Immobilization of human carbonic anhydrase on gold nanoparticles assembled onto amine/thiol-functionalized mesoporous SBA-15 for biomimetic sequestration of CO2

Carbonic anhydrase as a tool to mitigate global warming

The global average temperature breaks the record every year, and this unprecedented speed at which it is unfolding is causing serious climate change which in turn impacts the lives of humans and other living organisms. Thus, it is imperative to take immediate action to limit global warming. Increased CO2 emission from the industrial sector that relies on fossil fuels is the major culprit. Mitigating global warming is an uphill battle that involves an integration of technologies such as switching to renewable energy, increasing the carbon sink capacity, and implementing carbon capture and sequestration (CCS) on major sources of CO2 emissions. Among all these methods, CCS is globally accepted as a potential technology to address this climate change. CCS using carbonic anhydrase (CA) is gaining momentum due to its advantages over other conventional CCS technologies. CA is a metalloenzyme that catalyses a fundamental reaction for life, i.e. the interconversion of bicarbonate and protons from carbon dioxide and water. The practical application of CA requires stable CAs operating under harsh operational conditions. CAs from extremophilic microbes are the potential candidates for the sequestration of CO2 and conversion into useful by-products. The soluble free form of CA is expensive, unstable, and non-reusable in an industrial setup. Immobilization of CA on various support materials can provide a better alternative for application in the sequestration of CO2. The present review provides insight into several types of CAs, their distinctive characteristics, sources, and recent developments in CA immobilization strategies for application in CO2 sequestration.

Developing Enzyme Immobilization with Fibrous Membranes: Longevity and Characterization Considerations

Fibrous membranes offer broad opportunities to deploy immobilized enzymes in new reactor and application designs, including multiphase continuous flow-through reactions. Enzyme immobilization is a technology strategy that simplifies the separation of otherwise soluble catalytic proteins from liquid reaction media and imparts stabilization and performance enhancement. Flexible immobilization matrices made from fibers have versatile physical attributes, such as high surface area, light weight, and controllable porosity, which give them membrane-like characteristics, while simultaneously providing good mechanical properties for creating functional filters, sensors, scaffolds, and other interface-active biocatalytic materials. This review examines immobilization strategies for enzymes on fibrous membrane-like polymeric supports involving all three fundamental mechanisms of post-immobilization, incorporation, and coating. Post-immobilization offers an infinite selection of matrix materials, but may encounter loading and durability issues, while incorporation offers longevity but has more limited material options and may present mass transfer obstacles. Coating techniques on fibrous materials at different geometric scales are a growing trend in making membranes that integrate biocatalytic functionality with versatile physical supports. Biocatalytic performance parameters and characterization techniques for immobilized enzymes are described, including several emerging techniques of special relevance for fibrous immobilized enzymes. Diverse application examples from the literature, focusing on fibrous matrices, are summarized, and biocatalyst longevity is emphasized as a critical performance parameter that needs increased attention to advance concepts from lab scale to broader utilization. This consolidation of fabrication, performance measurement, and characterization techniques, with guiding examples highlighted, is intended to inspire future innovations in enzyme immobilization with fibrous membranes and expand their uses in novel reactors and processes.

Mesoporous SBA-15 supported gold nanoparticles for solvent-free oxidation of cyclohexane: superior catalytic activity with higher cyclohexanone selectivity

Surface modification of mesoporous SBA-15 with (3-mercaptopropyl) trimethoxysilane greatly enhances its capability to adsorb the tetrachloroauric anion (AuCl₄^-). The calcination of the sample after the adsorption experiment led to the generation of homogeneously dispersed, spherical, single crystalline gold nanoparticles (Au⁰ NPs) of less than 5 nm size, embedded on SBA-15 as observed from the TEM images. The as-prepared SBA-15/Au⁰ nanohybrid material has offered excellent catalytic activity for the selective oxidation of cyclohexane using TBHP as the oxidant in the absence of any solvent. A maximum of 48.7% cyclohexane conversion was achieved and surprisingly, cyclohexanone (K) has much higher selectivity (>95%) than cyclohexanol (A). The hot-filtration study confirmed the leach-resistant characteristics as well as the true heterogeneous catalytic activity of the SBA-15/Au⁰ nanohybrid catalyst. The catalyst was recycled up to four times without significant loss in its catalytic activity.

