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Description
An evolving understanding of elastomeric polymer nanocomposites continues to expand commercial, defense, and industrial products and applications. This work explores the thermomechanical properties of elastomeric nanocomposites prepared from bisphenol A diglycidyl ether (BADGE) and three amine-terminated poly(propylene oxides) (Jeffamines). The Jeffamines investigated include difunctional crosslinkers with molecular weights of 2,000 and 4,000 g/mol and a trifunctional crosslinker with a molecular weight of 3,000 g/mol. Additionally, carbon nanotubes (CNTs) were added, up to 1.25 wt%, to each thermoset. The findings indicate that the Tg and storage modulus of the polymer nanocomposites can be controlled independently within narrow concentration windows, and that effects observed following CNT incorporation are dependent on the crosslinker molecular weight.
Polymer matrix composites (PMCs) offer design solutions to produce smart sensing, conductive, or high performance composites for a number of critical applications. Nanoparticle additives, in particular, carbon nanotubes and metallic quantum dots, have been investigated for their ability to improve the conductivity, thermal stability, and mechanical strength of traditional composites. Herein we report the use of quantum dots (QDs) and fluorescently labeled carbon nanotubes (CNTs) to modify the thermomechanical properties of PMCs. Additionally, we find that pronounced changes in fluorescence emerge following plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the fluorescence occurs in response to mechanical activation.
Segmented ionenes are a class of thermoplastic elastomers that contain a permanent charged group within the polymer backbone and a spacer segment with a low glass transition temperature (Tg) to provide flexibility. Ionenes are of interest because of their synthetic versatility, unique morphologies, and ionic nature. Using phase changing ionene-based nanocomposites could be extended to create reversible mechanically, electrically, optically, and/or thermally responsive materials depending on constituent nanoparticles and polymers. This talk will discuss recent efforts to utilize the synthetic versatility of ionenes (e.g., spacer composition of PTMO or PEG) to prepare percolated ionic domains in microphase separated polymers that display a range of thermomechanical properties. Furthermore, by synthesizing two series of ionene copolymers with either PEG or PTMO spacers at various ratios with 1,12-dibromododecane will yield a range of ion contents (hard contents) and will impact nanoparticle dispersion.
Polymer matrix composites (PMCs) offer design solutions to produce smart sensing, conductive, or high performance composites for a number of critical applications. Nanoparticle additives, in particular, carbon nanotubes and metallic quantum dots, have been investigated for their ability to improve the conductivity, thermal stability, and mechanical strength of traditional composites. Herein we report the use of quantum dots (QDs) and fluorescently labeled carbon nanotubes (CNTs) to modify the thermomechanical properties of PMCs. Additionally, we find that pronounced changes in fluorescence emerge following plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the fluorescence occurs in response to mechanical activation.
Segmented ionenes are a class of thermoplastic elastomers that contain a permanent charged group within the polymer backbone and a spacer segment with a low glass transition temperature (Tg) to provide flexibility. Ionenes are of interest because of their synthetic versatility, unique morphologies, and ionic nature. Using phase changing ionene-based nanocomposites could be extended to create reversible mechanically, electrically, optically, and/or thermally responsive materials depending on constituent nanoparticles and polymers. This talk will discuss recent efforts to utilize the synthetic versatility of ionenes (e.g., spacer composition of PTMO or PEG) to prepare percolated ionic domains in microphase separated polymers that display a range of thermomechanical properties. Furthermore, by synthesizing two series of ionene copolymers with either PEG or PTMO spacers at various ratios with 1,12-dibromododecane will yield a range of ion contents (hard contents) and will impact nanoparticle dispersion.
ContributorsWang, Meng, Ph.D (Author) / Green, Matthew D (Thesis advisor) / Green, Alexander (Committee member) / Yarger, Jeffery (Committee member) / Arizona State University (Publisher)
Created2019
Temperature and polarizability effects on electron transfer in biology and artificial photosynthesis
Description
This study aims to address the deficiencies of the Marcus model of electron transfer
(ET) and then provide modifications to the model. A confirmation of the inverted energy
gap law, which is the cleanest verification so far, is presented for donor-acceptor complexes.
