a guest molecule undergoes in the clathrated cavity. ¹⁹F nuclear magnetic resonance spectra of Sulphur Hexafluoride Hydrate and Deuterate show isotropic rotation or reorientation about an axis at random of the SF₆molecule. ¹H magnetic resonance spectra of Cyclopropane Deuterate show highly restricted rotation up to 240°K, and then free rotation about C₃-axis at high temperatures. ¹H magnetic resonance spectra of Acetone Deuterate show that the CH₃-group is rotating even at 77°K, and self diffusion occurs at 172°K. The barrier hindering diffusion of the (CH₃)₂CO has been calculated and is 3.9 kcal/mole. / Science, Faculty of / Chemistry, Department of / Graduate

Experimental proof is presented that some of the motions responsible for transverse relaxation (T₂) in deuterium magnetic resonance (²H NMR) experiments on acyl chains of a model membrane in the liquid crystalline phase are extremely slow on the ²H NMR time scale being characterized by a correlation time T₂ > ѡq⁻¹. The experiments used to investigate these slow motions involve a form of the Carr-Purcell-Meiboom-Gill pulse sequence modified so as to be suitable for ²H NMR (q-CPMG). The most plausible mechanism responsible for T₂ relaxation is the gradual change in the average molecular orientation due to lateral diffusion of the phospholipid molecules along curved membrane surfaces. Presence of such diffusion is directly established by a selective inversion recovery experiment in which magnetization transfer across the spectrum is seen. The results of the T₂ relaxation as measured in the q-CPMG experiments are fitted to an average correlation time, T₂ ≈ 62 ms, yielding an estimate of the average effective radius of curvature of 1.2 µm for a typical model membrane system, in good agreement with other methods of measurement. The implications of this main result are examined for a number of model membranes; in particular, considerable changes are seen in the character of molecular motions in systems containing small concentrations of sterols. Similarly, changes caused by the topological differences between the lamellar L∝ and hexagonal H₁₁ phases are examined in a model membrane system which undergoes a L∝ to H₁₁ phase transition. A novel way of quantifying the differences in the orientational order parameters across the phase transition is used; the observed differences are consistent with the different symmetry properties of the two phases. Perdeuteriated polycrystalline hexamethylbenzene is used to demonstrate various methods of measuring ²H NMR relaxation. In addition, some aspects of orientation dependence of the relaxation rates are examined, and found to agree with the theory. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The theory of the quadrupole interaction in nuclear magnetic resonance spectroscopy and relaxation measurements is presented in detail, with applications to ²H-NMR studies of order and dynamics in bilayers of deuterated lipids. Investigations of lipid-protein interactions in reconstituted membrane systems and intact biological membranes are reviewed. An experimental program is described which uses a synthetic amphiphilic polypeptide, with known geometry and variable length, to isolate questions about the geometrical interpretation of orientational order in lipid-protein interactions. A report is presented of an investigation of the effects of this polypeptide, Lys₂-Gly-Leu₂₀-Lys₂-Ala-amide, on the mixed bilayer system: Dimyristoylphosphatidylcholine Di-per-deuterio-myristoylphosphatidic Acid. The addition of the peptide was found to have little effect (≤5%) on the first and second moments of the distribution of quadrupole splittings in the liquid crystalline phase. Similarly, the spin-lattice relaxation time constants were affected by ≤10% in the liquid crystalline phase. The time constant for the decay of the quadrupole echo decreased dramatically above the phase transition with the addition of peptide, a phenomenon which is explained in terms of the presence of a new slow motion in the lipid-peptide systems. A simple model of the slow motion induced by the peptide is proposed, in which the lipid molecules undergo a rapid exchange between boundary and bulk sites. An effective correlation time is determined from an average over the rotations on each of these sites. Using this model, estimates are made of the change in the second moment brought about by the onset of the rotations, and. of the number of binding sites on the peptide. These estimates are found to be in agreement with independent measurements of the change in the second moment, and the number of binding sites is within the range predicted by simple considerations of charge balance. The change in the lineshape with the variation of the spacing of the pulses in the quadrupole echo experiment was investigated, and it was determined that the transverse relaxation time constants have a slight orientation dependence. It was also determined that the addition of the peptide has no significant effect on the variation of the lineshape. Some experiments which could answer some of the questions raised by these results are suggested. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The work described in this thesis deals mainly with aspects related to two- and three-dimensional NMR imaging. A detailed discussion on frequency-selective excitation using amplitude modulated rf pulses in relation to slice selection in NMR imaging has been presented. This includes the analysis and implementation of the method as well as illustrative experimental results. Several radiofrequency probe designs suitable for high field NMR imaging have been experimentally evaluated and their modification and construction are also described. The comparative results obtained indicate the merits and demerits of different designs and provide necessary guidelines for selecting the most suitable design depending on the application. Practical aspects of two- and three-dimensional imaging have been discussed and NMR images of several intact systems have been presented. Experimental methods which enable slice selection in the presence of chemically shifted species and two-dimensional chemical shift resolved imaging have "been described and illustrated using phantoms. The use of three-dimensional chemical shift resolved imaging as a potential method to map the pH and temperature distribution within an object has also been demonstrated. A preliminary investigation of the application of ³¹P NMR spectroscopy to study the biochemical transformations of the rat kidney during periods of ischemia and reperfusion has been presented. / Science, Faculty of / Chemistry, Department of / Graduate