Carbonic anhydrase to boost CO2 sequestration: Improving carbon capture utilization and storage (CCUS)

CO₂ Capture Utilization and Storage (CCUS) is a fundamental strategy to mitigate climate change, and carbon sequestration, through absorption, can be one of the solutions to achieving this goal. In nature, carbonic anhydrase (CA) catalyzes the CO₂ hydration to bicarbonates. Targeting the development of novel biotechnological routes which can compete with traditional CO₂ absorption methods, CA utilization has presented a potential to expand as a promising catalyst for CCUS applications. Driven by this feature, the search for novel CAs as biocatalysts and the utilization of enzyme improvement techniques, such as protein engineering and immobilization methods, has resulted in suitable variants able to catalyze CO₂ absorption at relevant industrial conditions. Limitations related to enzyme recovery and recyclability are still a concern in the field, affecting cost efficiency. Under different absorption approaches, CA enhances both kinetics and CO₂ absorption yields, besides reduced energy consumption. However, efforts directed to process optimization and demonstrative plants are still limited. A recent topic with great potential for development is the CA utilization in accelerated weathering, where industrial residues could be re-purposed towards becoming carbon sequestrating agents. Furthermore, research of new solvents has identified potential candidates for integration with CA in CO₂ capture, and through techno-economic assessments, CA can be a path to increase the competitiveness of alternative CO₂ absorption systems, offering lower environmental costs. This review provides a favorable scenario combining the enzyme and CO₂ capture, with possibilities in reaching an industrial-like stage in the future.

Mesoporous silica nanoparticles: facile surface functionalization and versatile biomedical applications in oncology

Mesoporous silica nanoparticles (MSNs) have received increasing interest due to their tunable particle size, large surface area, stable framework, and easy surface modification. They are increasingly being used in varying applications as delivery vehicles including bio-imaging, drug delivery, biosensors and tissue engineering etc. Precise structure control and the ability to modify surface properties of MSNs are important for their applications. This review summarises the different synthetic methods for the preparation of well-ordered MSNs with tunable pore volume as well as the approaches of drugs loading, especially highlighting the facile surface functionalization for various purposes and versatile biomedical applications in oncology. Finally, the challenges of clinical transformation of MSNs-based nanomedicines are further discussed.

The Effect of Nanoparticles on the Structure and Enzymatic Activity of Human Carbonic Anhydrase I and II

Human carbonic anhydrases (hCAs) belong to a well characterized group of metalloenzymes that catalyze the conversion of carbonic dioxide into bicarbonate. There are currently 15 known human isoforms of carbonic anhydrase with different functions and distribution in the body. This links to the relevance of hCA variants to several diseases such as glaucoma, epilepsy, mountain sickness, ulcers, osteoporosis, obesity and cancer. This review will focus on two of the human isoforms, hCA I and hCA II. Both are cytosolic enzymes with similar topology and 60% sequence homology but different catalytic efficiency and stability. Proteins in general adsorb on surfaces and this is also the case for hCA I and hCA II. The adsorption process can lead to alteration of the original function of the protein. However, if the function is preserved interesting biotechnological applications can be developed. This review will cover the knowledge about the interaction between hCAs and nanomaterials. We will highlight how the interaction may lead to conformational changes that render the enzyme inactive. Moreover, the importance of different factors on the final effect on hCAs, such as protein stability, protein hydrophobic or charged patches and chemistry of the nanoparticle surface will be discussed.

Spatially arranging interfacial droplets at the oil-solid interface

The controlling and patterning of small droplets on a solid surface is of significant interest to understand interfacial phenomena and for practical applications. Among interfacial phenomena, the formation of interfacial droplets attracts scientists' attention, as the mechanism of this phenomenon where water molecules can spontaneously accumulate at the hydrophobic oil/solid interface is still not fully understood. Further investigation is needed to find out specifically where the driving force comes from and how to spatially arrange the interfacial droplets. Herein, self-assembled monolayers are formed on a gold substrate, and it turns out that the hydrophobic surface with a monolayer formed from HS(CH₂)₁₁CH₃ could inhibit the formation of interfacial droplets; by contrast, the hydrophilic surfaces with monolayers formed from HS(CH₂)₁₁COOH, HS(CH₂)₁₁NH₃·Cl and HS(CH₂)₁₁OH, all promote water accumulation. It suggests that the hydrogen bonding between the surface and water proves to be critical in inducing interfacial droplet formation but this has been neglected in past studies. Taking advantage of microcontact printing, the surface chemistry can be controlled at the micron scale and allows spatial arrangement of interfacial droplets at specific regions. This work moves a further step in understanding the mechanism of interfacial droplet formation, and can be potentially exploited for the collection of water and fabrication of microtemplates.