In addition to the macroscopic properties of the solvent, the physical properties of the solvent
are incorporated in the model via the microscopic solvation model. For the molecules
studied in this dissertation, the rate constant first increases with cooling, in contrast to the
prediction of the Arrhenius law, and then decreases at lower temperatures. Additionally,
the polarizability of solute, which was not considered in the original Marcus theory, is included
by the Q-model of ET. Through accounting for the polarizability of the reactants, the
Q-model offers an important design principle for achieving high performance solar energy
conversion materials. By means of the analytical Q-model of ET, it is shown that including
molecular polarizability of C60 affects the reorganization energy and the activation barrier
of ET reaction.
The theory and Electrochemistry of Ferredoxin and Cytochrome c are also investigated.
By providing a new formulation for reaction reorganization energy, a long-standing disconnect
between the results of atomistic simulations and cyclic voltametery experiments is
resolved. The significant role of polarizability of enzymes in reducing the activation energy
of ET is discussed. The binding/unbinding of waters to the active site of Ferredoxin leads
to non-Gaussian statistics of energy gap and result in a smaller activation energy of ET.
Furthermore, the dielectric constant of water at the interface of neutral and charged
C60 is studied. The dielectric constant is found to be in the range of 10 to 22 which is
remarkably smaller compared to bulk water( 80). Moreover, the interfacial structural
crossover and hydration thermodynamic of charged C60 in water is studied. Increasing the
charge of the C60 molecule result in a dramatic structural transition in the hydration shell,
which lead to increase in the population of dangling O-H bonds at the interface.
(ET) and then provide modifications to the model. A confirmation of the inverted energy
gap law, which is the cleanest verification so far, is presented for donor-acceptor complexes.
In addition to the macroscopic properties of the solvent, the physical properties of the solvent
are incorporated in the model via the microscopic solvation model. For the molecules
studied in this dissertation, the rate constant first increases with cooling, in contrast to the
prediction of the Arrhenius law, and then decreases at lower temperatures. Additionally,
the polarizability of solute, which was not considered in the original Marcus theory, is included
by the Q-model of ET. Through accounting for the polarizability of the reactants, the
Q-model offers an important design principle for achieving high performance solar energy
conversion materials. By means of the analytical Q-model of ET, it is shown that including
molecular polarizability of C60 affects the reorganization energy and the activation barrier
of ET reaction.
The theory and Electrochemistry of Ferredoxin and Cytochrome c are also investigated.
By providing a new formulation for reaction reorganization energy, a long-standing disconnect
between the results of atomistic simulations and cyclic voltametery experiments is
resolved. The significant role of polarizability of enzymes in reducing the activation energy
of ET is discussed. The binding/unbinding of waters to the active site of Ferredoxin leads
to non-Gaussian statistics of energy gap and result in a smaller activation energy of ET.
Furthermore, the dielectric constant of water at the interface of neutral and charged
C60 is studied. The dielectric constant is found to be in the range of 10 to 22 which is
remarkably smaller compared to bulk water( 80). Moreover, the interfacial structural
crossover and hydration thermodynamic of charged C60 in water is studied. Increasing the
charge of the C60 molecule result in a dramatic structural transition in the hydration shell,
which lead to increase in the population of dangling O-H bonds at the interface.
ContributorsWaskasi, Morteza M (Author) / Matyushov, Dmitry (Thesis advisor) / Richert, Ranko (Committee member) / Heyden, Matthias (Committee member) / Beckstein, Oliver (Committee member) / Arizona State University (Publisher)
Created2019
Description
Nanostructured zeolites, in particular nanocrystalline zeolites, are of great interest due to their efficient use in conventional catalysis, separations, and emerging applications. Despite the recent advances, fewer than 20 zeolite framework types have been synthesized in the form of nanocrystallites and their scalable synthesis has yet to be developed and understood. Geopolymers, claimed to be “amorphous cousins of zeolites”, are a class of ceramic-like aluminosilicate materials with prominent application in construction due to their unique chemical and mechanical properties. Despite the monolith form, geopolymers are fundamentally nanostructured materials and contain zeolite nanocrystallites.
