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Description
Natural photosynthesis features a complex biophysical/chemical process that requires sunlight to produce energy rich products. It is one of the most important processes responsible for the appearance and sustainability of life on earth. The first part of the thesis focuses on understanding the mechanisms involved in regulation of light harvesting, which is necessary to balance the absorption and utilization of light energy and in that way reduce the effect caused by photooxidative damage. In photosynthesis, carotenoids are responsible not only for collection of light, but also play a major role in protecting the photosynthetic system. To investigate the role of carotenoids in the quenching of the excited state of cyclic tetrapyrroles, two sets of dyads were studied. Both sets of dyads contain zinc phthalocyanine (Pc) covalently attached to carotenoids of varying conjugation lengths. In the first set of dyads, carotenoids were attached to the phthalocyanine via amide linkage. This set of dyads serves as a good model for understanding the molecular "gear-shift" mechanism, where the addition of one double bond can turn the carotenoid from a nonquencher to a very strong quencher of the excited state of a tetrapyrrole. In the second set of dyads, carotenoids were attached to phthalocyanine via a phenyl amino group. Two independent studies were performed on these dyads: femtosecond transient absorption and steady state fluorescence induced by two-photon excitation. In the transient absorption study it was observed that there is an instantaneous population of the carotenoid S1 state after Pc excitation, while two-photon excitation of the optically forbidden carotenoid S1 state shows 1Pc population. Both observations provide a strong indication of the existence of a shared excitonic state between carotenoid and Pc. Similar results were observed in LHC II complexes in plants, supporting the role of such interactions in photosynthetic down regulation. In the second chapter we describe the synthesis of porphyrin dyes functionalized with carboxylate and phosphonate anchoring groups to be used in the construction of photoelectrochemical cells containing a porphyrin-IrO2·nH2O complex immobilized on a TiO2 electrode. The research presented here is a step in the development of high potential porphyrin-metal oxide complexes to be used in the photooxidation of water. The last chapter focuses on developing synthetic strategies for the construction of an artificial antenna system consisting of porphyrin-silver nanoparticle conjugates, linked by DNA of varied length to study the distance dependence of the interaction between nanoparticles and the porphyrin chromophore. Preliminary studies indicate that at the distance of about 7-10 nm between porphyrin and silver nanoparticle is where the porphyrin absorption leading to fluorescence shows maximum enhancement. These new hybrid constructs will be helpful for designing efficient light harvesting systems.
ContributorsPillai, Smitha (Author) / Moore, Ana (Thesis advisor) / Moore, Thomas (Committee member) / Gould, Ian (Committee member) / Arizona State University (Publisher)
Created2011
Description
[FeFe]-hydrogenases are enzymes for the reduction of protons to hydrogen. They rely on only the earth abundant first-row transition metal iron at their active site (H cluster). In recent years, a multitude of diiron mimics of hydrogenases have been synthesized, but none of them catalyzes hydrogen production with the same exquisite combination of high turnover frequency and low activation energy as the enzymes. Generally, model complexes fail to include one or both of two features essential to the natural enzyme: an intricate array of outer coordination sphere contacts that constrain the coordination geometry to attain a catalytically optimal conformation, and the redox non-innocence of accessory [FeS] clusters found at or near the hydrogen-activating site. The work presented herein describes the synthesis and electrocatalytic characterization of iron-dithiolate models designed to incorporate these features. First, synthetic strategies are developed for constructing peptides with artificial metal-binding motifs, such as 1,3-dithiolate and phosphines, which are utilized to append diiron-polycarbonyl clusters onto a peptide. The phosphine-functionalized peptides are shown to be better electrocatalysts for proton reduction in water/acetonitrile mixtures than in neat acetonitrile. Second, we report the impact of redox non-innocent ligands on the electrocatalytic properties of two types of [FeFe]-hydrogenase models: dinuclear and mononuclear iron complexes. The bidentate, redox non-innocent α-diimine ligands (N-N), 2,2'-bipyridine and 2,2' bipyrimidine, are used to create complexes with the general formula (μ-SRS)Fe2(CO)4(N-N), new members of the well known family of asymmetric diiron carbonyls. While the 2,2'-bipyridine derivatives can act as electrocatalysts for proton reduction, surprisingly, the 2,2'-bipyrimidine analogues are found to be inactive towards catalysis. Electrochemical investigation of two related Fe(II) complexes, (bdt)Fe(CO)P2 for bdt = benzene-1,2-dithiolate and P2 = 1,1'-diphenylphosphinoferrocene or methyl-2-{bis(diphenylphosphinomethylamino}acetate, related to the distal iron in [FeFe]-hydrogenase show that these complexes catalyze the reduction of protons under mild conditions. However, their reactivities toward the external ligand CO are distinguished by gross geometrical differences.
