Cuticular and Suberin Polymers of Edible Plants. Analysis by Gas Chromatographic-Mass Spectrometric and Solid State Spectroscopic Methods

Cutin and suberin are structural and protective polymers of plant surfaces. The epidermal cells of the aerial parts of plants are covered with an extracellular cuticular layer, which consists of polyester cutin, highly resistant cutan, cuticular waxes and polysaccharides which link the layer to the epidermal cells. A similar protective layer is formed by a polyaromatic-polyaliphatic biopolymer suberin, which is present particularly in the cell walls of the phellem layer of periderm of the underground parts of plants (e.g. roots and tubers) and the bark of trees. In addition, suberization is also a major factor in wound healing and wound periderm formation regardless of the plants’ tissue. Knowledge of the composition and functions of cuticular and suberin polymers is important for understanding the physiological properties for the plants and for nutritional quality when these plants are consumed as foods. The aims of the practical work were to assess the chemical composition of cuticular polymers of several northern berries and seeds and suberin of two varieties of potatoes. Cutin and suberin were studied as isolated polymers and further after depolymerization as soluble monomers and solid residues. Chemical and enzymatic depolymerization techniques were compared and a new chemical depolymerization method was developed. Gas chromatographic analysis with mass spectrometric detection (GC-MS) was used to assess the monomer compositions. Polymer investigations were conducted with solid state carbon-13 cross polarization magic angle spinning nuclear magnetic resonance spectroscopy (13C CP-MAS NMR), Fourier transform infrared spectroscopy (FTIR) and microscopic analysis. Furthermore, the development of suberin over one year of post-harvest storage was investigated and the cuticular layers from berries grown in the North and South of Finland were compared. The results show that the amounts of isolated cuticular layers and cutin monomers, as well as monomeric compositions vary greatly between the berries. The monomer composition of seeds was found to differ from the corresponding berry peel monomers. The berry cutin monomers were composed mostly of long-chain aliphatic ω-hydroxy acids, with various mid-chain functionalities (double-bonds, epoxy, hydroxy and keto groups). Substituted α,ω-diacids predominated over ω-hydroxy acids in potato suberin monomers and slight differences were found between the varieties. The newly-developed closed tube chemical method was found to be suitable for cutin and suberin analysis and preferred over the solvent-consuming and laborious reflux method. Enzymatic hydrolysis with cutinase was less effective than chemical methanolysis and showed specificity towards α,ω-diacid bonds. According to ₁₃C CP-MAS NMR and FTIR, the depolymerization residues contained significant amounts of aromatic structures, polysaccharides and possible cutan-type aliphatic moieties. Cultivation location seems to have effect on cuticular composition. The materials studied contained significant amounts of different types of biopolymers that could be utilized for several purposes with or without further processing. The importance of the so-called waste material from industrial processes of berries and potatoes as a source of either dietary fiber or specialty chemicals should be further investigated in detail. The evident impact of cuticular and suberin polymers, among other fiber components, on human health should be investigated in clinical trials. These by-product materials may be used as value-added fiber fractions in the food industry and as raw materials for specialty chemicals such as lubricants and emulsifiers, or as building blocks for novel polymers.

Phenolics of the Inner Bark of Silver Birch: Characterization and Intraspecific Variation