Herein, a new cost-effective and scalable synthesis of various types of nanocrystalline zeolites based on geopolymer chemistry is presented. The study includes the synthesis of highly crystalline discrete nanorods of a CAN zeolite framework structure that had not been achieved hitherto, the exploration of the Na−Al−Si−H2O kinetic phase diagram of hydrogels that gives SOD, CAN and FAU nanocrystalline zeolites, and the discovery of a unique formation mechanism of highly crystalline nanostructured FAU zeolite with intermediate gel products that possess an unprecedented uniform distribution of elements. This study demonstrated the possibility of using high-concentration hydrogels for the synthesis of nanocrystalline zeolites of additional framework structures.
Moreover, a comprehensive study on nanostructured FAU zeolites ion-exchanged with Ag+, Zn2+, Cu2+ and Fe2+ for antibacterial applications is presented, which comprises metal ion release kinetics, antibacterial properties, and cytotoxicity. For the first time, superior metal ion release performance was confirmed for the nanostructured zeolites compared to their micron-sized counterparts. The metal ion-exchanged FAU nanostructured zeolites were established as new effective antibacterial materials featuring their unique physiochemical, antibacterial, and cytotoxic properties.
Herein, a new cost-effective and scalable synthesis of various types of nanocrystalline zeolites based on geopolymer chemistry is presented. The study includes the synthesis of highly crystalline discrete nanorods of a CAN zeolite framework structure that had not been achieved hitherto, the exploration of the Na−Al−Si−H2O kinetic phase diagram of hydrogels that gives SOD, CAN and FAU nanocrystalline zeolites, and the discovery of a unique formation mechanism of highly crystalline nanostructured FAU zeolite with intermediate gel products that possess an unprecedented uniform distribution of elements. This study demonstrated the possibility of using high-concentration hydrogels for the synthesis of nanocrystalline zeolites of additional framework structures.
Moreover, a comprehensive study on nanostructured FAU zeolites ion-exchanged with Ag+, Zn2+, Cu2+ and Fe2+ for antibacterial applications is presented, which comprises metal ion release kinetics, antibacterial properties, and cytotoxicity. For the first time, superior metal ion release performance was confirmed for the nanostructured zeolites compared to their micron-sized counterparts. The metal ion-exchanged FAU nanostructured zeolites were established as new effective antibacterial materials featuring their unique physiochemical, antibacterial, and cytotoxic properties.
ContributorsChen, Shaojiang (Author) / Seo, Dong Kyun (Thesis advisor) / Trovitch, Ryan (Committee member) / Thomas, MaryLaura Lind (Committee member) / Arizona State University (Publisher)
Created2019
Description
There is increasing interest and demand in biology studies for identifying and characterizing rare cells or bioparticle subtypes. These subpopulations demonstrate special function, as examples, in multipotent proliferation, immune system response, and cancer diagnosis. Current techniques for separation and identification of these targets lack the accuracy and sensitivity needed to interrogate the complex and diverse bioparticle mixtures. High resolution separations of unlabeled and unaltered cells is an emerging capability. In particular, electric field-driven punctuated microgradient separations have shown high resolution separations of bioparticles. These separations are based on biophysical properties of the un-altered bioparticles. Here, the properties of the bioparticles were identified by ratio of electrokinetic (EK) to dielectrophoretic (DEP) mobilities.
