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Enzymes can catalyze reactions with high selectivity under mild conditions, and are therefore especially suited for the upgrading of C1 feedstocks into value-added products. Linear carbon ligation routes are of particular interest due to their simplicity and potential for high carbon efficiencies. A linear carbon fixation pathway can be constructed using a combination of NADH-dependent oxidoreductase enzymes and a formaldehyde carboligation enzyme, through which CO₂ is upgraded into C₂ and C₃ products. The stoichiometric NADH requirement imposed by the oxidoreductase enzymes and the poor performance of two core pathway enzymes (formaldehyde dehydrogenase and formolase) are the main obstacles to the efficient application of this linear carbon fixation pathway. In this dissertation, a host of fundamental enzyme engineering and characterization techniques are applied to study and address the thermodynamic, transport, and kinetic challenges arising from the use of enzyme cascades for multistep catalysis. In Chapter 2, a modular approach for the design of cofactor-independent transhydrogenases was explored and developed to enable catalysis and cofactor recycling in a single protein. Individual, unmodified active sites were modularly assembled and their activity catalytically coupled using biomimetic PEG-NAD(H) swing arms. Protein engineering and molecular design were used to increase the swing arm content and increase the activity of the transhydrogenases without detriment to their selectivity, circumventing the typical tradeoffs associated with modifications to the active site. The modularity of this approach was illustrated through the creation and characterization of four novel transhydrogenase enzymes with behavior that was predictable from that of the parent enzyme active sites. In Chapter 3, the kinetic behavior of the formolase (FLS) enzyme was comprehensively characterized to facilitate its use in carbon fixation cascades. A mechanistic rate equation and theory-based figures of merit were derived from first principles and used to capture and rank the full catalytic performance of 8 FLS variants under different conditions. The transition state specificity constant derived in this chapter was used to quantify product preference. In Chapter 4, the limiting performance of the formaldehyde dehydrogenase enzyme was explored within the context of a NADH-dependent pathway for the reduction of CO₂ to methanol. Protein engineering experiments targeting the elongated cofactor binding loop of the enzyme were investigated as a means of enhancing NADH-dependent formate reductase activity, but all mutations in the loop region dramatically reduced protein expression levels. Pathway flux was increased through the substitution of the formaldehyde dehydrogenase with a better performing homolog. In Chapter 5, a model carboligation pathway for the conversion of formaldehyde to glycerol and ethylene glycol was built using the FLS, glycerol dehydrogenase, and phosphite dehydrogenase enzymes. The impact of the low activity and substrate affinity of the FLS enzyme on pathway carbon and energy efficiencies was examined in purified proteins and crude lysates. High energy efficiencies, as quantified through the efficiency of NADH utilization, were achieved only with purified proteins. Cofactor regeneration was also shown to lower the cofactor requirement from stoichiometric to catalytic concentrations. In this work, we utilized a range of characterization techniques to study the limitations and challenges involved in the use of both NADH-dependent oxidoreductases and designed enzymes for multistep catalysis, and used protein engineering to address them. The insights gained from this work will facilitate the efficient use of these enzymes for linear carbon fixation as well as other biocatalysis applications.

Seagrass is a valuable and important habitat, providing services such as coastal protection, supporting fisheries, and carbon sequestration. However, it is challenging to map accurately, as remote sensing has limits to how deep in the water column it can penetrate, and uncertainties such as distinguishing between algae and seagrass. Seagrass can exist at depths of theoretically 90 m deep in ultraoligotrophic waters, meaning that there is much of this habitat that cannot be mapped by remote sensing. Green turtles are an ideal candidate to help find seagrass blue carbon resources in the Red Sea. They go through an ontogenetic dietary shift to become almost completely herbivorous, and have a high fidelity to foraging sites. In this study we aim to assess the use of green turtles Chelonia mydas in discovering seagrass blue carbon. We use telemetry from 53 turtles tagged over 2018, 2019, and 2021 to map their foraging areas. 50 out of the 53 (94.34%) foraging sites had not been visited by previous seagrass studies in the Red Sea. We visited 18 locations in 14 of these foraging sites to ground truth them, and all 14 foraging sites (100%) had seagrass present. Comparatively, 18 out of 30 sites where seagrass was indicated by the remote sensing-based Allen Coral Atlas showed no seagrass. The turtles were seen to favour travelling shorter distances, thus it will be necessary to expand the area of tagging in order to achieve complete coverage of the Red Sea. Approximately 1/3 of the visited sites were deeper than 8 m, and so out of range of remote sensing, showing that considerable blue carbon resources may be discovered with the use of turtles. Samples were taken for carbon stock estimation from the ground truthed sites. A mean carbon stock of 4.89 ± 0.83 kg Corg m-2 was estimated for 1 m depth sediment. In the future it is important to develop methods for mapping the surface areas of the deep and inaccessible seagrass habitats that the turtles discover.