This thesis describes work related to the in-depth characterization of the phenolic compounds of silver birch (Betula pendula) inner bark. Phenolic compounds are the most ubiquitous class of plant secondary compounds. The unifying feature of this structurally diverse group is an aromatic ring containing at least one hydroxyl group. Due to the structural diversity, phenolics have various roles in the plant defense against biotic and abiotic stresses. In addition, they can confer several health-promoting properties to humans. Furthermore, the structural diversity of this class of compounds causes challenges for their analysis. The study species in the present work, silver birch, is economically the most important hard wood species in northern Europe. Its inner bark contains a high level of phenolic compounds and it has shown one of the strongest antioxidant activities among 92 Finnish plant materials. The literature review surveys the diversity and organ specific distribution of phenolic compounds in silver birch as well as the proposed ecological functions of phenolic compounds in nature. In addition, the basis for the characterization of phenolics by mass spectrometry (MS), nuclear magnetic resonance spectroscopy (NMR), and circular dichroism spectroscopy (CD) are reviewed. The objective of the experimental work was to extract, purify, characterize, and quantify the inner bark phenolic compounds. Overall 36 compounds were characterized by MS and ultraviolet spectroscopy (UV). 24 compounds were isolated and their structures confirmed by NMR and CD spectroscopy. Five novel natural compounds were identified. Special emphasis was placed on the establishment of a method for the characterization of proanthocyanidins (PAs). Hydrophilic interaction liquid chromatography (HILIC) was utilized because of its high resolution power and predictable elution order of oligomeric and polymeric PAs according to an increasing degree of polymerization. The combination of HILIC and high-resolution MS detection allowed the identification of procyanidin (PC) polymers up to the degree of polymerization of 22. In addition, a series of oligomeric and polymeric PC monoxylosides were observed for the first time in nature. Season and genotype influenced the quantities of the main inner bark phenolics, yet qualitative differences were not observed. However, manual wounding of the inner bark induced the production of ellagitannins (ETs) in the wounded tissues, i.e. callus. Since ETs were not detected in the intact inner bark, this finding may reflect the capacity of silver birch to exploit ellagitannins in its defense.

Chemical derivatization of galactoglucomannan for functional materials

In the framework of the biorefinery concept researchers aspire to optimize the utilization of plant materials, such as agricultural wastes and wood. For most of the known processes, the first steps in the valorisation of biomass are the extraction and purification of the individual components. The obtained raw products by means of a controlled separation can consecutively be modified to result in biofuels or biogas for energy production, but also in value-added products such as additives and important building blocks for the chemical and material industries. Considerable efforts are undertaken in order to substitute the use of oil-based starting materials or at least minimize their processing for the production of everyday goods. Wood is one of the raw materials, which have gained large attention in the last decades and its composition has been studied in detail. Nowadays, the extraction of water-soluble hemicelluloses from wood is well known and so for example xylan can be obtained from hardwoods and O-acetyl galactoglucomannans (GGMs) from softwoods. The aim of this work was to develop water-soluble amphiphilic materials of GGM and to assess their potential use as additives. Furthermore, GGM was also applied as a crosslinker in the synthesis of functional hydrogels for the removal of toxic metals and metalloid ions from aqueous solutions. The distinguished products were obtained by several chemical approaches and analysed by nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FTIR), size exclusion chromatography (SEC), thermal gravimetric analysis (TGA), scanning electron microscope SEM, among others. Bio-based surfactants were produced by applying GGM and different fatty acids as starting materials. On one hand, GGM-grafted-fatty acids were prepared by esterification and on the other hand, well-defined GGM-block-fatty acid derivatives were obtained by linking amino-functional fatty acids to the reducing end of GGM. The reaction conditions for the syntheses were optimized and the resultant amphiphilic GGM derivatives were evaluated concerning their ability to reduce the surface tension of water as surfactants. Furthermore, the block-structured derivatives were tested in respect to their applicability as additives for the surface modification of cellulosic materials. Besides the GGM surfactants with a bio-based hydrophilic and a bio-based hydrophobic part, also GGM block-structured derivatives with a synthetic hydrophobic tail, consisting of a polydimethylsiloxane chain, were prepared and assessed for the hydrophobization of surface of nanofibrillated cellulose films. In order to generate GGM block-structured derivatives containing a synthetic tail with distinguished physical and chemical properties, as well as a tailored chain length, a controlled polymerization method was used. Therefore, firstly an initiator group was introduced at the reducing end of the GGM and consecutively single electron transfer-living radical polymerization (SET-LRP) was performed by applying three different monomers in individual reactions. For the accomplishment of the synthesis and the analysis of the products, challenges related to the solubility of the reactants had to be overcome. Overall, a synthesis route for the production of GGM block-copolymers bearing different synthetic polymer chains was developed and several derivatives were obtained. Moreover, GGM with different molar masses were, after modification, used as a crosslinker in the synthesis of functional hydrogels. Hereby, a cationic monomer was used during the free radical polymerization and the resultant hydrogels were successfully tested for the removal of chromium and arsenic ions from aqueous solutions. The hydrogel synthesis was tailored and materials with distinguished physical properties, such as the swelling rate, were obtained after purification. The results generated in this work underline the potential of bio-based products and the urge to continue carrying out research in order to be able to use more green chemicals for the manufacturing of biorenewable and biodegradable daily products.