As part of this dissertation, high-resolution separations have been applied to neural stem and progenitor cells (NSPCs). The abundance of NSPCs captured with different range of ratio of EK to DEP mobilities are consistent with the final fate trends of the populations. This supports the idea of unbiased and unlabeled high-resolution separation of NSPCs to specific fates is possible. In addition, a new strategy to generate reproducible subpopulations using varied applied potential were employed for studying insulin vesicles from beta cells. The isolated subpopulations demonstrated that the insulin vesicles are heterogenous and showed different distribution of mobility ratios when compared with glucose treated insulin vesicles. This is consistent with existing vesicle density and local concentration data. Furthermore, proteins, which are accepted as challenging small bioparticles to be captured by electrophysical method, were concentrated by this technique. Proteins including IgG, lysozyme, alpha-chymotrypsinogen A were differentiated and characterized with the ratio factor. An extremely narrow bandwidth and high resolution characterization technique, which is experimentally simple and fast, has been developed for proteins. Finally, the native whole cell separation technique has also been applied for Salmonella serotype identification and differentiation for the first time. The technique generated full differentiation of four serotypes of Salmonella. These works may lead to a less expensive and more decentralized new tool and method for transplantation, proteomics, basic research, and microbiologists, working in parallel with other characterization methods.
As part of this dissertation, high-resolution separations have been applied to neural stem and progenitor cells (NSPCs). The abundance of NSPCs captured with different range of ratio of EK to DEP mobilities are consistent with the final fate trends of the populations. This supports the idea of unbiased and unlabeled high-resolution separation of NSPCs to specific fates is possible. In addition, a new strategy to generate reproducible subpopulations using varied applied potential were employed for studying insulin vesicles from beta cells. The isolated subpopulations demonstrated that the insulin vesicles are heterogenous and showed different distribution of mobility ratios when compared with glucose treated insulin vesicles. This is consistent with existing vesicle density and local concentration data. Furthermore, proteins, which are accepted as challenging small bioparticles to be captured by electrophysical method, were concentrated by this technique. Proteins including IgG, lysozyme, alpha-chymotrypsinogen A were differentiated and characterized with the ratio factor. An extremely narrow bandwidth and high resolution characterization technique, which is experimentally simple and fast, has been developed for proteins. Finally, the native whole cell separation technique has also been applied for Salmonella serotype identification and differentiation for the first time. The technique generated full differentiation of four serotypes of Salmonella. These works may lead to a less expensive and more decentralized new tool and method for transplantation, proteomics, basic research, and microbiologists, working in parallel with other characterization methods.
ContributorsLiu, Yameng (Author) / Hayes, Mark A. (Thesis advisor) / Wang, Xu (Committee member) / Borges, Chad (Committee member) / Arizona State University (Publisher)
Created2020
Eradication of multidrug-resistant bacteria using biomolecule-encapsulated two-dimensional materials
Description
The increasing pervasiveness of infections caused by multidrug-resistant bacteria (MDR) is a major global health issue that has been further exacerbated by the dearth of antibiotics developed over the past 40 years. Drug-resistant bacteria have led to significant morbidity and mortality, and ever-increasing antibiotic resistance threatens to reverse many of the medical advances enabled by antibiotics over the last 40 years. The traditional strategy for combating these superbugs involves the development of new antibiotics. Yet, only two new classes of antibiotics have been introduced to the clinic over the past two decades, and both failed to combat broad spectrum gram-negative bacteria. This situation demands alternative strategies to combat drug-resistant superbugs. Herein, these dissertation reports the development of potent antibacterials based on biomolecule-encapsulated two-dimensional inorganic materials, which combat multidrug-resistant bacteria using alternative mechanisms of strong physical interactions with bacterial cell membrane. These systems successfully eliminate all members of the ‘Superbugs’ set of pathogenic bacteria, which are known for developing antibiotic resistance, providing an alternative to the limited ‘one bug-one drug’ approach that is conventionally used. Furthermore, these systems demonstrate a multimodal antibacterial killing mechanism that induces outer membrane destabilization, unregulated ion movement across the membranes, induction of oxidative stress, and finally apoptotic-like cell death. In addition, a peptide-encapsulation of the two-dimensional material successfully eliminated biofilms and persisters at micromolar concentrations. Overall, these novel systems have great potential as next-generation antimicrobial agents for eradication of broad spectrum multidrug-resistant bacteria.
