Oxyamine Linkers for the Protecting Group Free Synthesis of Glycoconjugates

<p>Glycoconjugates, such as glycolipids and glycoproteins, are involved in a variety of cellular functions including cell-to-cell signalling and carbohydrate-protein recognition. Accordingly, glycoconjugates play important roles in health and disease and are promising new leads as carbohydrate-based therapeutics. However, for the development of glycoconjugates to study biological processes, or for the use of these adducts as therapeutics, the glycan needs to be conjugated to the carrier molecule or scaffold of choice. Many procedures for the conjugation of glycans involve lengthy protecting group strategies that install the aglycone at the start of glycan total synthesis and are therefore unsuitable for naturally derived sugars. Other glycan conjugation strategies can affect the integrity of the reducing end sugar or lead to adducts where the reducing end sugar adopts the ring-opened rather than the ring-closed form. N,O-Dialkyl oxyamine linkers, however, can be attached to the free reducing end of sugars in a single step without the need for protecting groups. This thesis therefore explores the synthesis and application of oxyamine linkers for the synthesis of glycoconjugates. First, the synthesis of an O-alkyl-N-methyl oxyamine linker (“Type A”) containing an amine at its terminus was improved by reducing the number of synthetic steps from six to four and by increasing the overall yield from 8% to 38%. This oxyamine linker was then conjugated to GlcNAc in 83% yield. The hydrolytic stability of this glycosyloxyamine was then compared to that of the analogous N-alkyl-O-methyl glycosyloxyamine (“Type B”). The stability of the two types of glycosyloxyamines has never been directly compared. Accordingly, it was not known whether the difference in substitution pattern between the two linkers affects their hydrolytic stability. To this end, the hydrolysis rates of the GlcNAc conjugated linkers were assessed at various pH values, glycoconjugate concentrations and buffer concentrations. In all instances, the “Type B” glycoside was found to have marginally better kinetic stability, while the “Type A” glycoside had marginally better thermodynamic stability, but overall, these differences were negligible. The pKa of the conjugate acid of these glycosyloxyamines was also determined to provide insight into the mechanism of hydrolysis. By considering this data, along with the observation that the rate of hydrolysis of these glycoconjugates increases with increasing buffer concentration, it was proposed that the hydrolysis of the oxyamines occurs via general acid catalysis at pH 4-6. A novel dithiol functionalised oxyamine linker was also designed and synthesised for the multivalent display of glycans on gold nanoparticles. With the successful attachment of this thiol linker to GlcNAc, the monomer unit of chitin, this work has paved the way for the future syntheses of chitin-functionalised gold nanoparticles. Such chitinfunctionalised AuNPs can be used to assess chitin’s ability to invoke the asthma allergic immune response, thereby bringing the possibility of an anti-asthma vaccine a step closer to fruition.</p>

Oxyamine Linkers for the Protecting Group Free Synthesis of Glycoconjugates

<p>Glycoconjugates, such as glycolipids and glycoproteins, are involved in a variety of cellular functions including cell-to-cell signalling and carbohydrate-protein recognition. Accordingly, glycoconjugates play important roles in health and disease and are promising new leads as carbohydrate-based therapeutics. However, for the development of glycoconjugates to study biological processes, or for the use of these adducts as therapeutics, the glycan needs to be conjugated to the carrier molecule or scaffold of choice. Many procedures for the conjugation of glycans involve lengthy protecting group strategies that install the aglycone at the start of glycan total synthesis and are therefore unsuitable for naturally derived sugars. Other glycan conjugation strategies can affect the integrity of the reducing end sugar or lead to adducts where the reducing end sugar adopts the ring-opened rather than the ring-closed form. N,O-Dialkyl oxyamine linkers, however, can be attached to the free reducing end of sugars in a single step without the need for protecting groups. This thesis therefore explores the synthesis and application of oxyamine linkers for the synthesis of glycoconjugates. First, the synthesis of an O-alkyl-N-methyl oxyamine linker (“Type A”) containing an amine at its terminus was improved by reducing the number of synthetic steps from six to four and by increasing the overall yield from 8% to 38%. This oxyamine linker was then conjugated to GlcNAc in 83% yield. The hydrolytic stability of this glycosyloxyamine was then compared to that of the analogous N-alkyl-O-methyl glycosyloxyamine (“Type B”). The stability of the two types of glycosyloxyamines has never been directly compared. Accordingly, it was not known whether the difference in substitution pattern between the two linkers affects their hydrolytic stability. To this end, the hydrolysis rates of the GlcNAc conjugated linkers were assessed at various pH values, glycoconjugate concentrations and buffer concentrations. In all instances, the “Type B” glycoside was found to have marginally better kinetic stability, while the “Type A” glycoside had marginally better thermodynamic stability, but overall, these differences were negligible. The pKa of the conjugate acid of these glycosyloxyamines was also determined to provide insight into the mechanism of hydrolysis. By considering this data, along with the observation that the rate of hydrolysis of these glycoconjugates increases with increasing buffer concentration, it was proposed that the hydrolysis of the oxyamines occurs via general acid catalysis at pH 4-6. A novel dithiol functionalised oxyamine linker was also designed and synthesised for the multivalent display of glycans on gold nanoparticles. With the successful attachment of this thiol linker to GlcNAc, the monomer unit of chitin, this work has paved the way for the future syntheses of chitin-functionalised gold nanoparticles. Such chitinfunctionalised AuNPs can be used to assess chitin’s ability to invoke the asthma allergic immune response, thereby bringing the possibility of an anti-asthma vaccine a step closer to fruition.</p>