Nuclear magnetic resonance experiments on impurity atoms in a ferromagnetic host have shown that the measured spin-lattice relaxation time of those nuclei located in domains is strongly dependent on the degree of magnetic saturation of the host material (1, 2, 3). The relaxation time increases as the applied magnetic field is increased and reaches a constant value for a magnetically saturated specimen. Wall nuclei show a much shorter relaxation time than those in the bulk. This fact, together with the increased number of walls present in a magnetically non-saturated specimen could explain the observed field-dependent decrease of the relaxation time if an increasing fraction of wall nuclei is observed. Nuclei located in walls experience a much larger enhancement than those in domains. Therefore special techniques have to be applied to exclusively observe nuclei located in the bulk (1, 4). For this reason some uncertainty exists in the interpretation of the nuclear magnetic resonance measurements. The theory of the spin-lattice relaxation in ferromagnetic metals (5) gives an estimate for the relaxation rate observed in magnetically saturated specimens. No field dependence the relaxation time is predicted. Partly due to the uncertainty in the NMR results, this theoretical problem has received little attention so far. We therefore have employed low temperature nuclear orientation which predominantly measures bulk nuclei to investigate this problem. In most of these experiments the combined technique of nuclear orientation and nuclear magnetic resonance (NMR/ON) (6) has been applied td prepare the initial state from which the relaxation takes place. Some experiments have also been performed by an entirely non-resonant technique (7). Our experimental results on ⁶⁰Co-Fe, ⁵⁴Mn-Fe and ⁵⁴Mn-Ni clearly confirm the field dependence of the relaxation time observed in nuclear magnetic resonance experiments (8). Thus the need for a detailed theoretical study is evident. Performing an NMR/ON experiment the resonance is detected by a change in the observed y-ray intensity. Resonance lines for ⁶⁰Co-Fe, ⁵⁴Mn-Fe and ⁵⁴Mn-Ni have been recorded. We have for the first time observed that their full widths at half maximum show a strong field dependence. An explanation in terms of a local distribution in the demagnetizing field is offered. We have also measured the intensity of the resonance line as a function of the applied field. An estimate shows that this is inadequately explained in terms of the expected field dependence of the enhancement factor. The distribution of hyperfine fields has never before been studied by NMR/ON. We have employed this technique successfully to investigate an alloy of one atomic percent ⁵⁹Co-Fe which has been doped with a small amount of ⁶⁰Co. A strong, well resolved satellite line of the impurity nuclei is observed. These data are interpreted in terms of the effect of near neighbor impurity nuclei on the hyperfine field (9, 10). We have computed a theoretical curve based on parameters given in the literature (10). This provides a moderately good fit for most portions of our spectra. This pilot study demonstrates that NMR/ON is indeed a valuable tool for the investigation of hyperfine field distributions. The advantages over nuclear magnetic resonance studies are that essentially only bulk as compared to wall nuclei are studied and that the sensitivity is independent of the alloy concentration. Based partially on our own data we present a short discussion of the question whether a spin temperature is maintained by the impurity nuclei during relaxation. Finally we offer a comparison between relaxation data measured by NMR/ON and other nuclear orientation techniques (11). For ⁶⁰Co-Fe the relaxation times measured by NMR/ON are found to be almost 50% longer than those measured by techniques in which the initial condition is known. This discrepancy is generally attributed to the incomplete knowledge of the initial conditions when the NMR/ON technique is employed. We have computed theoretical relaxation curves for a number of initial conditions and find that the resulting spread in relaxation time for those curves that allow a good fit to the measured curve is larger than the difference obtained from the experiments. Thus our model indeed could explain the observed discrepancy. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