ContributorsDebnath, Abhishek (Author) / Green, Alexander A (Thesis advisor) / Liu, Yan (Committee member) / Stephanopoulos, Nicholas (Committee member) / Arizona State University (Publisher)
Created2019
Description
Chemical modification of (semi)conducting surfaces with soft-material coatings containing electrocatalysts provides a strategy for developing integrated constructs that capture, convert, and store solar energy as fuels. However, a lack of effective strategies for interfacing electrocatalysts with solid-state materials, and an incomplete understanding of performance limiting factors, inhibit further development. In this work, chemical modification of a nanostructured transparent conductive oxide, and the III-V semiconductor, gallium phosphide, is achieved by applying a thin-film polymer coating containing appropriate functional groups to direct, template, and assemble molecular cobalt catalysts for activating fuel-forming reactions. The heterogeneous-homogeneous conducting assemblies enable comparisons of the structural and electrochemical properties of these materials with their homogeneous electrocatalytic counterparts. For these hybrid constructs, rational design of the local soft-material environment yields a nearly one-volt span in the redox chemistry of the cobalt metal centers. Further, assessment of the interplay between light absorption, charge transfer, and catalytic activity in studies involving molecular-catalyst-modified semiconductors affords models to describe the rates of photoelectrosynthetic fuel production as a function of the steady-state concentration of catalysts present in their activated form. These models provide a conceptual framework for extracting kinetic and thermodynamic benchmarking parameters. Finally, investigation of molecular ‘proton wires’ inspired by the Tyrosine Z-Histidine 190 redox pair in Photosystem II, provides insight into fundamental principles governing proton-coupled electron transfer, a process essential to all fuel-forming reactions relevant to solar fuel generation.
ContributorsWadsworth, Brian Lawrence (Author) / Moore, Gary F (Thesis advisor) / Moore, Thomas A. (Committee member) / Trovitch, Ryan J (Committee member) / Arizona State University (Publisher)
Created2020
Description
Global industrialization and urbanization have led to increased levels of air pollution. The costs to society have come in the form of environmental damage, healthcare expenses, lost productivity, and premature mortality. Measuring pollutants is an important task for identifying its sources, warning individuals about dangerous exposure levels, and providing epidemiologists with data to link pollutants with diseases. Current methods for monitoring air pollution are inadequate though. They rely on expensive, complex instrumentation at limited fixed monitoring sites that do not capture the true spatial and temporal variation. Furthermore, the fixed outdoor monitoring sites cannot warn individuals about indoor air quality or exposure to chemicals at worksites. Recent advances in manufacturing and computing technology have allowed new classes of low-cost miniature gas sensor to emerge as possible alternatives. For these to be successful however, there must be innovations in the sensors themselves that improve reliability, operation, and their stability and selectivity in real environments. Three novel gas sensor solutions are presented. The first is the development of a wearable personal exposure monitor using all commercially available components, including two metal oxide semiconductor gas sensors. The device monitors known asthma triggers: ozone, total volatile organic compounds, temperature, humidity, and activity level. Primary focus is placed on the ozone sensor, which requires special circuits, heating algorithm, and calibration to remove temperature and humidity interferences. Eight devices are tested in multiple field tests. The second is the creation of a new compact optoelectronic gas sensing platform using colorimetric microdroplets printed on the surface of a complementary-metal-oxide-semiconductor (CMOS) imager. The nonvolatile liquid microdroplets provide a homogeneous, uniform environment that is ideal for colorimetric reactions and lensless optical measurements. To demonstrate one type of possible indicating system gaseous ammonia is detected by complexation with Cu(II). The third project continues work on the CMOS imager optoelectronic platform and develops a more robust sensing system utilizing hydrophobic aerogel particles. Ammonia is detected colorimetrically by its reaction with a molecular dye, with additives and surface treatments enhancing uniformity of the printed films. Future work presented at the end describes a new biological particle sensing system using the CMOS imager.