Flavonoids and other phenolic compounds: characterization and interactions with lepidopteran and sawfly larvae

This thesis focuses on flavonoids, a subgroup of phenolic compounds produced by plants, and how they affect the herbivorous larvae of lepidopterans and sawflies. The first part of the literature review examines different techniques to analyze the chemical structures of flavonoids and their concentrations in biological samples. These techniques include, for example, ultraviolet-visible spectroscopy, mass spectrometry, and nuclear magnetic resonance spectroscopy. The second part of the literature review studies how phenolic compounds function in the metabolism of larvae. The harmful oxidation reactions of phenolic compounds in insect guts are also emphasized. In addition to the negative effects, many insect species have evolved the use of phenolic compounds for their own benefit. In the experimental part of the thesis, high concentrations of complex flavonoid oligoglycosides were found in the hemolymph (the circulatory fluid of insects) of birch and pine sawflies. The larvae produced these compounds from simple flavonoid precursors present in the birch leaves and pine needles. Flavonoid glycosides were also found in the cocoon walls of sawflies, which suggested that flavonoids were used in the construction of cocoons. The second part of the experimental work studied the modifications of phenolic compounds in conditions that mimicked the alkaline guts of lepidopteran larvae. It was found that the 24 plant species studied and their individual phenolic compounds had variable capacities to function as oxidative defenses in alkaline conditions. The excrements of lepidopteran and sawfly species were studied to see how different types of phenolics were processed by the larvae. These results suggested that phenolic compounds were oxidized, hydrolyzed, or modified in other ways during their passage through the digestive tract of the larvae.

Partial characterization and anticoagulant activity of a heterofucan from the brown seaweed Padina gymnospora

The brown algae Padina gymnospora contain different fucans. Powdered algae were submitted to proteolysis with the proteolytic enzyme maxataze. The first extract of the algae was constituted of polysaccharides contaminated with lipids, phenols, etc. Fractionation of the fucans with increasing concentrations of acetone produced fractions with different proportions of fucose, xylose, uronic acid, galactose, and sulfate. One of the fractions, precipitated with 50% acetone (v/v), contained an 18-kDa heterofucan (PF1), which was further purified by gel-permeation chromatography on Sephadex G-75 using 0.2 M acetic acid as eluent and characterized by agarose gel electrophoresis in 0.05 M 1,3 diaminopropane/acetate buffer at pH 9.0, methylation and nuclear magnetic resonance spectroscopy. Structural analysis indicates that this fucan has a central core consisting mainly of 3-ß-D-glucuronic acid 1-> or 4-ß-D-glucuronic acid 1 ->, substituted at C-2 with alpha-L-fucose or ß-D-xylose. Sulfate groups were only detected at C-3 of 4-alpha-L-fucose 1-> units. The anticoagulant activity of the PF1 (only 2.5-fold lesser than low molecular weight heparin) estimated by activated partial thromboplastin time was completely abolished upon desulfation by solvolysis in dimethyl sulfoxide, indicating that 3-O-sulfation at C-3 of 4-alpha-L-fucose 1-> units is responsible for the anticoagulant activity of the polymer.

Classification of brain tumor extracts by high resolution ¹H MRS using partial least squares discriminant analysis

High resolution proton nuclear magnetic resonance spectroscopy (¹H MRS) can be used to detect biochemical changes in vitro caused by distinct pathologies. It can reveal distinct metabolic profiles of brain tumors although the accurate analysis and classification of different spectra remains a challenge. In this study, the pattern recognition method partial least squares discriminant analysis (PLS-DA) was used to classify 11.7 T ¹H MRS spectra of brain tissue extracts from patients with brain tumors into four classes (high-grade neuroglial, low-grade neuroglial, non-neuroglial, and metastasis) and a group of control brain tissue. PLS-DA revealed 9 metabolites as the most important in group differentiation: γ-aminobutyric acid, acetoacetate, alanine, creatine, glutamate/glutamine, glycine, myo-inositol, N-acetylaspartate, and choline compounds. Leave-one-out cross-validation showed that PLS-DA was efficient in group characterization. The metabolic patterns detected can be explained on the basis of previous multimodal studies of tumor metabolism and are consistent with neoplastic cell abnormalities possibly related to high turnover, resistance to apoptosis, osmotic stress and tumor tendency to use alternative energetic pathways such as glycolysis and ketogenesis.