Idification and structural characterisation of a catecholate-type siderophore produced by Stenotrophomonas maltophilia K279a

Siderophores are produced by several bacteria that utilise iron in various environments. Elucidating the structure of a specific siderophore may have valuable applications in drug development. Stenotrophomonas maltophilia , a Gram-negative bacterium that inhabits a wide range of environments and can cause pneumonia, produces siderophores. However, the structure was unknown, and therefore, in this study, we aimed to elucidate it. We purified siderophores from cultures of S. maltophilia K279a using preparative reversed-phase HPLC. The structure was analysed through LC-MS and 1H and 13C NMR. The results demonstrated that S. maltophilia K279a produces 2,3-dihydroxybenzoylserine (DHBS), a monomer unit of enterobactin. We suggested the uptake of Iron(III) by the DHBS complex. DHBS production by S. maltophilia K279a could be attributed to an incomplete enterobactin pathway. Drugs targeting DHBS synthesis could prevent S. maltophilia infection.

Tau strains shape disease

Tauopathies consist of over 25 different neurodegenerative diseases that include argyrophilic grain disease (AGD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and Pick’s disease (PiD). Tauopathies are defined by brain accumulation of microtubule-associated protein tau in fibrillar aggregates, whose prevalence strongly correlates with dementia. Dominant mutations in tau cause neurodegenerative diseases, and most increase its aggregation propensity. Pathogenesis of tauopathies may involve pathological tau conformers that serve as templates to recruit native protein into growing assemblies and also move between brain cells to cause disease progression, similar to prions. Prions adopt pathological conformations, termed “strains,” that stably propagate in living systems, and create unique patterns of neuropathology. Data from multiple laboratories now suggest that tau acts as a prion. It propagates unique strains indefinitely in cultured cells, and when these are inoculated into mouse models, they create defined neuropathological patterns, which establish a direct link between conformation and disease. In humans, distinct fibril structures are associated with different diseases, but causality has not been established as in mice. Cryo-EM structures of tau fibrils isolated from tauopathy brains reveal distinct fibril cores across disease. Interestingly, the conformation of the tau monomer unit within different fibril subtypes from the same patient appears relatively preserved. This is consistent with data that the tau monomer samples an ensemble of conformations that act as distinct pathologic templates in the formation of restricted numbers of strains. The propensity of a tau monomer to adopt distinct conformations appears to be linked to defined local motifs that expose different patterns of amyloidogenic amino acid sequences. The prion hypothesis, which predicts that protein structure dictates resultant disease, has proved particularly useful to understand the diversity of human tauopathies. The challenge now is to develop methods to rapidly classify patients according to the structure of the underlying pathological protein assemblies to achieve more accurate diagnosis and effective therapy.

Influence of Salt on the Self-Organization in Solutions of Star-Shaped Poly-2-alkyl-2-oxazoline and Poly-2-alkyl-2-oxazine on Heating

The water–salt solutions of star-shaped six-arm poly-2-alkyl-2-oxazines and poly-2-alkyl-2-oxazolines were studied by light scattering and turbidimetry. The core was hexaaza[26]orthoparacyclophane and the arms were poly-2-ethyl-2-oxazine, poly-2-isopropyl-2-oxazine, poly-2-ethyl-2-oxazoline, and poly-2-isopropyl-2-oxazoline. NaCl and N-methylpyridinium p-toluenesulfonate were used as salts. Their concentration varied from 0–0.154 M. On heating, a phase transition was observed in all studied solutions. It was found that the effect of salt on the thermosensitivity of the investigated stars depends on the structure of the salt and polymer and on the salt content in the solution. The phase separation temperature decreased with an increase in the hydrophobicity of the polymers, which is caused by both a growth of the side radical size and an elongation of the monomer unit. For NaCl solutions, the phase separation temperature monotonically decreased with growth of salt concentration. In solutions with methylpyridinium p-toluenesulfonate, the dependence of the phase separation temperature on the salt concentration was non-monotonic with minimum at salt concentration corresponding to one salt molecule per one arm of a polymer star. Poly-2-alkyl-2-oxazine and poly-2-alkyl-2-oxazoline stars with a hexaaza[26]orthoparacyclophane core are more sensitive to the presence of salt in solution than the similar stars with a calix[n]arene branching center.