ContributorsRoy, Souvik (Author) / Jones, Anne K (Thesis advisor) / Moore, Thomas (Committee member) / Trovitch, Ryan (Committee member) / Arizona State University (Publisher)
Created2013
Description
Developing a system capable of using solar energy to drive the conversion of an abundant and available precursor to fuel would profoundly impact humanity's energy use and thereby the condition of the global ecosystem. Such is the goal of artificial photosynthesis: to convert water to hydrogen using solar radiation as the sole energy input and ideally do so with the use of low cost, abundant materials. Constructing photoelectrochemical cells incorporating photoanodes structurally reminiscent of those used in dye sensitized photovoltaic solar cells presents one approach to establishing an artificial photosynthetic system. The work presented herein describes the production, integration, and study of water oxidation catalysts, molecular dyes, and metal oxide based photoelectrodes carried out in the pursuit of developing solar water splitting systems.
ContributorsSherman, Benjamin D (Author) / Moore, Thomas (Thesis advisor) / Moore, Ana (Committee member) / Buttry, Daniel (Committee member) / Arizona State University (Publisher)
Created2013
Description
The development of sustainable catalysts that exhibit exceptional activity has become a major goal of organometallic chemists. Considering their low cost and environmentally benign nature, the use of base metals in catalysis has recently been explored. This dissertation is focused on the development of manganese catalysts for organic transformations and inorganic polymerizations. Previous advances in Mn-based hydrosilylation and hydroboration catalysis are reviewed in Chapter 1 and set the stage for the experimental work described herein.In Chapter 2, the electronic structure of [(2,6-iPr2PhBDI)Mn(μ-H)]2 is explored. This compound was evaluated by density functional theory calculations, SQUID magnetometry and EPR spectroscopy at low temperature. Single crystal X-ray diffraction data was collected for related compounds that feature bridging X-type ligands. The data revealed how bridging ligands impact the Mayer bond order between the two Mn atoms and explained why [(2,6-iPr2PhBDI)Mn(μ-H)]2 is an active catalyst for organic transformations.
Chapter 3 spotlights the first study to systematically demonstrate commercial aminosilane CVD precursor synthesis by way of SiH4 and amine dehydrocoupling using [(2,6-iPr2PhBDI)Mn(μ-H)]2. In addition, the study provided an efficient and halogen-free preparation of highly cross-linked polycarbosilazanes under ambient conditions. Furthermore, exceptionally pure perhydropolysilazane was directly prepared from ammonia and silane at room temperature through dehydrogenative coupling. These are also the first reported examples of Mn-catalyzed Si–N dehydrocoupling.
This research was then extended to the Mn-catalyzed dehydrogenative coupling of NH3 and diamines to organic silanes. Organic polysilazanes and polycarbosilazanes were synthesized and the structures were characterized by NMR, FT-IR, and MALDI-TOF spectroscopy. The thermal properties and coating applications of the products were evaluated by TGA, DSC, X-ray powder diffraction, SEM and EDX. A turnover frequency (TOF) experiment using 0.01 mol% of [(2,6-iPr2PhBDI)Mn(μ-H)]2 revealed a maximum TOF of 300 s-1, which is the highest activity ever reported for this transformation.