Manual and semi-automatic quantification of in vivo ¹H-MRS data for the classification of human primary brain tumors

In vivo proton magnetic resonance spectroscopy (¹H-MRS) is a technique capable of assessing biochemical content and pathways in normal and pathological tissue. In the brain, ¹H-MRS complements the information given by magnetic resonance images. The main goal of the present study was to assess the accuracy of ¹H-MRS for the classification of brain tumors in a pilot study comparing results obtained by manual and semi-automatic quantification of metabolites. In vivo single-voxel ¹H-MRS was performed in 24 control subjects and 26 patients with brain neoplasms that included meningiomas, high-grade neuroglial tumors and pilocytic astrocytomas. Seven metabolite groups (lactate, lipids, N-acetyl-aspartate, glutamate and glutamine group, total creatine, total choline, myo-inositol) were evaluated in all spectra by two methods: a manual one consisting of integration of manually defined peak areas, and the advanced method for accurate, robust and efficient spectral fitting (AMARES), a semi-automatic quantification method implemented in the jMRUI software. Statistical methods included discriminant analysis and the leave-one-out cross-validation method. Both manual and semi-automatic analyses detected differences in metabolite content between tumor groups and controls (P < 0.005). The classification accuracy obtained with the manual method was 75% for high-grade neuroglial tumors, 55% for meningiomas and 56% for pilocytic astrocytomas, while for the semi-automatic method it was 78, 70, and 98%, respectively. Both methods classified all control subjects correctly. The study demonstrated that ¹H-MRS accurately differentiated normal from tumoral brain tissue and confirmed the superiority of the semi-automatic quantification method.

NAA and NAAG variation in neuronal activation during visual stimulation

N-acetyl-aspartyl-glutamate (NAAG) and its hydrolysis product N-acetyl-L-aspartate (NAA) are among the most important brain metabolites. NAA is a marker of neuron integrity and viability, while NAAG modulates glutamate release and may have a role in neuroprotection and synaptic plasticity. Investigating on a quantitative basis the role of these metabolites in brain metabolism in vivo by magnetic resonance spectroscopy (MRS) is a major challenge since the main signals of NAA and NAAG largely overlap. This is a preliminary study in which we evaluated NAA and NAAG changes during a visual stimulation experiment using functional MRS. The paradigm used consisted of a rest period (5 min and 20 s), followed by a stimulation period (10 min and 40 s) and another rest period (10 min and 40 s). MRS from 17 healthy subjects were acquired at 3T with TR/TE = 2000/288 ms. Spectra were averaged over subjects and quantified with LCModel. The main outcomes were that NAA concentration decreased by about 20% with the stimulus, while the concentration of NAAG concomitantly increased by about 200%. Such variations fall into models for the energy metabolism underlying neuronal activation that point to NAAG as being responsible for the hyperemic vascular response that causes the BOLD signal. They also agree with the fact that NAAG and NAA are present in the brain at a ratio of about 1:10, and with the fact that the only known metabolic pathway for NAAG synthesis is from NAA and glutamate.

Modification of commercial poly-lactide for extrusion coated food packaging applications

