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Abstract: This thesis is mainly concerned with understanding the flotation behaviors of the polymorphic pyrrhotite (Fe1-xS, 0 < x ≤ 0.125) and pentlandite ((Ni,Fe)9S8) to find better measures for their flotation separation. The flotation separation of pentlandite from pyrrhotite is a complicated issue due to the complex chemical environment of the real flotation system. To learn more about the complex chemical environment, we conducted two plant surveys in the Strathcona Mill to learn about the flotation performance of polymorphic pyrrhotite and pentlandite and to find factors that impacted their flotation performance (Chapter 3). Two important phenomena were noticed: 1. the hexagonal pyrrhotite showed higher floatability than the monoclinic pyrrhotite in the Strathcona Mill; 2. the copper adsorption enhanced the flotation recovery of pyrrhotite. For further understanding these phenomena, pyrrhotite oxidation and copper activation were studied in Chapter 4 and Chapter 6, respectively. Importantly, the flotation separation of pentlandite from pyrrhotite was achieved with selective oxidation using hydrogen peroxide, which is shown in Chapter 5. Pyrrhotite floatability is mainly related to its oxidation level. The oxidation rate of both pyrrhotites was investigated by cyclic voltammetry (CV) test, and oxidation level difference was evaluated with X-ray photoelectron spectroscopy (XPS) and Time-of-Flight secondary ion mass spectroscopy (ToF-SIMS). The CV tests demonstrated a higher oxidation rate of the monoclinic pyrrhotite than the hexagonal pyrrhotite, further explained by the different variations of the Fe-S bond strength. Investigations of the oxidized polymorphic pyrrhotite surfaces with ToF-SIMS showed that the Fe-S bond strength decreased gradually over a ‘defective layer’ under the surface. Over this ‘defective layer,’ the Fe-S bond strength of the monoclinic pyrrhotite declined more steeply than that of the hexagonal pyrrhotite, which is mainly due to the faster incorporation of the oxygen atoms into the monoclinic pyrrhotite than into the hexagonal pyrrhotite. For the flotation separation of the hexagonal pyrrhotite and pentlandite, hydrogen peroxide was employed to enlarge the oxidation difference between hexagonal pyrrhotite and pentlandite. The surface reactions of the hexagonal pyrrhotite and pentlandite towards the hydrogen peroxide conditioning were examined with electrochemical tests, XPS, ToF-SIMS, and dissolved oxygen (DO) studies. It was found that they responded differently towards the reduction reaction of hydrogen peroxide. On the hexagonal pyrrhotite, the reduction of the hydrogen peroxide was mainly balanced by the surface oxidation of the hexagonal pyrrhotite. While, on the pentlandite, the reduction of the hydrogen peroxide was balanced primarily by the oxidation of hydrogen peroxide. The more severe surface oxidation of the hexagonal pyrrhotite than the pentlandite rendered the hexagonal pyrrhotite lower floatability than the pentlandite. As a critical factor in the surrounding chemical environment, copper activation effects were firstly confirmed with micro-flotation studies. To fully understand copper activation effects on pyrrhotite flotation, copper’s effects on protecting pyrrhotite oxidation were investigated via the CV and XPS depth profile. It was found that copper protected pyrrhotite from severe oxidation by hindering the dissolution of sulfur. Meanwhile, the XPS depth profiles of the pyrrhotite showed that the Cu(I)S is the first and foremost copper activation species formed on pyrrhotite surfaces, which gradually oxidized to Cu(II)S and CuO as oxidation progresses. Cu(I)S is formed through the interaction between Cu2+ and surface reactive sulfur anions, which suggested that the copper adsorption can partially occupy the available sulfur anions to reduce the sulfur dissolution rate. In summary, this study explained the flotation performance of polymorphic pyrrhotite and pentlandite with their different oxidation behaviors under specific chemical environments. Such fundamental understandings revealed the challenges in the floatation separation of pentlandite from hexagonal pyrrhotite and are valuable for exploring for more effective measures.