The last chapter highlights the first examples of nitrile dihydroboration mediated by a manganese catalyst. Using 0.5 mol% of [(2,6-iPr2PhBDI)Mn(μ-H)]2, 14 nitriles were reduced with HBPin at 80 ℃ to afford N,N-diborylamines after 24 h. A mechanism was proposed based on the isolation of [(2,6-iPr2PhBDI)Mn(NCHPh)]2 as an intermediate and further substantiated by DFT.
ContributorsNguyen, Thu Thao (Author) / Trovitch, Ryan (Thesis advisor) / Jones, Anne (Committee member) / Ackerman, Laura (Committee member) / Arizona State University (Publisher)
Created2022
Description
Due to the potential synergistic properties from combining inorganic and organic moieties, inorganic/organic hybrids materials have recently attracted great attention. These hybrids are critical components in coating and nanocomposite additive technologies and have potential for future application in catalysis, energy production or storage, environmental remediation, electronic, and sensing technologies. Most of these hybrids utilize low dimensional metal oxides as a key ingredient for the inorganic part. Generally, clay materials are used as inorganic components, however, the use of low dimensional transition metal oxides may provide additional properties not possible with clays. Despite their potential, few methods are known for the use of low dimensional transition metal oxides in the construction of inorganic/organic hybrid materials.Herein, new synthetic routes to produce hybrid materials from low dimensional early transition metal oxides are presented. Included in this thesis is a report on a destructive, chemical exfoliation method designed specifically to exploit the Brønsted acidity of hydrated early transition metal oxides. The method takes advantage of (1) the simple acid-base reaction principle applied to strong two-dimensional Brønsted solid acids and mildly basic, high-polarity organic solvents, (2) the electrostatic repulsion among exfoliated nanosheets, and (3) the high polarity of the organic solvent to stabilize the macroanionic metal oxide nanosheets in the solvent medium. This exfoliation route was applied to tungstite (WO3∙H2O) and vanadium phosphate hydrate (VOPO4∙H2O) to produce stable dispersions of metal oxide nanosheets. The nanosheets were then functionalized by adduct formation or silane surface modification. Both functionalization methods resulted in materials with unique properties, which demonstrates the versatility of the new exfoliation methods in preparing novel hybrid materials.
Further extension of the method to aqueous systems allowed discovery of a new synthetic method for electrically-conducting polyaniline-polyoxometalate hybrid materials. Namely, destructive dissolution of MoO2(HPO4)(H2O) in water produces protons and Strandberg-type phosphomolybdate clusters, and in the presence of aniline and an oxidizing agent, the clusters self-assemble with protonated anilines and selectively form polyaniline-phosphomolybdate hybrids on various types of surfaces through in situ oxidative chemical polymerization. New conductive nanocomposite materials were produced by selectively coating the surface of silica nanoparticles.