Poly-L-lactide (PLLA) is a widely used sustainable and biodegradable alternative to replace synthetic non-degradable plastic materials in the packaging industry. Conversely, its processing properties are not always optimal, e.g. insufficient melt strength at higher temperatures (necessary in extrusion coating processes). This thesis reports on research to improve properties of commercial PLLA grade (3051D from NatureWorks), to satisfy and extend end-use applications, such as food packaging by blending with modified PLLA. Adjustment of the processability by chain branching of commercial poly-L-lactide initiated by peroxide was evaluated. Several well-defined branched structures with four arms (sPLLA) were synthesized using pentaerythritol as a tetra-functional initiator. Finally, several block copolymers consisting of polyethylene glycol and PLLA (i.e. PEGLA) were produced to obtain a well extruded material with improved heat sealing properties. Reactive extrusion of poly-L-lactide was carried out in the presence of 0.1, 0.3 and 0.5 wt% of various peroxides [tert-butyl-peroxybenzoate (TBPB), 2,5-dimethyl-2,5-(tert-butylperoxy)-hexane (Lupersol 101; LOL1) and benzoyl peroxide (BPO)] at 190C. The peroxide-treated PLLAs showed increased complex viscosity and storage modulus at lower frequencies, indicating the formation of branched/cross linked architectures. The material property changes were dependent on the peroxide, and the used peroxide concentration. Gel fraction analysis showed that the peroxides, afforded different gel contents, and especially 0.5 wt% peroxide, produced both an extremely high molar mass, and a cross linked structure, not perhaps well suited for e.g. further use in a blending step. The thermal behavior was somewhat unexpected as the materials prepared with 0.5 wt% peroxide showed the highest ability for crystallization and cold crystallization, despite substantial cross linking. The peroxide-modified PLLA, i.e. PLLA melt extruded with 0.3 wt% of TBPB and LOL1 and 0.5 wt% BPO was added to linear PLLA in ratios of 5, 15 and 30 wt%. All blends showed increased zero shear viscosity, elastic nature (storage modulus) and shear sensitivity. All blends remained amorphous, though the ability of annealing was improved slightly. Extrusion coating on paperboard was conducted with PLLA, and peroxide-modified PLLA blends (90:10). All blends were processable, but only PLLA with 0.3 wt% of LOL1 afforded a smooth high quality surface with improved line speed. Adhesion levels between fiber and plastic, as well as heat seal performance were marginally reduced compared with pure 3051D. The water vapor transmission measurements (WVTR) of the blends containing LOL1 showed acceptable levels, only slightly lower than for comparable PLLA 3051D. A series of four-arm star-shaped poly-L-lactide (sPLLA) with different branch length was synthesized by ring opening polymerization (ROP) of L-lactide using pentaerythritol as initiator and stannous octoate as catalyst. The star-shaped polymers were further blended with its linear resin and studied for their melt flow and thermal properties. Blends containing 30 wt% of sPLLA with low molecular weight (30 wt%; Mwtotal: 2500 g mol-1 and 15000 g mol-1) showed lower zero shear viscosity and significantly increased shear thinning, while at the same time slightly increased crystallization of the blend. However, the amount of crystallization increased significantly with the higher molecular weight sPLLA, therefore the star-shaped structure may play a role as nucleating agent. PLLA-polyethylene glycol–PLLA triblock copolymers (PEGLA) with different PLLA block length were synthesized and their applicability as blends with linear PLLA (3051D NatureWorks) was investigated with the intention of improving heat-seal and adhesion properties of extrusion-coated paperboard. PLLA-PEG-PLLA was obtained by ring opening polymerization (ROP) of L-lactide using PEG (molecular weight 6000 g mol-1) as an initiator, and stannous octoate as catalyst. The structures of the PEGLAs were characterized by proton nuclear magnetic resonance spectroscopy (1H-NMR). The melt flow and thermal properties of all PEGLAs and their blends were evaluated using dynamic rheology, and differential scanning calorimeter (DSC). All blends containing 30 wt% of PEGLAs showed slightly higher zero shear viscosity, higher shear thinning and increased melt elasticity (based on tan delta). Nevertheless, no significant changes in thermal properties were distinguished. High molecular weight PEGLAs were used in extrusion coating line with 3051D without problems.

Optimization of the extraction of carrageenan from Kappaphycus alvarezii using response surface methodology

This study aims to optimize an alternative method of extraction of carrageenan without previous alkaline treatment and ethanol precipitation using Response Surface Methodology (RSM). In order to introduce an innovation in the isolation step, atomization drying was used reducing the time for obtaining dry carrageenan powder. The effects of extraction time and temperature on yield, gel strength, and viscosity were evaluated. Furthermore, the extracted material was submitted to structural analysis, by infrared spectroscopy and nuclear magnetic resonance spectroscopy (¹H-NMR), and chemical composition analysis. Results showed that the generated regression models adequately explained the data variation. Carrageenan yield and gel viscosity were influenced only by the extraction temperature. However, gel strength was influenced by both, extraction time and extraction temperature. Optimal extraction conditions were 74 ºC and 4 hours. In these conditions, the carrageenan extract properties determined by the polynomial model were 31.17%, 158.27 g.cm-2, and 29.5 cP for yield, gel strength, and viscosity, respectively, while under the experimental conditions they were 35.8 ± 4.68%, 112.50 ± 4.96 g.cm-2, and 16.01 ± 1.03 cP, respectively. The chemical composition, nuclear magnetic resonance spectroscopy, and infrared spectroscopy analyses showed that the crude carrageenan extracted is composed mainly of κ-carrageenan.

