Universiẗat, Diss., 2000--Kaiserslautern.

Techn. Hochsch., Diss., 2001--Aachen.

Universiẗat, Diss., 2000--Tübingen.

Techn. Hochsch., Diss., 2003--Aachen. / Parallelt.: Generalized image reconstruction for magnetic resonance imaging.

Universiẗat, Diss., 2005--Kiel.

Proefschrift Rijksuniversiteit Groningen. / Met lit. opg. - Met samenvatting in het Nederlands.

This work details the synthesis of low density porous polymers, and characterisation with an emphasis on Time Domain Nuclear Magnetic Resonance (TD-NMR). High energy laser physics utilises low density porous polymers to study astrophysical phenomena at high pressures and temperatures in the form of plasma. Low Z, low density Polymerised High Internal Phase Emulsions (PolyHIPE) and aerogels form a large part of these capabilities, however increasingly stringent laser target parameters are now required to develop new capabilities. For low density porous polymers, this demands greater control over properties such as pore size, density, composition (C[sub]nH[sub](>n)) and homogeneity through novel synthesis and characterisation. Microstructure inhomogeneity of styrene-co-divinyl benzene (S-co-DVB) polyHIPEs in conjunction with novel t-butyl styrene and para divinyl benzene polyHIPEs were investigated using ¹H spin-lattice (T₁) and spin-lock (T[sub](1ρ)) NMR relaxation experiments using TD-NMR. The strong relationship between ¹H spin-lattice relaxation times and Dynmaic Mechanical Analysis (DMA) data, and application of relaxation experiments at varied temperatures reveal that structural inhomogeneity is based on poor emulsion stability and clustering of DVB polymer affecting bulk molecular motion. Divinyl benzene (DVB) aerogels and a range of innovative C[sub]nH[sub](>n) aerogels such as poly-5-vinyl-2-norbornene were synthesised using free-radical, cationic or ring opening metathesis polymerisation techniques. A one-step synthesis of homogeneous density gradient DVB aerogels was developed for the first time, which is fundamental to be able to study plasma shock fronts. Characterisation using X-ray tomography revealed the homogeneous density gradient. Successful carbonisation of dichloroparaxylene (DCPX) aerogels has similar properties to resorcinol-formaldehyde (RF) aerogels, but is produced in significantly less time and shrinkage, presents as a candidate for future laser experiments. Correlation between NMR relaxation times to established techniques of DMA and mercury porosimetry was explored to determine the suitability of TD-NMR in characterisation of low density porous polymers.

Abstract Nuclear magnetic resonance spectroscopic structure determination of proteins has been under rapid development during the last decade. The size limitation impeding structural studies of biological macromolecules in solution has increased from 10 kDa to 30 kDa thanks to exploitation of 15N/13C enrichment. Perdeuteration of non-exchangeable protons has pushed this limit even further, allowing backbone resonance assignment of 40 to 50 kDa proteins. Most recently, transverse relaxation optimized spectroscopy (TROSY) has been demonstrated to lengthen 15N and 1HN spin transverse relaxation times significantly, especially in large perdeuterated proteins, thus extending the size limit beyond 100 kDa systems. However, determination of structurally important nuclear Overhauser enhancements (NOE) suffers from perdeuteration, due to the lower density of proton spins available, eventually leading to imprecise protein structures. Very recently, residual dipolar couplings have been used to supplement NOE information, enabling accurate molecular structures to also be obtained with perdeuterated proteins. This thesis focuses on the measurement of the structurally important 3J-coupling between 1HN and 1Hα spins, and determination of residual dipolar couplings by utilizing the novel spin-state-selective subspectral editing together with the TROSY methodology. This approach allows precise measurement of a large number of dipolar couplings in larger protonated or perdeuterated proteins.