Membrane-integral adenylyl cyclases (ACs) are key enzymes in mammalian G protein- dependent signal transduction, which is important in many cellular processes. Signals received by the G protein-coupled receptors are conveyed to adenylyl cyclases through G proteins, modulating the levels of cellular cyclic adenosine monophosphate (cAMP). Until now, the structural biology studies of adenylyl cyclase have been limited to crystal structures of the chimeric soluble domains of these enzymes. The function of the transmembrane (TM) domain and the detailed catalysis mechanism of the adenylyl cyclases have remained a mystery. My PhD project was aimed at determination of the full-length structure of adenylyl cyclase and adenylyl cyclase-based signal transduction complexes and elucidation of their molecular mechanisms of catalysis and regulation. In the first part of my PhD project (chapter 3), using cryo-electron microscopy (cryo-EM), I determined the first full-length structure of the mammalian membrane adenylyl cyclase AC9 bound to an activated G protein as subunit at 3.4 Å resolution. I found that AC9 is sensitive to forskolin after activation by Gas, based on biochemical assays. The structure revealed the organization of the transmembrane domain and the helical domain that connects the membrane and catalytic domains of AC9. The C-terminal extension of the catalytic domain occludes both the catalytic and the allosteric sites of AC9, inducing a conformation distinct from the substrate- and activator-bound state, suggesting a regulatory role in cAMP production. I determined the structure of a truncated AC9, AC91250, in complex with Gas, forskolin and MANT-GTP. This structure confirmed the existence of the occluded state of AC9. By comparing the AC9-Gas and AC91250-Gas-forskolin-MANT-GTP, we could determine the differences in the conformations of these two structures. The manuscript describing these results has been published in Science. In the second of part of my PhD project (chapter 4), I determined the structure of AC1250-Gas in MANT-GTP bound state. The structure of AC1250-Gas-MANT-GTP provided further insights into the substrate-bound and occluded state of AC9. Comparisons of all available structures showed the conformational changes induced by forskolin. In addition, in collaboration with the group of Andreas Pluckthun (University of Zurich) we developed an artificial AC9 binding protein, DARPin-C4, capable of activating AC9 (chapter 5). We determined the cryo-EM structure of AC9-C4 complex and showed that C4 uses a binding site and activation mechanism similar to that of Gas. In conclusion, combining methods of biochemistry and structural biology I systematically studied the structure and function of a central enzyme in mammalian signal transduction, the membrane AC (AC9) and its complex with Gas protein. This work allowed us to propose the mechanism of forskolin and Gas activation. These results, together with the new tools that I developed in the course of my PhD project (such as the DARPin C4), will not only be helpful for the fundamental research on the principles of cellular signaling, but may also facilitate the future development of new therapies directed at the ACs.

Concise syntheses of the natural products brosimones A and B have been achieved using sequential dehydrogenative Diels-Alder (DHDA) cycloadditions. The syntheses employ either Pt/C-cyclopentene or DDQ to effect dehydrogenation of prenylchalcone substrates in combination with silver nanoparticles (AgNP’s) to promote subsequent Diels-Alder cycloadditions. A concise, biomimetic approach to sorbiterrin A has been developed employing consecutive Michael additions of a 4-hydroxypyrone to a sorbicillinol derivative and silver nanoparticle-mediated bridged aldol/dehydration to construct the [3.3.1] ring system. The relative stereochemistry of sorbiterrin A was unambiguously confirmed by X-ray crystallographic analysis. Metal-catalyzed, double Claisen rearrangement of a bis-allyloxyflavone has been utilized to enable a concise synthesis of the hydrobenzofuro[3,2-b]chromenone core structure of the natural products sanggenon A and sanggenol F. In addition, catalytic, enantioselective [4+2] cycloadditions of 2’-hydroxychalcones have been accomplished using B(OPh)3/BINOL complexes. Asymmetric syntheses of the flavonoid Diels-Alder natural products sanggenons C and O have been achieved employing a stereodivergent reaction of a racemic mixture (stereodivergent RRM) involving [4+2] cycloaddition. Diaporine is a natural product containing a novel epoxyquinol dimer framework. An efficient annulation involving pyrone addition to a quinone has been developed for rapid assembly of the γ-naphthopyrone core structure. Dimerization was achieved through a Pd(II)-mediated dehydrogenative coupling. A natural product and precursor to diaporine, aurofusarin, was synthesized in excellent yield through an oxidation and demethylation sequence. In addition, diastereoselective epoxidation of aurofusarin was achieved using a phase transfer catalytic system.

The redistribution of melt in partially molten rocks during deformation plays an important role in the evolution and dynamics of Earth's mantle. Previous studies discovered different scales of melt redistribution: melt alignment and melt segregation to form melt-enriched bands , both of which have demonstrated their importance to the deformation of the mantle. In this dissertation, two new forms of stress-driven melt redistribution in deformed partially molten rocks are produced: a formation of pressure shadows around rigid particles and a large-scale, base-state melt segregation. For pressure shadows, observations on the microstructure around the rigid particles revealed the melt distribution and solid flow field, which will provide a constraint on the bulk viscosity of the partially molten rock, if associated with theoretical studies. The presence of base-state melt segregation validated a hypothesis of viscous anisotropy, which provides explanations for melt segregation processes and will cause a significant impact to the dynamic of the mantle. Therefore, the studies of stress-driven melt redistribution in this dissertation are of great significance that will influence the future studies of Earth's mantle.