ContributorsCiota, David (Author) / Seo, Dong-Kyun (Thesis advisor) / Trovitch, Ryan (Committee member) / Birkel, Christina (Committee member) / Arizona State University (Publisher)
Created2022
Description
Exploration of long-range conductance in non-redox-active proteins at the single molecule scale is aided by the development of innovative, tailor-made quantitative data analysis techniques. This thesis details the rationale behind the proposed approaches, the steps taken to design and implement every method, and the validation of the methodologies using appropriate experiments, benchmarks, and rigorous statistical data analysis. The first chapter conducts a thorough literature review, sets the stage for the subsequent investigation, and underscores the importance of the research questions addressed in this thesis. The second chapter describes the solvent effects on the electronic conductance of a series of Consensus Tetratricopeptide Repeat proteins (CTPR) measured with Scanning Tunneling Microscopy (STM). The study reveals a reversible reduction in electronic conductance when water (H2O) is replaced with heavy water (D2O) due to a ~6-fold decrease in the carrier diffusion constant as proteins become solvated by D2O. Similar observations are made in a ~7 nm long tryptophan zipper protein, while a phenylalanine zipper protein of comparable length remains unchanged in D2O, highlighting the critical role of aromatic residues in proteins lacking redox cofactors. As an extension to this finding, the third chapter describes the development of a machine-learning model to detect the presence of a protein and identify essential features helping in the detection. For this purpose, a solid-state device was engineered to measure the conductance of CTPR-16 protein wires. This approach addresses the limitations in characterizing the STM gap, enables the collection of stable current vs. time data, and provides a statistical understanding of the electronic transport through a protein. The final chapter investigates real-time changes in conductance in response to protein conformation alterations. A deoxyribonucleic acid (DNA) polymerase Φ29 was chosen for its potential utility as a single-molecule DNA sequencing device. The modified enzyme was bound to electrodes functionalized with streptavidin. Φ29 connected by one biotinylated contact and a second nonspecific contact showed rapid small fluctuations in current when activated. Signals were greatly enhanced with two specific contacts. Features in the distributions of conductance increased by a factor of 2 or more over the open-to-closed conformational transition of the polymerase.
ContributorsMukherjee, Sohini (Author) / Lindsay, Stuart (Thesis advisor) / Moore, Thomas (Committee member) / Qing, Quan (Committee member) / Arizona State University (Publisher)
Created2023
Description
Mitochondria are energy-producing organelles present in eukaryotic cells. Energy as adenosine triphosphate (ATP) is produced at the end of a series of electron transfers called the electron transport chain (ETC). Such a highly coordinated and regulated series of electron transfer reactions give rise to a small percentage of electron leakage which, by the subsequent reduction of molecular oxygen, produce superoxide anions (O2.-). These anions initiate the production of additional highly reactive oxygen-containing radicals commonly known as reactive oxygen species (ROS). Although cells are equipped with endogenous antioxidant systems to minimize ROS accumulation, these endogenous defense systems become inadequate when ROS generation is increased. When ROS production occurs in excess, the cell is said to be under oxidative stress. Unchecked ROS production causes damage to cellular macromolecules, which in turn leads to cell death. Dysfunctional mitochondria and subsequent cell degeneration are a common cause of neurodegenerative diseases such as Friedreich’s ataxia (FRDA) and Alzheimer’s disease (AD). Therefore, targeting the mitochondria by neuroprotective drugs is imperative for the treatment of such diseases. In Chapter 1, the functioning of the ETC is described. Moreover, excessive ROS production and its consequences are also described.
FRDA is a progressive neurodegenerative disease caused by insufficient expression of frataxin (FXN). FXN is instrumental in the assembly of iron-sulfur clusters, which in turn are critical for the functioning of the ETC enzyme complexes. Therapeutic agents which, in addition to being antioxidants also increase FXN, can be good drugs to counter FRDA. In Chapter 2, the synthesis of phenothiazine analogues are described. Moreover, their efficacy as antioxidants and their ability to increase FXN are described. Finally, the synthesis of a reduced salt form of one analogue and its ability to cross the blood brain barrier (BBB) in mouse models of the disease is also described.
In Chapter 3, to discover potent neuroprotective drugs, a pair of regioisomeric benzoquinone analogues has been synthesized. The compounds were tested for their efficacy as antioxidants. Additionally, two pyrimidinol based redox cores were analyzed electrochemically to enable a better understanding of the mechanism of action of the multifunctional radical quencher (MRQ) class of antioxidants.