A Zinc Coordination Complex Mimicking Carbonic Anhydrase for CO2 Hydrolysis and Sequestration

Carbonic anhydrase (CA) mimicking is an effective and environmentally friendly strategy for carbon dioxide sequestration. Herein, we developed a nonanuclear CA-mimetic zinc coordination complex (1) which possesses a coordination environment similar to that of CA's catalytically active zinc sites. Complex 1 exhibited excellent reusability, solvent and thermal stability, and gram-scale synthesis, which are essential for practical applications. It was found that complex 1 exhibited outstanding catalytic performance that is much better in comparison to that of the popular CA-mimetic compound Zn-cyclen and comparable to that of the reported metal-organic frameworks (e.g., CFA-1). Moreover, we found that its catalytic activity can be significantly improved via OAc^-/OH^- exchange and particle size reduction treatment. This study provides important guidance for the design of highly efficient CA-mimetic materials.

Novel bovine carbonic anhydrase encapsulated in a metal–organic framework: a new platform for biomimetic sequestration of CO2

In this work, maximizing the utilization of CO₂ and its precipitation as CaCO₃ by using immobilized bovine carbonic anhydrase (BCA) was evaluated. In this way, selection of suitable carriers which have a gas adsorption function would enhance the CO₂ sequestration efficiency of the carbonic anhydrase (CA). So a metal–organic framework (MOF), an excellent material for gas adsorption and enzyme immobilization was used. In this manner, BCA was encapsulated into the microporous zeolite imidazolate framework, ZIF-8, for the first time, using a bottle-around-a-ship method. Systematic characterization including powder X-ray diffraction (PXRD), UV-vis, and Fourier transform infrared (FT-IR) spectroscopies, BET, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray (EDX) confirmed that the entrapment of BCA molecules was successfully achieved during the crystal growth of ZIF-8 with an enzyme loading of ca. 100 ± 1.2 mg g⁻¹ of BCA–ZIF-8. Optimization of the matrix for increasing the stability of the enzyme in an encapsulated form is the main aim of the present study. The de novo approach was proposed because this method provides better enzyme protection from degradation, minimizes enzyme leaching and enables multiple reuse. Then, the influence of different parameters, including pH, temperature, storage and reusability, was evaluated for enzyme@MOF composites versus free enzymes. The prepared biocatalyst exhibited outstanding activity in a wide pH and temperature range and demonstrates high storage stability up to 37 days. This efficient and simple association procedure seems well-adapted to produce an enzymatic bio-catalyst for biocatalytic hydration of CO₂. The FT-IR analysis revealed that the structure of BCA was well maintained during the encapsulation process. The thermal stability and reusability of the BCA–ZIF-8 increased noticeably due to the structural rigidity and confinement of the ZIF-8 scaffolds. These two parameters are very important for practical applications.

A novel heterogeneous bio-catalyst was developed for CO₂ capture by encapsulating bovine carbonic anhydrase into ZIF-8.

Markov-state model for CO2 binding with carbonic anhydrase under confinement

Enzyme immobilization with a nanostructure material can enhance its stability and facilitate reusability. However, the apparent activity is often compromised due to additional diffusion barriers and complex interactions with the substrates and solvent molecules. The present study elucidates the effects of the surface hydrophobicity of nano-confinement on CO₂ diffusion to the active site of human carbonic anhydrase II (CA), an enzyme that is able to catalyze CO₂ hydration at extremely high turnover rates. Using the Markov-state model in combination with coarse-grained molecular dynamics simulations, we demonstrate that a hydrophobic cage increases CO₂ local density but hinders its diffusion towards the active site of CA under confinement. By contrast, a hydrophilic cage hinders CO₂ adsorption but promotes its binding with CA. An optimal surface hydrophobicity can be identified to maximize both the CO₂ occupation probability and the diffusion rate. The simulation results offer insight into understanding enzyme performance under nano-confinement and help us to advance broader applications of CA for CO₂ absorption and recovery.