Extracting Ramachandran torsional angle distributions from 2D NMR data using Tikhonov regularization /

Solid state nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for studying structural and dynamical properties of disordered and partially ordered materials, such as glasses, polymers, liquid crystals, and biological materials. In particular, twodimensional( 2D) NMR methods such as ^^C-^^C correlation spectroscopy under the magicangle- spinning (MAS) conditions have been used to measure structural constraints on the secondary structure of proteins and polypeptides. Amyloid fibrils implicated in a broad class of diseases such as Alzheimer's are known to contain a particular repeating structural motif, called a /5-sheet. However, the details of such structures are poorly understood, primarily because the structural constraints extracted from the 2D NMR data in the form of the so-called Ramachandran (backbone torsion) angle distributions, g{^,'4)), are strongly model-dependent. Inverse theory methods are used to extract Ramachandran angle distributions from a set of 2D MAS and constant-time double-quantum-filtered dipolar recoupling (CTDQFD) data. This is a vastly underdetermined problem, and the stability of the inverse mapping is problematic. Tikhonov regularization is a well-known method of improving the stability of the inverse; in this work it is extended to use a new regularization functional based on the Laplacian rather than on the norm of the function itself. In this way, one makes use of the inherently two-dimensional nature of the underlying Ramachandran maps. In addition, a modification of the existing numerical procedure is performed, as appropriate for an underdetermined inverse problem. Stability of the algorithm with respect to the signal-to-noise (S/N) ratio is examined using a simulated data set. The results show excellent convergence to the true angle distribution function g{(j),ii) for the S/N ratio above 100.

Chelated fluoroboron cations : synthesis and multinuclear nmr studies

The preparation of chelated difluoroboron cations (DD)BF2+, where DD is a saturated polydentate tertiary-amine or polydentate aromatic ligand, has been systematically studied by using multinuclear solution and solid state nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. Three new methods of synthesis of (DD)BF2+ cations are reported, and compared with the previous method of reacting a chelating donor with Et20.BF3. The methods most effective for aromatic donors such as 1,1O-phenanthroline are ineffective for saturated polydentate tertiary-amines like N,N,N' ,Nil ,Nil-pentamethyldiethylenetriamine. Polydentate tertiary-amine donors that form 5-membered rings upon bidentate chelation were found to chelate effectively when the BF2 source contained two leaving groups (a heavy halide and a Lewis base such as pyridine =pyr or isoxazole =ISOX), i.e., pyr.BF2X (X = CI or Br), ISOX.BF2X and (pyr)2BF2+. Those that would form 6membered rings upon chelation do not chelate by any of the four methods. Polydentate aromatic ligands chelate effectively when the BF2 source contained a weak Lewis base, e.g., ISOX.BF3, ISOX.BF2X and Et20.BF3. Bidentate chelation by polydentate tertiaryamine and aromatic donors leads to nmr parameters that are significantly different then their (D)2BF2+ relatives (D =monod~ntate t-amines or pyridines). The chelated haloboron cations (DD)BFCI+, and (DD)BFBr+ were generated from D.BFX2 adducts for all ligands that form BF2+ cations above. In addition, the (DD)BCI2+ and (DD)BBr2+ cations were formed from D.BX3 adducts by the chelating aromatic ligands, except for the aromatic ligand 1,8-bis(dimethylamino)naphthalene, which formed only the (DD)BF2+ cation, apparently due to its extreme steric hindrance. Chelation by a donor is a two-step reaction. For polydentate tertiary-amine ligands, the two rates appear to be very dependent on the two possible leaving groups on the central boron atom. The order of increasing ease of displacement for the donors was: pyr < Cl < Br < ISOX. The rate of chelation by polydentate aromatic ligands appears to be dependent on the displacement of the first ligand from the boron. The order of increasing ease of displacement for the donors was: pyr < CI < ISOX ~ Br < Et20.

