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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
[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
Thiol functionalization is one potentially useful way to tailor physical and chemical properties of graphene oxides (GOs) and reduced graphene oxides (RGOs). Despite the ubiquitous presence of thiol functional groups in diverse chemical systems, efficient thiol functionalization has been challenging for GOs and RGOs, or for carbonaceous materials in general. In this work, thionation of GOs has been achieved in high yield through two new methods that also allow concomitant chemical reduction/thermal reduction of GOs; a solid-gas metathetical reaction method with boron sulfides (BxSy) gases and a solvothermal reaction method employing phosphorus decasulfide (P4S10). The thionation products, called "mercapto reduced graphene oxides (m-RGOs)", were characterized by employing X-ray photoelectron spectroscopy, powder X-ray diffraction, UV-Vis spectroscopy, FT-IR spectroscopy, Raman spectroscopy, electron probe analysis, scanning electron microscopy, (scanning) transmission electron microscopy, nano secondary ion mass spectrometry, Ellman assay and atomic force microscopy. The excellent dispersibility of m-RGOs in various solvents including alcohols has allowed fabrication of thin films of m-RGOs. Deposition of m-RGOs on gold substrates was achieved through solution deposition and the m-RGOs were homogeneously distributed on gold surface shown by atomic force microscopy. Langmuir-Blodgett (LB) films of m-RGOs were obtained by transferring their Langmuir films, formed by simple drop casting of m-RGOs dispersion on water surface, onto various substrates including gold, glass and indium tin oxide. The m-RGO LB films showed low sheet resistances down to about 500 kΩ/sq at 92% optical transparency. The successful results make m-RGOs promising for applications in transparent conductive coatings, biosensing, etc.
ContributorsJeon, Kiwan (Author) / Seo, Dong-Kyun (Thesis advisor) / Jones, Anne K (Committee member) / Yarger, Jeffery (Committee member) / Arizona State University (Publisher)
Created2013
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
Increased global demand for energy has led to prolific use of fossil fuels, which produce and release greenhouse gases, such as carbon dioxide. This increase in atmospheric carbon dioxide affects the global weather system and has been cited as a cause for global warming. For humans to continue to meet demands for energy while reducing greenhouse emission, a sustainable, carbon-neutral energy source must be developed. The sun provides energy for the majority of life on earth, as well as the energy stored in the chemical bonds of fossil fuels. This dissertation investigates systems inspired by the biological mechanism of solar energy capture and storage. In natural photosynthesis, organisms use chlorophyll as a chromophore to absorb the sun's energy. Bio-inspired systems use close analogues like porphyrins and phthalocyanines. In this dissertation, a soluble, semiconducting porphyrin is reported. The polymer was synthesized via a Buchwald-Hartwig style coupling of porphyrin monomers which produced a polyaniline-like chain with porphyrins incorporated into the backbone. Spectroscopic and electrochemical studies were performed, which show evidence of excited state charge transfer and a first oxidation state of 0.58 V (vs SCE). These properties suggest that the polymer could be involved in excited state electron donation to fullerenes and other electron acceptors, which could be beneficial in organic photovoltaics, sensors, and other applications. Molecular dyads and triads capable of charge separation have been studied for decades, and the spectroscopic properties of two novel systems are reported in this dissertation. A peripherally-connected zinc-phthalocyanine-C60 dyad was studied, and showed excited state electron transfer from the phthalocyanine excited state to the C60, with a long-lived charge separated state. An axially-linked carotene-Si-pthalocyanine-C60 triad was studied, showing excited state electron transfer from the phthalocyanine to the C60, but fast recombination before hole transfer can occur to the carotene. Analogues of the electron transport mechanisms used in many biological systems use iron-sulfur clusters to shuttle electrons from donors to acceptors. In this dissertation, the spectroscopic properties of a de novo protein were studied. Nanosecond transient absorption was used to characterize the electron and energy transfer of an excited water-soluble porphyrin to the oxidized [FeS] clusters incorporated in the de novo protein. The triplet state of the porphyrin was strongly quenched with the holo-protein without a rise in porphyrin plus signal, suggesting that only Dexter-type energy transfer occurs between the sensitized porphyrin and the [FeS] clusters.
ContributorsSchmitz, Robert (Author) / Gust, John D (Thesis advisor) / Jones, Anne K (Committee member) / Buttry, Daniel (Committee member) / Arizona State University (Publisher)
Created2014
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
Molybdenum (Mo) is a key trace nutrient for biological assimilation of nitrogen, either as nitrogen gas (N2) or nitrate (NO3-). Although Mo is the most abundant metal in seawater (105 nM), its concentration is low (<5 nM) in most freshwaters today, and it was scarce in the ocean before 600 million years ago. The use of Mo for nitrogen assimilation can be understood in terms of the changing Mo availability through time; for instance, the higher Mo content of eukaryotic vs. prokaryotic nitrate reductase may have stalled proliferation of eukaryotes in low-Mo Proterozoic oceans. Field and laboratory experiments were performed to study Mo requirements for NO3- assimilation and N2 fixation, respectively. Molybdenum-nitrate addition experiments at Castle Lake, California revealed interannual and depth variability in plankton community response, perhaps resulting from differences in species composition and/or ammonium availability. Furthermore, lake sediments were elevated in Mo compared to soils and bedrock in the watershed. Box modeling suggested that the largest source of Mo to the lake was particulate matter from the watershed. Month-long laboratory experiments with heterocystous cyanobacteria (HC) showed that <1 nM Mo led to low N2 fixation rates, while 10 nM Mo was sufficient for optimal rates. At 1500 nM Mo, freshwater HC hyperaccumulated Mo intercellularly, whereas coastal HC did not. These differences in storage capacity were likely due to the presence in freshwater HC of the small molybdate-binding protein, Mop, and its absence in coastal and marine cyanobacterial species. Expression of the mop gene was regulated by Mo availability in the freshwater HC species Nostoc sp. PCC 7120. Under low Mo (<1 nM) conditions, mop gene expression was up-regulated compared to higher Mo (150 and 3000 nM) treatments, but the subunit composition of the Mop protein changed, suggesting that Mop does not bind Mo in the same manner at <1 nM Mo that it can at higher Mo concentrations. These findings support a role for Mop as a Mo storage protein in HC and suggest that freshwater HC control Mo cellular homeostasis at the post-translational level. Mop's widespread distribution in prokaryotes lends support to the theory that it may be an ancient protein inherited from low-Mo Precambrian oceans.