NMR studies of paramagnetic hemoproteins have improved significantly our understanding of the structure-function relationship ofhemoproteins in general. Up to date most of the studies focus on low-spin ferric systems which are characterized by relatively narrow resonance peaks and concomitant better resolution. However, characterizing in detail the NMR spectra of high-spin ferric hemoproteins is important since there are several hemoproteins, such as peroxidases, catalases, oxygenases, and some ferricytochromes that contain high-spin iron (III) in their biologically active forms. Yet assigning resonances from heme peripheral protons and/or heme pocket residues in high-spin myoglobins is a daunting undertaking. Only a sparse number of active site residues are assigned in such instances, even for metaquo-myoglobin. The protons from the heme and heme pocket residues in high-spin complexes experience extremely fast relaxation and very broad linewidths, which impede the 2D methods that detect through-space and through-bond connectivities. It is the intention of this study to develop an effective strategy to gain more resonance assignments for fast-relaxing protons in hemoproteins. We have set out to use a combined strategy, using two-dimensional exchange spectroscopy (2D-EXSY) with two dimensional nuclear Overhauser effect spectroscopy I correlation spectroscopy I total correlation spectroscopy (NOESY/COSY/TOCSY). I demonstrate here that 2D EXSY experiments can be used to obtain assignment correlations for the heme protons of methydroxy-, metthiocyano-, metaquo-, and metimidazole-myoglobin forms. All these assignments are unambiguous and straightforward. Moreover, saturation-transfer experiments allow determination of ligand binding kinetics. Thus, the exchange rates between the metaquo- and metimidazole- or methyl substituted imidazole myoglobin complexes are estimated. The differences between the exchange rates reflect the differences in the hydrophobic and steric interactions between the ligands and the protein moiety. Although I only demonstrate the feasibility of2D EXSY for the myoglobin case, this assignment strategy should to be applicable to other hemoprotein systems.

Tin protoporphyrin IX (SnPP) is currently under investigation for the treatment of hyperbilirubinemia. The study of the complex between SnPP and equine myoglobin (EqMb) by ¹H and ¹¹⁹Sn nuclear magnetic resonance spectroscopy (NMR) can be viewed as a general model for SnPP interaction with hemoproteins. The complex formed from the equilibrium mixture of SnPP and EqMb, SnPP•EqMb, was found to have essentially the same porphyrin-binding pocket as EqMbCO and SwMbCO, including the same porphyrin orientation in the major form of the two species. ¹¹⁹Sn NMR spectroscopy was used to demonstrate that the proximal His(93)F8-metal coordination is likely to be intact in SnPP•EqMb. Minor shifts in the side chain positions of some of the residues were observed, possibly reflecting the presence of water in the sixth coordination site. SnPP•EqMb appears to be stable; it persists at room temperature for weeks and exhibits very slow exchange rates (²Hfor ¹H) for a large number of amide protons in the pH range 7-9. Events during the reconstitution of apomyoglobin (apoMb) with SnPP were probed. Thus interactions between tin(IV)protoporphyrin IX (SnPP) and equine apoMb, and between tin(IV) protoporphyrin IX dimers (SnPP)₂ and apoMb were observed by ¹H NMR and optical spectroscopic techniques. The products and intermediates observed in this situation were related to the equilibrium structure of SnPP•EqMb. Reactions of apoEqMb with SnPP and (SnPP)₂ produce different intermediates, although the final product, SnPP•EqMb, is the same for each. An intermediate observed for the reaction of SnPP with apoEqMb at pH 10 is in exchange with free SnPP, with the observed rate constant Koff ~ 1 sˉ¹; meso-proton resonances were assigned for this intermediate by correlation to SnPP resonances via chemical exchange. The intermediate observed for the reaction of (SnPP)₂ with apoEqMb at neutral pH produces another species which may be the alternate porphyrin-insertion isomer arising from a 180° rotation about the α,γ-meso axis of the porphyrin. Although optical absorbance spectroscopy of the Soret region shows evidence for the reaction of SnPP and (SnPP)₂ with apoMb, only in combination with ¹H NMR are the various processes assigned. T his study of the complex SnPP•EqMb facilitated the investigation of the more complex heme binding protein, hemopexin (Hx). Proton NMR spectroscopy is reported for the first time for the hemin complex of hemopexin, a serum protein that binds heme exceptionally tightly. Hx from cow, rat, rabbit, and human was isolated, and data for the protein were reported. Heme-bound Hx has spectral characteristics for being low-spin, paramagnetic. Deuterium isotope labels reveal the positions for the heme 1-, 3-, and 8-methyls; the 5-methyl lies in the -5 to 12 ppm region. Furthermore, two-dimensional nuclear Overhauser effect spectroscopy was used to locate other heme periphery protons, including those from the 2-vinyl and the 7-propionate. Upfield resonances are identified that are very strongly relaxed, and so are assigned to protons on the axial ligands. The information reported here contributes to the understanding of Hx as an antioxidant at the cellular level.

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