In Situ Analysis of the Growth and Dielectric Properties of Organic Self-Assembled Monolayers: A Way To Tailor Organic Layers for Electronic Applications

Organic nanoscale science and technology relies on the control of phenomena occurring at the molecular level. This is of particular importance for the self-assembly of molecular monolayers (SAM) that can be used in various applications ranging from organic electronics to bioelectronic applications. However, the understanding of the elementary nanoscopic processes in molecular film growth is still in its infancy. Here, we developed a novel in situ and extremely sensitive detection method for the analysis of the electronic properties of molecular layer during molecular layer deposition. This low-frequency sensor (1 kHz) is employed to analyze the standard vapor deposition process of SAMs of molecules and, subsequently, it is used to optimize the growth process itself. By combining this method with an ex situ determination of the effective thickness of the resulting layers via ellipsometry, we observe a large difference of the permittivity (1 kHz) of the examined aminosilanes in the liquid state (εliquid = 5.5-8.8) and in SAMs (εSAM = 22-52, electric field in the plane of the layer). We ascribe this difference to either the different orientation and order of the molecules, the different density of molecules, or a combination of both effects. Our novel in situ analyses not only allows monitoring and optimizing the deposition of organic layers but also demonstrates the high potential of organic SAMs as organic high-k layers in electronic devices.

Accelerating Mineral Carbonation Using Carbonic Anhydrase

Carbonic anhydrase (CA) enzymes have gained considerable attention for their potential use in carbon dioxide (CO2) capture technologies because they are able to catalyze rapidly the interconversion of aqueous CO2 and bicarbonate. However, there are challenges for widespread implementation including the need to develop mineralization process routes for permanent carbon storage. Mineral carbonation of highly reactive feedstocks may be limited by the supply rate of CO2. This rate limitation can be directly addressed by incorporating enzyme-catalyzed CO2 hydration. This study examined the effects of bovine carbonic anhydrase (BCA) and CO2-rich gas streams on the carbonation rate of brucite [Mg(OH)2], a highly reactive mineral. Alkaline brucite slurries were amended with BCA and supplied with 10% CO2 gas while aqueous chemistry and solids were monitored throughout the experiments (hours to days). In comparison to controls, brucite carbonation using BCA was accelerated by up to 240%. Nesquehonite [MgCO3·3H2O] precipitation limited the accumulation of hydrated CO2 species, apparently preventing BCA from catalyzing the dehydration reaction. Geochemical models reproduce observed reaction progress in all experiments, revealing a linear correlation between CO2 uptake and carbonation rate. Data demonstrates that carbonation in BCA-amended reactors remained limited by CO2 supply, implying further acceleration is possible.

Electrospun hematite nanofiber/mesoporous silica core/shell nanomaterials as an efficient adsorbent for heavy metals

Functionalized nanomaterials hold tremendous promise for water treatment because their high surface area makes them ideal sorbents for pollutants like heavy metal ions that are pervasive in global water supplies.

The salt-based catalytic enhancement of CO2 absorption by a tertiary amine medium

An improved CO₂ sequestration method is proposed that uses new catalysts, specifically a series of tertiary amine nitrate salts, in an aqueous tertiary amine medium.

Morphology Control in the Synthesis of CaCO₃ Microspheres with a Novel CO₂Storage Material

A green and template-free method was applied to control the morphology of CaCO3 microspheres with a layered nanostructure surface and pure crystalline phase of vaterite via the hydrothermal reaction between Ca(OH)2 saturated limpid solution and a novel CO2-storage material (CO2SM). Morphologies of the as-prepared CaCO3 crystals could be tuned with CO2SM concentration, reaction temperature, or crystallization time. After the precipitation of CaCO3 crystals, the filtered solution could not only be used to absorb CO2 but also to produce CaCO3 microspheres repeatedly with the addition of Ca(OH)2 solution. Furthermore, an aggregation and self-assembly mechanism for the formation CaCO3 microspheres had been proposed. As a result, this novel synthesis strategy of CaCO3 microspheres with CO2SM again emphasized that was possible to synthesize inorganic/organic hybrid materials with exquisite morphology and offered an alternative way for comprehensive utilization of CO2.