Abstract In this work, the homogeneity of both the B0 and B1 fields was studied. Both B0 and B1 field homogeneities are the basic assumptions of high resolution liquid state NMR. Although some inhomogeneity of both of the fields is always present, the spectrometers can be operated, with the help of the developed spectral purging techniques, without giving any thought to the field inhomogeneities or the necessary actions to minimize their adverse effects. Although the effect of B0 inhomogeneity can occasionally be seen, the B1 fieldin a modern probe head is often assumed to be sufficiently homogenous for any practical purpose. By using the method used in this study the B1 field strength along one axis, typically the z-axis, can be easily mapped. Based on the information gathered from a single experiment, one can obtain reliable and valuable information about the B1 field distribution, e.g. homogeneity of the coil. From such information, the degree of required artifact suppressing methods for successful NMR experiments can be determined. Since normal pulse length calibration also requires the acquisition of several 1-D spectra, the required experimentation time is not increased. Although the maximum amount of signal from an NMR experiment is obtained when the signal is acquired from a maximum number of resonating spins, the results presented show that significantly more homogenous B1 field along the active sample volume is achieved by rejection of the signal originating from the outer parts of the coil length. Although the total amount of signal obtained from the outer parts of the RF-coil is not very high, some loss of signal is associated with the spatially selective acquisition. The rejected signal, however, is a significant source of artifacts, and if no precautions were taken, the artifacts would severely decrease the quality of the acquired data. If the sample concentration can be increased, it would be advantageous to dissolve the amount of sample available in as small an amount of solvent as is possible and place the sample in the most B1 homogenous part of the probe-head RF-coil. With the same amount of nuclear spins concentrated into a smaller volume, the sensitivity of an NMR experiment can be increased manifold. As an application of a spatially selective data acquisition, a versatile method capable of producing a map of the B0 field strength and its variation along the sample volume is presented.

La capacité de mesurer les rapports isotopiques par spectroscopie RMN du 13C (irm-13C NMR) donne un accès direct à la distribution d’isotopes possédant une position spécifique au sein de molécules. Dans cette thèse, cette approche a été développée dans le but d’élucider le fractionnement isotopique au cours de la biosynthèse dans des plantes de différents alcaloïdes (nicotine, tropine, tramadol) qui ont certaines caractéristiques communes lors de leurs biosynthèses. L’une des caractéristiques clés de ces composés est la présence de groupes O-méthyle et/ou N-méthyle. En général, le rapport 13C/12C dans les groupes O-méthyle et N-méthyle de produits naturels est exceptionnellement faible comparé aux autres carbones dans la molécule, ces travaux ont été axés sur l’explication de ce phénomène. La grande majorité de ces groupes méthyles dans les produits naturels proviennent du transfert d’un groupe Sméthyle de L-méthionine (L-Met) via la Sadénosylméthionine (AdoMet). Il a été prouvé par irm-13C NMR que, dans la molécule donneuse, L-Met, le groupe Sméthyle est appauvrit. La cause de ce phénomène a été explorée par le biais de l’étude de petit modèle théorique pour la cobalamin-independent méthionine synthase, l’enzyme responsable du transfert du groupement méthyle durant la biosynthèse de L-met. Ces calculs ont montré une barrière d’énergie élevée pour le transfert du groupe méthyle et un fort effet isotopique cinétique du 13C y associé. De plus, une méthodologie générique de l’étude du ratio 13C/12C dans les aminoacides a été développée, ce qui permet une meilleure compréhension du fractionnement isotopique intervenant durant la biosynthèse d’acides aminés. Une caractéristique supplémentaire de ces travaux est que les données permettent : (i) une comparaison entre les produits naturels et commerciaux, permettant de distinguer ces deux sources, et (ii) une interprétation du modèle isotopique en terme d’origine biosynthétique de composés naturels. Pour le tramadol, il a été possible de proposer un chemin hypothétique pour ce produit naturel récemment découvert. / The ability to carry out isotope ratio monitoring by 13C NMR spectrometry (irm-13C NMR) gives direct access to position-specific isotope distributions in whole molecules. In this thesis this approach has been developed with the aim of elucidating isotopic fractionation during the biosynthetic pathways in plants of a number of alkaloids (nicotine, tropine and tramadol) that have certain features of their biosynthesis in common. One key common feature of these compounds is the presence of O-methyl and/or N–methyl groups. As it is generally found that the 13C/12C ratio in the O-methyl and N-methyl groups of natural products is exceptionally low relative to the other carbon positions in the molecule, the work focused on explaining this phenomenon. The vast majority of these methyl groups in natural products are derived by the transfer of the S-methyl group from L-methionine (L-Met) via S-adenosyl methionine (AdoMet). It is shown by irm-13C NMR that in the donor molecule, L-met, the S-methyl group is impoverished. The cause of this was investigated by the study of a small theoretical model for the cobalaminindependent methionine synthase, the enzyme responsible for methyl group transfer in L-met biosynthesis. These calculations showed a high energy barrier for methyl group transfer and an associated large 13C kinetic isotope effect. In addition, a generic methodology to study the 13C/12C ratios in amino acids has been developed, which allows insight into the isotopic fractionation occurring during amino acid biosynthesis. A further feature of the work is that the data allow: (i) a comparison of natural and commercial products, which enables distinguishing between sources, and (ii) an interpretation of the isotopic pattern in terms of the biosynthetic origin of natural compounds. For tramadol, this made it possible to propose a hypothetical pathway for this newly-discovered natural product.