FRDA is a progressive neurodegenerative disease caused by insufficient expression of frataxin (FXN). FXN is instrumental in the assembly of iron-sulfur clusters, which in turn are critical for the functioning of the ETC enzyme complexes. Therapeutic agents which, in addition to being antioxidants also increase FXN, can be good drugs to counter FRDA. In Chapter 2, the synthesis of phenothiazine analogues are described. Moreover, their efficacy as antioxidants and their ability to increase FXN are described. Finally, the synthesis of a reduced salt form of one analogue and its ability to cross the blood brain barrier (BBB) in mouse models of the disease is also described.
In Chapter 3, to discover potent neuroprotective drugs, a pair of regioisomeric benzoquinone analogues has been synthesized. The compounds were tested for their efficacy as antioxidants. Additionally, two pyrimidinol based redox cores were analyzed electrochemically to enable a better understanding of the mechanism of action of the multifunctional radical quencher (MRQ) class of antioxidants.
ContributorsBandyopadhyay, Indrajit (Author) / Hecht, Sidney M. (Thesis advisor) / Gould, Ian R (Committee member) / Trovitch, Ryan (Committee member) / Arizona State University (Publisher)
Created2019
Description
A vast amount of energy emanates from the sun, and at the distance of Earth, approximately 172,500 TW reaches the atmosphere. Of that, 80,600 TW reaches the surface with 15,600 TW falling on land. Photosynthesis converts 156 TW in the form of biomass, which represents all food/fuel for the biosphere with about 20 TW of the total product used by humans. Additionally, our society uses approximately 20 more TW of energy from ancient photosynthetic products i.e. fossil fuels. In order to mitigate climate problems, the carbon dioxide must be removed from the human energy usage by replacement or recycling as an energy carrier. Proposals have been made to process biomass into biofuels; this work demonstrates that current efficiencies of natural photosynthesis are inadequate for this purpose, the effects of fossil fuel replacement with biofuels is ecologically irresponsible, and new technologies are required to operate at sufficient efficiencies to utilize artificial solar-to-fuels systems. Herein a hybrid bioderived self-assembling hydrogen-evolving nanoparticle consisting of photosystem I (PSI) and platinum nanoclusters is demonstrated to operate with an overall efficiency of 6%, which exceeds that of land plants by more than an order of magnitude. The system was limited by the rate of electron donation to photooxidized PSI. Further work investigated the interactions of natural donor acceptor pairs of cytochrome c6 and PSI for the thermophilic cyanobacteria Thermosynechococcus elogantus BP1 and the red alga Galderia sulphuraria. The cyanobacterial system is typified by collisional control while the algal system demonstrates a population of prebound PSI-cytochrome c6 complexes with faster electron transfer rates. Combining the stability of cyanobacterial PSI and kinetics of the algal PSI:cytochrome would result in more efficient solar-to-fuel conversion. A second priority is the replacement of platinum with chemically abundant catalysts. In this work, protein scaffolds are employed using host-guest strategies to increase the stability of proton reduction catalysts and enhance the turnover number without the oxygen sensitivity of hydrogenases. Finally, design of unnatural electron transfer proteins are explored and may introduce a bioorthogonal method of introducing alternative electron transfer pathways in vitro or in vivo in the case of engineered photosynthetic organisms.
ContributorsVaughn, Michael David (Author) / Moore, Thomas (Thesis advisor) / Fromme, Petra (Thesis advisor) / Ghirlanda, Giovanna (Committee member) / Redding, Kevin (Committee member) / Arizona State University (Publisher)
Created2014
Description
Redox reactions are crucial to energy transduction in biology. Protein film electrochemistry (PFE) is a technique for studying redox proteins in which the protein is immobilized at an electrode surface so as to allow direct exchange of electrons. Establishing a direct electronic connection eliminates the need for redoxactive mediators, thus allowing for interrogation of the redox protein of interest. PFE has proven a versatile tool that has been used to elucidate the properties of many technologically relevant redox proteins including hydrogenases, laccases, and glucose oxidase.