ContributorsGlass, Jennifer (Author) / Anbar, Ariel D (Thesis advisor) / Shock, Everett L (Committee member) / Jones, Anne K (Committee member) / Hartnett, Hilairy E (Committee member) / Elser, James J (Committee member) / Fromme, Petra (Committee member) / Arizona State University (Publisher)
Created2011
Description
Hydrogenases, the enzymes that reversibly convert protons and electrons to hydrogen, are used in all three domains of life. [NiFe]-hydrogenases are considered best suited for biotechnological applications because of their reversible inactivation with oxygen. Phylogenetically, there are four groups of [NiFe]-hydrogenases. The best characterized group, "uptake" hydrogenases, are membrane-bound and catalyze hydrogen oxidation in vivo. In contrast, the group 3 [NiFe]-hydrogenases are heteromultimeric, bifunctional enzymes that fulfill various cellular roles. In this dissertation, protein film electrochemistry (PFE) is used to characterize the catalytic properties of two group 3 [NiFe]-hydrogenases: HoxEFUYH from Synechocystsis sp. PCC 6803 and SHI from Pyrococcus furiosus. First, HoxEFUYH is shown to be biased towards hydrogen production. Upon exposure to oxygen, HoxEFUYH inactivates to two states, both of which can be reactivated on the timescale of seconds. Second, we show that PfSHI is the first example of an oxygen tolerant [NiFe]-hydrogenase that produces two inactive states upon exposure to oxygen. Both inactive states are analogous to those characterized for HoxEFUYH, but oxygen exposed PfSHI produces a greater fraction that reactivates at high potentials, enabling hydrogen oxidation in the presence of oxygen. Third, it is shown that removing the NAD(P)-reducing subunits from PfSHI leads to a decrease in bias towards hydrogen oxidation and renders the enzyme oxygen sensitive. Both traits are likely due to impaired intramolecular electron transfer. Mechanistic hypotheseses for these functional differences are considered.
ContributorsMcIntosh, Chelsea Lee (Author) / Jones, Anne K (Thesis advisor) / Ghirlanda, Giovanna (Committee member) / Buttry, Daniel (Committee member) / Arizona State University (Publisher)
Created2012
Description
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is widely accepted as the world's most abundant enzyme and represents the primary entry point for inorganic carbon into the biosphere. Rubisco's slow carboxylation rate of ribulose-1,5-bisphosphate (RuBP) and its susceptibility to inhibition has led some to term it the "bottle neck" of photosynthesis. In order to ensure that Rubisco remains uninhibited, plants require the catalytic chaperone Rubisco activase. Activase is a member of the AAA+ superfamily, ATPases associated with various cellular activities, and uses ATP hydrolysis as the driving force behind a conformational movement that returns activity to inhibited Rubisco active sites. A high resolution activase structure will be an essential tool for examining Rubisco/activase interactions as well as understanding the activase self-association phenomenon. Rubisco activase has long eluded crystallization, likely due to its infamous self-association (polydispersity). Therefore, a limited proteolysis approach was taken to identify soluble activase subdomains as potential crystallization targets. This process involves using proteolytic enzymes to cleave a protein into a few pieces and has previously proven successful in identifying crystallizable protein fragments. Limited proteolysis, utilizing two different proteolytic enzymes (alpha-chymotrypsin and trypsin), identified two tobacco activase products. The fragments that were identified appear to represent most of what is considered to be the AAA+ C-terminal all alpha-domain and some of the AAA+ N-terminal alpha beta alpha-domain. Identified fragments were cloned using the pET151/dTOPO. The project then moved towards cloning and recombinant protein expression in E. coli. NtAbeta(248-383) and NtAbeta(253-354) were successfully cloned, expressed, purified, and characterized through various biophysical techniques. A thermofluor assay of NtAbeta(248-383) revealed a melting temperature of about 30°C, indicating lower thermal stability compared with full-length activase at 43°C. Size exclusion chromatography suggested that NtAbeta(248-383) is monomeric. Circular dichroism was used to identify the secondary structure; a plurality of alpha-helices. NtAbeta(248-383) and NtAbeta(253-354) were subjected to crystallization trials.
ContributorsConrad, Alan (Author) / Wachter, Rebekka (Thesis advisor) / Moore, Thomas (Committee member) / Redding, Kevin (Committee member) / Arizona State University (Publisher)
Created2012