Superior thermal stability and fast crystallization behavior of a novel, biodegradable α-methylated bacterial polyester

Given their ubiquity in modern society, the development of biodegradable and renewably sourced plastics is essential for the creation of an environmentally sustainable society. One of the drawbacks for currently available biodegradable plastics such as poly(l-lactic acid) (PLLA) and polyhydroxyalkanoates (PHAs) is that it is difficult to simultaneously achieve mechanical flexibility and certain crystallization behavior in these materials, which limits their use as replacements for established petroleum-based plastics such as isotactic polypropylene (iPP). Here, we report the synthesis and characterization of a new biodegradable plastic, poly(3-hydroxy-2-methylbutyrate) [P(3H2MB)], which is a member of the bacterial PHA family whose members include an α-methylated monomer unit. Biosynthesis of P(3H2MB) was achieved using recombinant Escherichiacoli expressing an engineered pathway. Biosynthesized P(3H2MB) exhibited the highest melting temperature (197 °C) among the biosynthesized PHAs and improved thermal resistance. It also exhibited improved crystallization behavior and mechanical flexibility nearly equal to those of iPP. The primary nucleation rate of P(3H2MB) was faster than that of P(3HB), and the spherulite morphology of P(3H2MB) was much finer than that of P(3HB). This crystal morphology may result in more rapid crystallization behavior, increased transparency, and enhanced mechanical properties. The superior physical properties of P(3H2MB) have the potential to open new avenues for the production of high-performance biodegradable plastics for replacing petroleum-based bulk commodity plastics.

WOOD COMPONENTS AS SOURCES OF PENTO-CONTAINING RAW MATERIALS FOR SYNTHESIS OF USEFUL COMPOUNDS, PRODUCTS AND REAGENTS

The paper considers the main components and products of wood processing, agricultural waste, pulp and paper industry waste and qualifies them as sources of pentose-containing resource-renewable domestic raw materials. The article describes in detail the structural components of wood as a natural polymer, which contains aromatic and carbohydrate parts. It is noted that these poly-mers are promising as raw materials for the production of useful chemical products. The role of lignin, cellolose and hemicellulose in the design of mechanical and structural properties of wood is considered. The article considers the features of the sulfonation reactions of the lignin monomer unit depending on the pH of the medium: acidic, neutral and alkaline. There are three main reac-tions that occur simultaneously with lignin in the process of wood delignification during sulfite cooking, such as the sulfonation reaction, the hydrolytic destruction reaction, and the condensation reaction. It is shown that the lignin-hemicellulose matrix contains three types of interconnected mesh structures: the lignin itself; a network of covalent bonds of lignin with hemicelluloses, and a network whose structure is obtained due to the hydrogen bond and the forces of the physical inter-action of lignin and hemicelluloses. The features of chemical transformations of the monomeric aromatic link of lignosulfonate – phenylpropane unit in the processes of wood delignification, the main chemical reactions of wood raw material delignification under the conditions of sulfite and neutral-sulfite brews are shown. The method of quantitative determination of monosaccharides in the composition of the carbohydrate part is proposed.

Photochemistry of carbon nitrides and heptazine derivatives

This article highlights the photochemistry of heptazine derivatives, a structural monomer unit of carbon nitride photocatalysts.

Adhesion and Energy Characteristics of Rigid-Chain Polymer Surface: Polyamidoimides

The adhesion characteristics and surface energies of two series of polyamidoimides (PAI) with different molecular weights, monomer unit structures, hinge groups in the main chain of the macromolecules, and thermal prehistory were determined via delamination at 180° and test fluids contact angles. We found that PAI are high-energy polymers, the surface energy of which varies in the range from 32 to 45 mJ/m2. In contrast to flexible-chain polymers, the exponent in the McLeod equation is two, which is due to the flat parallel orientation of the macromolecular chains in the surface layers. The main contribution to the change in surface characteristics of these polymers is the change in the packing density of PAI macromolecules, which is reflected mainly in the change in the polymers’ dispersion component. We found that the adhesion properties of PAI with respect to high- and low-energy substrates are determined mainly by the macromolecules packing density in the surface layers with their conformation state unchanged.

Supramolecular Cocrystals Built Through Redox-Triggered ion Intercalation in π-Conjugated Polymers

Self-organization in π-conjugated polymers gives rise to a highly ordered lamellar structure, in which inter-chain stacking spontaneously forms two-dimensional conjugated sheets. This multi-layer stacked nature of semicrystalline polymers results not only in effective charge transport, but also allows the inclusion of various functional molecules and ions. In particular, redox-triggered ion-intercalation in a polymer's lamellae is an ideal system for molecular doping, for which extremely high charge carrier density, corresponding to one carrier per monomer unit, has been achieved. We conducted a detailed structural analysis and electron density simulation to pinpoint exactly where the guest dopants are located periodically in the restricted void space in a polymer's lamellae. Our findings are indicative of an intercalation compound of layered polymers and a guest intercalant. In addition, we show that a homogeneous cocrystal structure can be realized throughout the host polymer medium, which is proved unambiguously by the observation of coherent carrier transport across microscopic-scale films. Also, the intercalation cocrystal nature gives the best achievable doping level in semicrystalline conjugated polymers and excellent environmental stability. These findings should open up new possibilities for tuning the collective dynamics of functional molecules and ions through intercalation phenomena.