This dissertation is comprised of two parts: extension of PFE to a novel electrode material and application of PFE to the investigation of a new type of hydrogenase. In the first part, mesoporous antimony-doped tin oxide (ATO) is employed for the first time as an electrode material for protein film electrochemistry. Taking advantage of the excellent optical transparency of ATO, spectroelectrochemistry of cytochrome c is demonstrated. The electrochemical and spectroscopic properties of the protein are analogous to those measured for the native protein in solution, and the immobilized protein is stable for weeks at high loadings. In the second part, PFE is used to characterize the catalytic properties of the soluble hydrogenase I from Pyrococcus furiosus (PfSHI). Since this protein is highly thermostable, the temperature dependence of catalytic properties was investigated. I show that the preference of the enzyme for reduction of protons (as opposed to oxidation of hydrogen) and the reactions with oxygen are highly dependent on temperature, and the enzyme is tolerant to oxygen during both oxidative and reductive catalysis.
This dissertation is comprised of two parts: extension of PFE to a novel electrode material and application of PFE to the investigation of a new type of hydrogenase. In the first part, mesoporous antimony-doped tin oxide (ATO) is employed for the first time as an electrode material for protein film electrochemistry. Taking advantage of the excellent optical transparency of ATO, spectroelectrochemistry of cytochrome c is demonstrated. The electrochemical and spectroscopic properties of the protein are analogous to those measured for the native protein in solution, and the immobilized protein is stable for weeks at high loadings. In the second part, PFE is used to characterize the catalytic properties of the soluble hydrogenase I from Pyrococcus furiosus (PfSHI). Since this protein is highly thermostable, the temperature dependence of catalytic properties was investigated. I show that the preference of the enzyme for reduction of protons (as opposed to oxidation of hydrogen) and the reactions with oxygen are highly dependent on temperature, and the enzyme is tolerant to oxygen during both oxidative and reductive catalysis.
ContributorsKwan, Patrick Karchung (Author) / Jones, Anne K (Thesis advisor) / Francisco, Wilson (Committee member) / Moore, Thomas (Committee member) / Arizona State University (Publisher)
Created2014
Description
Since the discovery of graphene, two dimensional materials (2D materials) have become a focus of interest for material research due to their many unique physical properties embedded in their 2D structure. While they host many exciting potential applications, some of these 2D materials are subject to environmental instability issues induced by interaction between material and gas molecules in air, which poses a barrier to further application and manufacture. To overcome this, it is necessary to understand the origin of material instability and interaction with molecules commonly found in air, as well as developing a reproducible and manufacturing compatible method to post-process these materials to extend their lifetime. In this work, the very first investigation on environmental stability on Te containing anisotropic 2D materials such as GaTe and ZrTe3 is reported. Experimental results have demonstrated that freshly exfoliated GaTe quickly deteriorate in air, during which the Raman spectrum, surface morphology, and surface chemistry undergo drastic changes. Environmental Raman spectroscopy and XPS measurements demonstrate that H2O molecules in air interact strongly on the surface while O2, N2, and inert gases don't show any detrimental effects on GaTe surface. Moreover, the anisotropic properties of GaTe slowly disappear during the aging process. To prevent this gas/material interaction based surface transformation, diazonium based surface functionalization is adopted on these Te based 2D materials. Environmental Raman spectroscopy results demonstrate that the stability of functionalized Te based 2D materials exhibit much higher stability both in ambient and extreme conditions. Meanwhile, PL spectroscopy, angle resolved Raman spectroscopy, atomic force microscopy measurements confirm that many attractive physical properties of the material are not affected by surface functionalization. Overall, these findings unveil the degradation mechanism of Te based 2D materials as well as provide a way to significantly enhance their environmental stability through an inexpensive and reproducible surface chemical functionalization route.
ContributorsYang, Sijie (Author) / Tongay, Sefaattin (Thesis advisor) / Gould, Ian (Thesis advisor) / Trovitch, Ryan (Committee member) / Ghirlanda, Giovanna (Committee member) / Arizona State University (Publisher)
Created2017