Metal-uronide interactions and their relevance to the thermolysis of kelp

This thesis describes investigations into the coordination- and thermo-chemistry of metal-uronide complexes (namely alginates). The work probes the importance of such metal-saccharide interactions in the thermolysis of kelp, with a view to deriving valuable fuels and chemicals from this aquatic bioresource. In this regard, Chapter 1 justifies the model-compound approach adopted in this thesis, and outlines the wider context in which the work is set. Chapter 2 describes the isolation of the composite monosaccharides of alginate (D-mannuronate and L-guluronate) and their characterisation by NMR spectroscopy. A combination of 1- and 2-D 1H and 13C experiments were utilised to provide the most comprehensive assignments of algal mono-uronates to-date. Subsequently, the well-defined mono-uronate spectra were used to probe the conditions that favour hydrothermal uronolactone formation. Chapter 3 probed the response of the algal mono-saccharides (prepared and characterised in Chapter 2) to a range of metal ions (Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Zn2+, and Cu2+) to explore the validity of the well-known “Egg-box model” of metal ion/alginate coordination. By observing changes to anomeric equilibria, α-L-gulopyranuronate was found to coordinate (via an axial-equatorial-axial arrangement of hydroxyl groups) to large, divalent cations, in a manner consistent with “Egg-box binding”. In contrast, in the presence of Na+, Mg2+, Zn2+, and, Cu2+ α-L-gulopyranuronate interacted via its carboxylate moiety (and possibly ring oxygen), demonstrating the unsuitability of these ions for Egg-box binding. Chapter 4 describes the impact of the metal ions discussed in Chapter 3 on the subsequent pyrolysis behaviour of alginates (and related mono- and poly-uronides). Thermogravimetric analysis (TGA), pyrolysis-gas chromatography mass spectrometry (Py-GCMS), and solid-state studies of the post-thermolysis chars were all conducted. The uronides were generally found to demonstrate unfavourable thermal behaviour (high yields of char, CO2, and H2O, and low yields of condensable hydrocarbons). Complexation of Cu2+, however, had a beneficial impact on subsequent thermolysis, by increasing the low-temperature, selective formation of 2-furfural. The presence of Cu0 in the alginate char is indicative of Cu(II)-mediated alginate decomposition occurring via a Hofer-Moest type decarboxylation. Chapter 5 tested the validity of the model compound approach by enriching samples of kelp with either Cu2+ or Ca2+ ions and studying the thermal degradation of the resulting materials by TGA and Py-GCMS. The thermochemical outcomes for the whole biomass mirrored those found for the model compounds studied in Chapter 4. Finally, in Chapter 6, the results of Chapter 2 – 5 are analysed synoptically, and the success of the model compound approach is appraised. Ultimately, it is concluded that the ability of a metal ion to inhibit or promote thermal decarboxylation of a uronide (in isolation or within kelp) is more important in dictating pyrolysis behaviour than any differences in coordination to various hydroxyl groups around the saccharide ring. The results could find application in the development of a phytoremediative kelp-based thermal biorefinery

Hybrid Chemo-, Bio-, and Electrocatalysis for Atom-Efficient Deuteration of Cofactors in Heavy Water

Deuterium-labeled nicotinamide cofactors such as [4-2H]-NADH can be used as mechanistic probes in biological redox processes and offer a route to the synthesis of selectively [2H] labeled chemicals via biocatalytic reductive deuteration. Atom-efficient routes to the formation and recycling of [4-2H]-NADH are therefore highly desirable but require careful design in order to alleviate the requirement for [2H]-labeled reducing agents. In this work, we explore a suite of electrode or hydrogen gas driven catalyst systems for the generation of [4-2H]-NADH and consider their use for driving reductive deuteration reactions. Catalysts are evaluated for their chemoselectivity, stereoselectivity, and isotopic selectivity, and it is shown that inclusion of an electronically coupled NAD+-reducing enzyme delivers considerable advantages over purely metal based systems, yielding exclusively [4S-2H]-NADH. We further demonstrate the applicability of these types of [4S-2H]-NADH recycling systems for driving reductive deuteration reactions, regardless of the facioselectivity of the coupled enzyme

Opening the Egg Box: NMR spectroscopic analysis of the interactions between s-block cations and kelp monosaccharides

The best-known theory accounting for metal-alginate complexation is the so-called “Egg Box” model. In order to gain greater insight into the metal-saccharide interactions that underpin this model, the coordination chemistry of the corresponding monomeric units of alginate, L-guluronate (GulA) and D-mannuronate (ManA) have been studied herein. GulA and ManA were exposed to solutions of different s-block cations and then analysed by 1H and 13C NMR spectroscopy. It was found that the α/β ratio of the pyranose anomeric equilibria of GulA showed large pertubations from the starting value (α/β = 0.21 ± 0.01) upon contact with 1.0 M Ca2+, Sr2+, and Ba2+ (α/β = 1.50 ± 0.03, 1.20 ± 0.02, and 0.58 ± 0.02, respectively) at pD 7.9, but remained almost constant in the presence of Na+, K+, and Mg2+ (α/β = 0.24 ± 0.01, 0.19 ± 0.01, and 0.26 ± 0.01, respectively). By comparison, no significant changes were observed in the α/β ratios of ManA and related mono-uronates D-glucuronate (GlcA) and D-galacturonate (GalA) in the presence of all of the metal ions surveyed. Analysis of the 1H and 13C coordination chemical shift patterns indicate that the affinity of α-GulA for larger divalent cations is a consequence of the unique ax-eq-ax arrangement of hydroxyl groups found for this uronate anomer

Bringing Biocatalysis into the Deuteration Toolbox

Chemicals labelled with the heavy hydrogen isotope deuterium (2H) have long been used in chemical and biochemical mechanistic studies, spectroscopy, and as analytical tracers. More recently, demonstration of selectively deuterated drug candidates that exhibit advantageous pharmacological traits has spurred innovations in metal-catalysed 2H insertion at targeted sites, but asymmetric deuteration remains a key challenge. Here we demonstrate an easy-to-implement biocatalytic deuteration strategy, achieving high chemo-, enantio- and isotopic selectivity, requiring only 2H2O (D2O) and unlabelled dihydrogen under ambient conditions. The vast library of enzymes established for NADH-dependent C=O, C=C, and C=N bond reductions have yet to appear in the toolbox of commonly employed 2H-labelling techniques due to requirements for suitable deuterated reducing equivalents. By facilitating transfer of deuterium atoms from 2H2O solvent to NAD+, with H2 gas as a clean reductant, we open up biocatalysis for asymmetric reductive deuteration as part of a synthetic pathway or in late stage functionalisation. We demonstrate enantioselective deuteration via ketone and alkene reductions and reductive amination, as well as exquisite chemo-control for deuteration of compounds with multiple unsaturated sites.</p