ContributorsMallires, Kyle Reed (Author) / Tao, Nongjian (Thesis advisor) / Forzani, Erica (Thesis advisor) / Wiktor, Peter (Committee member) / Wang, Di (Committee member) / Alford, Terry (Committee member) / Xian, Xiaojun (Committee member) / Arizona State University (Publisher)
Created2020
Description当前，民营企业已成为中国重要支撑力量，而未来5到10年，约有300多万家民营企业面临传承困境。但学术研究领域在传承整体框架、配套机制建设方面有完整论述、有成功案例的所见不多。首先，针对以上民营企业的传承现状，本文将研究、回答五个问题：1、成功传承的标准和要素是什么？2、传承模式有哪几种，每种模式配套的传承机制是什么，该如何建立？3、民营企业应选择何种传承模式，如何选择？4、民营企业的整套传承方案如何落地搭建？5、是否有普适性的、可借鉴的民营企业传承模型，包含哪些要素？
其次，本文主要使用文献研究、案例研究、实证分析，选取中、美、德、日四家不同传承阶段、不同传承模式的知名民营企业，对其传承情况进行深入研究。在此基础上，归纳总结出传承的关键要素，对前述五个问题进行系统解答。同时，本文创新性地结合理论研究、案例研究及企业实践，提出适合我国大部分民营企业的传承全周期管理框架。
最后，根据以上研究，本文总结出关于中国民营企业传承的八大结论及建议：1、本质：权力的交接和义务的传递；2、两大风险：继任风险（继任人的能力要求）、代理风险（继任人对企业核心理念的意愿/忠诚度）；3、降低风险的四大机制：领袖锻造、人才梯队、管控治理、激励机制；4、两大成功要素：“选领袖”和“建机制”；5、四大机制是并行推进、相辅相成的，要尽早构建、持续优化；6、三大模式：家族成员继承、内生培养经理人、外聘职业经理人；7、民营企业传承模型包含七大要素、五大步骤；8、民营企业在制定传承方案时，除了要注意传承模型中的要素，还要注意其他关键要素。
其次，本文主要使用文献研究、案例研究、实证分析，选取中、美、德、日四家不同传承阶段、不同传承模式的知名民营企业，对其传承情况进行深入研究。在此基础上，归纳总结出传承的关键要素，对前述五个问题进行系统解答。同时，本文创新性地结合理论研究、案例研究及企业实践，提出适合我国大部分民营企业的传承全周期管理框架。
最后，根据以上研究，本文总结出关于中国民营企业传承的八大结论及建议：1、本质：权力的交接和义务的传递；2、两大风险：继任风险（继任人的能力要求）、代理风险（继任人对企业核心理念的意愿/忠诚度）；3、降低风险的四大机制：领袖锻造、人才梯队、管控治理、激励机制；4、两大成功要素：“选领袖”和“建机制”；5、四大机制是并行推进、相辅相成的，要尽早构建、持续优化；6、三大模式：家族成员继承、内生培养经理人、外聘职业经理人；7、民营企业传承模型包含七大要素、五大步骤；8、民营企业在制定传承方案时，除了要注意传承模型中的要素，还要注意其他关键要素。
ContributorsCao, Jianwei (Author) / Huang, Xiaochuan (Thesis advisor) / Liang, Bing (Thesis advisor) / Cheng, Shijun (Committee member) / Arizona State University (Publisher)
Created2020
Description
Coronaviruses are the causative agents of SARS, MERS and the ongoing COVID-19 pandemic. Coronavirus envelope proteins have received increasing attention as drug targets, due to their multiple functional roles during the infection cycle. The murine coronavirus mouse hepatitis virus strain A59, a hepatic and neuronal tropic coronavirus, is considered a prototype of the betacoronaviruses. The envelope protein of the mouse hepatitis virus (MHV-E) was extensively screened with various membrane mimetics by solution state nuclear magnetic resonance spectroscopy to find a suitable mimetic, which allowed for assignment of ~97% of the backbone atoms in the transmembrane region. Following resonance assignments, the binding site of the ion channel inhibitor hexamethylene amiloride (HMA) was mapped to MHV-E using chemical shift perturbations in both amide and aromatic transverse relaxation optimized spectroscopy (TROSY) spectra, which indicated the inhibitor binding site is located at the N-terminal opening of the channel, in accord with one of the proposed HMA binding sites in the envelope protein from the related SARS (severe acute respiratory syndrome) betacoronavirus. Structure calculation of residues M1-K38 of MHV-E, encompassing the transmembrane region, is currently in progress using dihedral angle restraints obtained from isotropic chemical shifts and distance restraints obtained from manually assigned NOE cross-peaks, with the ultimate aim of generating a model of the MHV-E viroporin bound to the inhibitor HMA. This work outlines the first NMR studies on MHV-E, which have provided a foundation for structure based drug design and probing interactions, and the methods can be extended, with suitable modifications, to other coronavirus envelope proteins.