Surface Engineering of Polypropylene Membranes with Carbonic Anhydrase-Loaded Mesoporous Silica Nanoparticles for Improved Carbon Dioxide Hydration

Carbonic anhydrase (CA) is a native enzyme that facilitates the hydration of carbon dioxide into bicarbonate ions. This study reports the fabrication of thin films of active CA enzyme onto a porous membrane substrate using layer-by-layer (LbL) assembly. Deposition of multilayer films consisting of polyelectrolytes and CA was monitored by quartz crystal microgravimetry, while the enzymatic activity was assayed according to the rates of p-nitrophenylacetate (p-NPA) hydrolysis and CO2 hydration. The fabrication of the films onto a nonporous glass substrate showed CO2 hydration rates of 0.52 ± 0.09 μmol cm(-2) min(-1) per layer of bovine CA and 2.6 ± 0.7 μmol cm(-2) min(-1) per layer of a thermostable microbial CA. The fabrication of a multilayer film containing the microbial CA on a porous polypropylene membrane increased the hydration rate to 5.3 ± 0.8 μmol cm(-2) min(-1) per layer of microbial CA. The addition of mesoporous silica nanoparticles as a film layer prior to enzyme adsorption was found to increase the activity on the polypropylene membranes even further to a rate of 19 ± 4 μmol cm(-2) min(-1) per layer of microbial CA. The LbL treatment of these membranes increased the mass transfer resistance of the membrane but decreased the likelihood of membrane pore wetting. These results have potential application in the absorption of carbon dioxide from combustion flue gases into aqueous solvents using gas-liquid membrane contactors.

Peptide-induced affinity binding of carbonic anhydrase to carbon nanotubes

Although affinity binding between short chain peptides and carbon nanotube (CNT) has been reported, little is known for the study of proteins with CNT recognition and specific binding capabilities. Herein, carbonic anhydrase (CA) was functionalized via protein fusion with a single-walled carbon nanotube (SWNTs)-binding peptide, thereby forming a bioactive protein with high affinity binding capability. TEM and AFM analyses showed that the fusion CA could firmly coat to SWNTs with a surface coverage over 51%, while the enzyme maintained its catalytic activity. Structural analysis revealed that slight conformation changes were induced as a result of the fusion; however, the affinity binding of CA to the hydrophobic surface of SWNTs restored the native structure of the protein, with the conformation of the SWNT-bound CA largely resembling that of the native parent enzyme. Interfacial interactions between the fusion CA and SWNT were further investigated with Raman spectrometry and microscopic analysis. The results suggested that such peptide-induced CNT-protein binding allows the development of bioactive hybrid materials with the native structures of the protein moieties largely undisrupted.

Carbonic anhydrase-mimetic bolaamphiphile self-assembly for CO2 hydration and sequestration

A biomimetic catalyst was prepared through the self-assembly of a bolaamphiphilic molecule with histidine moieties for the sequestration of carbon dioxide. The histidyl bolaamphiphilic molecule bis(N-α-amidohistidine)-1,7-heptane dicarboxylate has been synthesized and self-assembled to produce analogues of the active sites of carbonic anhydrase (CA) after association with Zn(2+) ions. Spectroscopic analysis demonstrated the coordination of the Zn(2+) ions with histidine imidazole moieties, which is the core conformation of CA active sites. The Zn-associated self-assembly worked as a CA-mimetic catalyst that shows catalytic activity for CO2 hydration. Evaluation of the kinetics of using para-nitrophenylacetate revealed that the kinetic parameters of the CA-mimetic catalyst were maximized at the optimal Zn concentration and that excess Zn ions resulted in deteriorated catalytic activity. The performance of the CA-mimetic catalyst was enhanced by changing the pH value and temperature of the reaction, which implies that the hydrolysis of the substrate is the rate-determining step. The catalyst-assisted sequestration of CO2 was demonstrated by CaCO3 precipitation upon the addition of Ca(2+) ions. This study offers an easy way to prepare enzyme analogues for CO2 sequestration through the self-assembly of bolaamphiphile molecules with designer biochemical moieties.

Inactivation and adsorption of human carbonic anhydrase II by nanoparticles

The enzymatic activity of human carbonic anhydrase II (HCAII) was studied in the presence of nanoparticles of different nature and charge. Negatively charged nanoparticles inhibit HCAII whereas no effect is seen for positively charged particles. The kinetic effects were correlated with the strength of binding of the enzyme to the particle surface as measured by ITC and adsorption assays. Moreover, conformational changes upon adsorption were observed by circular dichroism. The main initial driving force for the adsorption of HCAII to nanoparticles is of electrostatic nature whereas the hydrophobic effect is not strong enough to drive the initial binding. This is corroborated by the fact that HCAII do not adsorb on positively charged hydrophobic polystyrene nanoparticles. Furthermore, the dehydration of the particle and protein surface seems to play an important role in the inactivation of HCAII by carboxyl-modified polystyrene nanoparticles. On the other hand, the inactivation by unmodified polystyrene nanoparticles is mainly driven by intramolecular interactions established between the protein and the nanoparticle surface upon conformational changes in the protein.