NMR studies of mixed tetrahaloborates and some related boron trihalide complexes /|nGary John Schrobilgen. -- 260 St. Catharines, Ont. : [s. n.],

Nuclear magnetic resonance spectroscopy has been used to study donor-acceptor complexes of boron trifluoride with several ureas, tetramethylthiourea, tetramethylselenourea, and tetramethylquanidine as well as adducts of tetramethyl- -urea with BF2Cl, BFC1 2 , and BC1 3 - A large number of mixed tetrahaloborate ions, including some of the ternary ones such as BF2CIBr-,have been obtained by ligand exchange reactions and studied by NMR techniques. The bonding in these ions is of the same inherent interest as the bonding in the isoelectronic tetrahalomethanes which have been the subject of many detailed studies and have been involved in a controversy concerning the existence of and the nature of "fluorine hyperconjugation" or C-F P1T- Pn bonding_ Ligand exchange reactions also gave rise to the difluoroboron cation, (TMU)20BF2+o The difluoroboron cation has been observed in solutions of TMU-BF3 , and has been proposed as a possible intermediate for fluorine exchange reactions in BF3 adducts.

NMR studies of strong hydrogen bond to fluoride ion

Systems such as MF/diol (M = alkali metal) and }1F/carboxylic acid were subjected to IH, I9F and 13C nmr study to investigate the nature of the very strong H-bonding of fluoride ions with these systems. Evidence indicates a strong H-bond in diol-fluoride systems (~H ~ -(56) kJ mol-I) which is stronger than most 'typical' H-bonds (~H = -(12-40) kJ mol-I), but weaker than that reported for carboxylic acid-fluoride systems (~H ~ -(120) kJ mol-I). Approximate fluoride H-bonded shifts (o(OH)OHF) were evaluated for MF/diol systems from IH chemical shift measurements. No direct correlation was observed between I9F chemical shift and H-bond strength. Thermodynamic parameters were calculated from temperature dependent IH and 19F shifts. Preliminary studies of BUn 4NF-acetylacetone by I9F nmr were conducted at low temperatures and a possible Jmax (ca. 400 Hz) is reported for the fluoride ion H-bonded to acetylacetone. Highfield shift for non-protonated carbons and downfield shift for protonated carbons were observed in carboxylic acid/KF systems. Significant decreas$in I3C TI due to strong H-bonding to fluoride ions were also detected in both diol and carboxylic acid systems. Anomalous results were obtained, such as increasing NOE with increasing temperature in neat 1,2-ethanediol (values above the theoretical maximum of 1.988) and in 1,2-ethanediol/KF. The large 13C NOE's for carboxy carbons in neat carboxylic acids which are. further enhanced by the addition of KF are also unusual.

Simultaneous extraction of order parameters and orientational distribution fuctions from 31[superscript]P NMR spectra of magnetically partially oriented phospholipid bilayers

Order parameter profiles extracted from the NMR spectra of model membranes are a valuable source of information about their structure and molecular motions. To al1alyze powder spectra the de-Pake-ing (numerical deconvolution) ~echnique can be used, but it assumes a random (spherical) dist.ribution of orientations in the sample. Multilamellar vesicles are known to deform and orient in the strong magnetic fields of NMR magnets, producing non-spherical orientation distributions. A recently developed technique for simultaneously extracting the anisotropies of the system as well as the orientation distributions is applied to the analysis of partially magnetically oriented 31p NMR spectra of phospholipids. A mixture of synthetic lipids, POPE and POPG, is analyzed to measure distortion of multilamellar vesicles in a magnetic field. In the analysis three models describing the shape of the distorted vesicles are examined. Ellipsoids of rotation with a semiaxis ratio of about 1.14 are found to provide a good approximation of the shape of the distorted vesicles. This is in reasonable agreement with published experimental work. All three models yield clearly non-spherical orientational distributions, as well as a precise measure of the anisotropy of the chemical shift. Noise in the experimental data prevented the analysis from concluding which of the three models is the best approximation. A discretization scheme for finding stability in the algorithm is outlined