<p> The results of an investigation of the electronic absorption spectrum of carbon diselenide are presented. The synthesis of the compound from individual isotopes of selenium is described.. Some new observations and results from a reinvestigation of the infrared and Raman spectra are reported. The electronic absorption spectra of individual isotopic molecules were recorded and a detailed vibrational analysis of one of the observed systems in the spectrum has been made o The rotational structure of bands in this system has been examined. A preliminary study of two other systems are discussed. The spectra of carbon diselenide and the analogous carbon disulphide are compared and the similarity between the two spectra is shown. </p> / Thesis / Doctor of Philosophy (PhD)

My work presents a novel approach to fabricate binder free three-dimensional carbon nanotubes/sulfur (3DCNTs/S) hybrid composite by a facile and scalable method increasing the loading amount from 1.86 to 8.33 mg/cm2 highest reported to date with excellent electrochemical performance exhibiting maximum specific energy of ~1233Wh/kg and specific power of ~476W/kg, with respect to the mass of the cathode. Such an excellent performance is attributed to the fact that 3DCNTs offers higher loading amount of sulfur, and confine polysulfide within the structure. In second part of the thesis, molybdenum disulfide (MoS2) is typically studied for three electrochemical energy storage devices including supercapacitors, Li-ion batteries, and hybrid Li-ion capacitors. The intrinsic sheet like morphology of MoS2 provides high surface area for double layer charge storage and a layered structure for efficient intercalation of H+/ Li+ ions. My work demonstrates the electrochemical analysis of MoS2 grown on different substrates including copper (conducting), and carbon nanotubes. MoS2 film on copper was investigated as a supercapacitor electrode in three electrode system exhibiting excellent volumetric capacitance of ~330F/cm3 along with high volumetric power and energy density in the range of 40-80 W/cm3 and 1.6-2.4 mWh/cm3, respectively. Furthermore, we have developed novel binder-free 3DCNTs/ MoS2 as an anode materials in half cell Li-ion batteries. The vertically oriented morphology of MoS2 offers high surface area and active electrochemical sites for efficient intercalation of Li+ ions and demonstrating excellent electrochemical performance with high specific capacity and cycling stability. This 3DCNTs/ MoS2 anode was coupled with high surface area southern yellow pine derived activated carbon (SYAC) cathode to obtain hybrid 3DCNTs/ MoS2 || SYAC Li-ion capacitor (LIC), which delivers large operating voltage window of 1-4.0V with excellent cycling stability exhibiting capacitance retention of ~80% after 5000 cycles.

The application of membranes in liquid and gas separation is attractive because of their energy efficiency. Synthesis of membranes with well-defined nanostructure is necessary to achieve highly permeability and selectivity for separation processes. Recently, carbon nanomaterials such as graphene oxide nanoplatelets (GONPs) and carbon nanodots (CNDs) have emerged as an interesting class of nanomaterials due to their unique properties and tailorable functionalities. Incorporation of these nanomaterials in the membranes has been shown to improve membrane selectivity, mechanical robustness, and chemical stability. This dissertation elaborates on developing CNDs or GONPs embedded thin film composite (TFC) membranes using layer-by-layer (LbL) synthesis technique. Regarding the water desalination applications, GONPs were used to enhance the TFC membranes’ selectivity, chlorine resistant properties, and surface hydrophilicity. Incorporation of GONPs in the polyamide layer via LbL method resulted in an increase of surface hydrophilicity and salt rejection properties. Upon exposure to chlorine, GONPs embedded membranes retained salt rejection performance better than the pristine membranes (without GONPs). The LbL assembly was used to synthesize CNDs based TFC membranes for organic solvent nanofiltration (OSN) applications. Using the LbL framework, amineunctionalized CNDs were covalently crosslinked with trimesoyl chloride monomer to obtain nanoscale membranes. The synthesized membranes manifested high selectivity (up to 90%) when tested for dye molecules such as brilliant blue and disperse red in methanol. As the CNDs synthesized here are fluorescent under UV light, the resultant film is also fluorescent. This property can be harnessed for diagnostic purposes, such as tracking mechanical failure and fouling of the membranes. Based on the results, it can be concluded that the incorporation of carbon nanomaterials in the polymeric membranes has enhanced the hydrophilicity, mechanical stability, and chlorine resistant properties of the membranes. Overall, the LbL platform can be considered as a modular method in embedding nanoparticles in TFC membranes.

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