Carbonic anhydrases in industrial applications

Carbonic anhydrases (CAs) catalyze a fundamental reaction: the reversible hydration and dehydration of carbon dioxide (CO2) and bicarbonate ([Formula: see text]), respectively. Current methods for CO2 capture and sequestration are harsh, expensive, and require prohibitively large energy inputs, effectively negating the purpose of removing CO2 from the atmosphere. Due to CA's activity on CO2 there is increasing interest in using CAs for industrial applications such as carbon sequestration and biofuel production. A lot of work in the last decade has focused on immobilizing CA onto various supports for incorporation into CO2 scrubbing applications or devices. Although the proof of principle has been validated, current CAs being tested do not withstand the harsh industrial conditions. The advent of large-scale genome sequencing projects has resulted in several emerging efforts seeking out novel CAs from a variety of microorganisms, including bacteria, micro-, and macro-algae. CAs are also being investigated for their use in medical applications, such drug delivery systems and artificial lungs. This review also looks at possible downstream uses of captured and sequestered CO2, from using it to enhance oil recovery to incorporating it into useful and financially viable products.

CO2 absorption and sequestration as various polymorphs of CaCO3 using sterically hindered amine

One aspect of the attempt to restrain global warming is the reduction of the levels of atmospheric CO2 produced by fossil fuel power systems. This study attempted to develop a method that reduces CO2 emissions by investigating the absorption of CO2 into sterically hindered amine 2-amino-2-methyl-1-propanol (AMP), the acceleration of the absorption rate by using the enzyme carbonic anhydrase (CA), and the conversion of the absorption product to stable carbonates. CO2 absorbed by AMP is converted via a zwitterion mechanism to bicarbonate species; the presence of these anions was confirmed with (1)H and (13)C NMR spectral analysis. The catalytic efficiency (kcat/Km), CO2 absorption capacities, and enthalpy changes (ΔHabs) of aqueous AMP in the presence or absence of CA were found to be 2.61 × 10(6) or 1.35 × 10(2) M(-1) s(-1), 0.97 or 0.96 mol/mol, and -69 or -67 kJ/mol, respectively. The carbonation of AMP-absorbed CO2 was performed by using various Ca(2+) sources, viz., CaCl2 (CAC), Ca(OOCCH3)2 (CAA), and Ca(OOCCH2CH3)2 (CAP), to obtain various polymorphs of CaCO3. The yields of CaCO3 from the Ca(2+) sources were found in the order CAP > CAA > CAC as a result of the effects of the corresponding anions. CAC produces pure rhombohedral calcite, and CAA and CAP produce the unusual phase transformation of calcite to spherical vaterite crystals. Thus, AMP in combination with CAA and CAP can be used as a CO2 absorbent and buffering agent for the sequestration of CO2 in porous CaCO3.

Effect of small molecule osmolytes on the self-assembly and functionality of globular protein-polymer diblock copolymers

Blending the small molecule osmolytes glycerol and trehalose with the model globular protein-polymer block copolymer mCherry-b-poly(N-isopropyl acrylamide) (mCherry-b-PNIPAM) is demonstrated to improve protein functionality in self-assembled nanostructures. The incorporation of either additive into block copolymers results in functionality retention in the solid state of 80 and 100% for PNIPAM volume fractions of 40 and 55%, respectively. This represents a large improvement over the 50-60% functionality observed in the absence of any additive. Furthermore, glycerol decreases the thermal stability of block copolymer films by 15-20 °C, while trehalose results in an improvement in the thermal stability by 15-20 °C. These results suggest that hydrogen bond replacement is responsible for the retention of protein function but suppression or enhancement of thermal motion based on the glass transition of the osmolyte primarily determines thermal stability. While both osmolytes are observed to have a disordering effect on the nanostructure morphology with increasing concentration, this effect is less pronounced in materials with a larger polymer volume fraction. Glycerol preferentially localizes in the protein domains and swells the nanostructures, inducing disordering or a change in morphology depending on the PNIPAM coil fraction. In contrast, trehalose is observed to macrophase separate from the block copolymer, which results in nanodomains becoming more disordered without changing significantly in size.