ContributorsBaravati, Bobby (Author) / Fromme, Petra (Thesis advisor) / Hansen, Debra (Thesis advisor) / Van Horn, Wade (Committee member) / Wang, Xu (Committee member) / Arizona State University (Publisher)
Created2020
Description
Oxidoreductases catalyze transformations important in both bioenergetics and microbial technologies. Nonetheless, questions remain about how to tune them to modulate properties such as preference for catalysis in the oxidative or reductive direction, the potential range of activity, or coupling of multiple reactions. Using protein film electrochemistry, the features that control these properties are defined by comparing the activities of five [FeFe]-hydrogenases and two thiosulfate reductases. Although [FeFe]-hydrogenases are largely described as hydrogen evolution catalysts, the catalytic bias of [FeFe]-hydrogenases, i.e. the ratio of maximal reductive to oxidative activities, spans more than six orders of magnitude. At one extreme, two [FeFe]-hdyrogenases, Clostridium pasteuriaunum HydAII and Clostridium symbiosum HydY, are far more active for hydrogen oxidation than hydrogen evolution. On the other extreme, Clostridium pasteurianum HydAI and Clostridium acetobutylicum HydA1 have a neutral bias, in which both proton reduction and hydrogen oxidation are efficient. By investigating a collection of site-directed mutants, it is shown that the catalytic bias of [FeFe]-hydrogenases is not trivially correlated with the identities of residues in the primary or secondary coordination sphere. On the other hand, the catalytic bias of Clostridium acetobutylicum HydAI can be modulated via mutation of an amino acid residue coordinating the terminal [FeS] cluster. Simulations suggest that this change in catalytic bias may be linked to the reduction potential of the cluster.
Two of the enzymes examined in this work, Clostridium pasteurianum HydAIII and Clostridium symbiosum HydY, display novel catalytic properties. HydY is exclusively a hydrogen oxidizing catalyst, and it couples this activity to peroxide reduction activity at a rubrerythrin center in the same enzyme. On the other hand, CpIII operates only in a narrow potential window, inactivating at oxidizing potentials. This suggests it plays a novel physiological role that has not yet been identified. Finally, the electrocatalytic properties of Pyrobaculum aerophilum thiosulfate reductase with either Mo or W in the active site are compared. In both cases, the onset of catalysis corresponds to reduction of the active site. Overall, the Mo enzyme is more active, and reduces thiosulfate with less overpotential.
Two of the enzymes examined in this work, Clostridium pasteurianum HydAIII and Clostridium symbiosum HydY, display novel catalytic properties. HydY is exclusively a hydrogen oxidizing catalyst, and it couples this activity to peroxide reduction activity at a rubrerythrin center in the same enzyme. On the other hand, CpIII operates only in a narrow potential window, inactivating at oxidizing potentials. This suggests it plays a novel physiological role that has not yet been identified. Finally, the electrocatalytic properties of Pyrobaculum aerophilum thiosulfate reductase with either Mo or W in the active site are compared. In both cases, the onset of catalysis corresponds to reduction of the active site. Overall, the Mo enzyme is more active, and reduces thiosulfate with less overpotential.
ContributorsWilliams, Samuel Garrett (Author) / Jones, Anne K (Thesis advisor) / Hayes, Mark A. (Committee member) / Trovitch, Ryan J (Committee member) / Arizona State University (Publisher)
Created2020