Engineered Escherichia coli with periplasmic carbonic anhydrase as a biocatalyst for CO2 sequestration

Carbonic anhydrase is an enzyme that reversibly catalyzes the hydration of carbon dioxide (CO2). It has been suggested recently that this remarkably fast enzyme can be used for sequestration of CO2, a major greenhouse gas, making this a promising alternative for chemical CO2 mitigation. To promote the economical use of enzymes, we engineered the carbonic anhydrase from Neisseria gonorrhoeae (ngCA) in the periplasm of Escherichia coli, thereby creating a bacterial whole-cell catalyst. We then investigated the application of this system to CO2 sequestration by mineral carbonation, a process with the potential to store large quantities of CO2. ngCA was highly expressed in the periplasm of E. coli in a soluble form, and the recombinant bacterial cell displayed the distinct ability to hydrate CO2 compared with its cytoplasmic ngCA counterpart and previously reported whole-cell CA systems. The expression of ngCA in the periplasm of E. coli greatly accelerated the rate of calcium carbonate (CaCO3) formation and exerted a striking impact on the maximal amount of CaCO3 produced under conditions of relatively low pH. It was also shown that the thermal stability of the periplasmic enzyme was significantly improved. These results demonstrate that the engineered bacterial cell with periplasmic ngCA can successfully serve as an efficient biocatalyst for CO2 sequestration.

Structural and catalytic characterization of a thermally stable and acid-stable variant of human carbonic anhydrase II containing an engineered disulfide bond

The carbonic anhydrases (CAs) are a family of mostly zinc metalloenzymes that catalyze the reversible hydration of CO2 to bicarbonate and a proton. Recently, there has been industrial interest in utilizing CAs as biocatalysts for carbon sequestration and biofuel production. The conditions used in these processes, however, result in high temperatures and acidic pH. This unfavorable environment results in rapid destabilization and loss of catalytic activity in CAs, ultimately resulting in cost-inefficient high-maintenance operation of the system. In order to negate these detrimental industrial conditions, cysteines at residues 23 (Ala23Cys) and 203 (Leu203Cys) were engineered into a wild-type variant of human CA II (HCAII) containing the mutation Cys206Ser. The X-ray crystallographic structure of the disulfide-containing HCAII (dsHCAII) was solved to 1.77 Å resolution and revealed that successful oxidation of the cysteine bond was achieved while also retaining desirable active-site geometry. Kinetic studies utilizing the measurement of (18)O-labeled CO2 by mass spectrometry revealed that dsHCAII retained high catalytic efficiency, and differential scanning calorimetry showed acid stability and thermal stability that was enhanced by up to 14 K compared with native HCAII. Together, these studies have shown that dsHCAII has properties that could be used in an industrial setting to help to lower costs and improve the overall reaction efficiency.

Carbonic anhydrase promotes the absorption rate of CO2 in post-combustion processes

The rate of carbon dioxide (CO2) absorption by monoethanol amine (MEA), diethanol amine (DEA), N-methyl-2,2'-iminodiethanol (MDEA), and 2-amino-2-methyl 1-propanol (AMP) solutions was found to be enhanced by the addition of bovine carbonic anhydrase (CA), has been investigated using a vapor-liquid equilibrium (VLE) device. The enthalpy (-ΔHabs) of CO2 absorption and the absorption capacities of aqueous amines were measured in the presence and/or absence of CA enzyme via differential reaction calorimeter (DRC). The reaction temperature (ΔT) under adiabatic conditions was determined based on the DRC analysis. Bicarbonate and carbamate species formation mechanisms were elucidated by (1)H and (13)C NMR spectral analysis. The overall CO2 absorption rate (flux) and rate constant (kapp) followed the order MEA > DEA > AMP > MDEA in the absence or presence of CA. Hydration of CO2 by MDEA in the presence of CA directly produced bicarbonate, whereas AMP produced unstable carbamate intermediate, then underwent hydrolytic reaction and converted to bicarbonate. The MDEA > AMP > DEA > MEA reverse ordering of the enhanced CO2 flux and kapp in the presence of CA was due to bicarbonate formation by the tertiary and sterically hindered amines. Thus, CA increased the rate of CO2 absorption by MDEA by a factor of 3 relative to the rate of absorption by MDEA alone. The thermal effects suggested that CA yielded a higher activity at 40 °C.