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901.
Epoxy-norbornane (EPO-NBE) is a crucial building block for the synthesis of various biologically active heterocyclic systems. To develop an efficient protocol for producing EPO-NBE using norbornene (NBE) as a substrate, cytochrome P450 enzyme from Pseudomonas putida (CYP238A1) was examined and its crystal structure (PDB code: 7X53) was resolved. Molecular mechanism analysis showed a high energy barrier related to iron-alkoxy radical complex formation. Therefore, a protein engineering strategy was developed and an optimal CYP238A1NPV variant containing a local hydrophobic “fence” at the active site was obtained, which increased the H2O2-dependent epoxidation activity by 7.5-fold compared with that of CYP238A1WT. Among the “fence”, Glu255 participates in an efficient proton transfer system. Whole-cell transformation using CYP238A1NPV achieved an EPO-NBE yield of 77.6 g ⋅ L−1 in a 30-L reactor with 66.3 % conversion. These results demonstrate the potential of this system for industrial production of EPO-NBE and provides a new biocatalytic platform for epoxidation chemistry.  相似文献   
902.
Total chemical protein synthesis provides access to entire D-protein enantiomers enabling unique applications in molecular biology, structural biology, and bioactive compound discovery. Key enzymes involved in the central dogma of molecular biology have been prepared in their D-enantiomeric forms facilitating the development of mirror-image life. Crystallization of a racemic mixture of L- and D-protein enantiomers provides access to high-resolution X-ray structures of polypeptides. Additionally, D-enantiomers of protein drug targets can be used in mirror-image phage display allowing discovery of non-proteolytic D-peptide ligands as lead candidates. This review discusses the unique applications of D-proteins including the synthetic challenges and opportunities.  相似文献   
903.
L-2-aminobutyrate (L-ABA) is an important chiral drug intermediate with a key role in modern medicinal chemistry. Here, we describe the development of an efficient method for the asymmetric synthesis of L-ABA in a tri-enzymatic cascade in Escherichia coli BL21 (DE3) using a cost-effective L-Thr. Low activity of leucine dehydrogenase from Bacillus thuringiensis (BtLDH) and unbalanced expression of enzymes in the cascade were major challenges. Mechanism-based protein engineering generated the optimal triple variant BtLDH M3 (A262S/V296C/P150M) with 20.7-fold increased specific activity and 9.6-fold increased kcat/Km compared with the wild type. Optimizing plasmids with different copy numbers regulated enzymatic expression, thereby increasing the activity ratio (0.3 : 1:0.6) of these enzymes in vivo close to the optimal ratio (0.4 : 1 : 1) in vitro. Importing the optimal triple mutant BtLDH M3 into our constructed pathway in vivo and optimization of transformation conditions achieved one-pot conversion of L-Thr to 130.2 g/L L-ABA, with 95 % conversion, 99 % e.e. and 10.9 g L−1 h−1 productivity (the highest to date) in 12 h on a 500 mL scale. These results describe a potential biosynthesis approach for the industrial production of L-ABA.  相似文献   
904.
Synthetic polymer materials have been surged to the forefront of research in the fields of tissue engineering, drug delivery, and biomonitoring in recent years. Biodegradable synthetic polymers are increasingly needed as transient substrates for tissue regeneration and medicine delivery. In contrast to commonly used polymers including polyesters, polylactones, polyanhydrides, poly(propylene fumarates), polyorthoesters, and polyurethanes, biodegradable polyphosphazenes (PPZs) hold great potential for the purposes indicated above. PPZ's versatility in the synthetic process has enabled the production of a variety of polymers with various physico-chemical, and biological properties have been produced, making them appropriate for biomedical applications. Biocompatible PPZs are often used as scaffolds in the regeneration of skeleton, bones, and other tissues. PPZs have also received special attention as potential drug vehicles of high-value biopharmaceuticals such as anticancer drugs. Additionally, by incorporating fluorophores into the PPZ backbone to produce photoluminescent biodegradable PPZs, the utility of polyphosphazenes is further expanded as they are used in tracking the regeneration of the target tissue as well as the fate of PPZ based scaffolds or drug delivery vehicles. This review provides a summary of the evolution of PPZ applications in the fields of tissue engineering, drug delivery, and bioimaging in recent 5 years.  相似文献   
905.
Therapeutic monoclonal antibodies (mAbs) are typically manufactured using mammalian cell cultures in fed-batch bioreactors, with increasing emphasis on meeting productivity and product quality attribute targets that depend strongly on such process variables as nutrient feed rates and bioreactor operating conditions. In this article, we identify, categorize, and address the challenges of achieving both productivity and product quality goals simultaneously, by developing a multivariable, model-based control system that can satisfy multiple production objectives in a fed-batch cell culture process. Here, we discuss model development and present theoretical concepts of observability and controllability that are essential to understanding and handling effectively these intrinsic challenges. Subsequently, we evaluate via simulation the performance of the outer-loop model predictive control and demonstrate the overall capability to satisfy complex production objectives in a laboratory scale bioreactor, as a first step toward the ultimate goal of creating an advanced control system for fed-batch mAb manufacturing processes.  相似文献   
906.
Developing electrocatalysts for CO2 electroreduction with high activity and superior selectivity is extremely important and desirable from both academic and industrial perspectives. However, owing to competition with hydrogen evolution, highly efficient CO2 reduction is mostly achieved with high CO selectivity in a narrow potential range, which is incompatible with a large cell voltage required for industrial-level CO2 reduction. Herein, we report an effective strategy to regulate CO2 reduction performances of single-atom Ni electrocatalysts over a broad potential window by engineering their pore structures (micropores, mesopores, or hierarchical pores with both micropores and mesopores). It is revealed that hierarchically pores can significantly promote CO2 reduction efficiency of single-atom Ni electrocatalysts. The hierarchically porous electrocatalyst achieves a maximum CO Faradaic efficiency (FECO) of 97.4% at −1.2 V (vs. RHE) and shows high FECO of >85% over a broad potential window from −0.7 to −1.7 V, much superior to electrocatalysts with other pore structures. More impressively, turnover frequency of the hierarchically porous electrocatalyst increases rapidly with increasing the applied potential and reaches 50,067 h−1 at −1.7 V. Such CO2 electroreduction promotion could be attributed to a synergistic effect of micropores for enhancing CO2 adsorption and mesopores for facilitating rapid release of product bubbles, which significantly improves CO2 reduction and suppresses hydrogen evolution.  相似文献   
907.
Statistical techniques coupled with genetic algorithm (GA) were used to identify optimal values of key operational variables in fluid catalytic cracking (FCC) process. A Kellog Orthoflow F fluid catalytic cracking process model was considered. It is known as a highly nonlinear process with a large number of variables with strong interactions among them. A reduced process model was obtained through factorial design technique to be used as a process function in the optimisation work giving as result the operational conditions that maximise conversion without infringing operational restrictions with savings in computational burden and time. An increase of 8.71% in process conversion was achieved applying GA as optimisation technique. © 2012 Canadian Society for Chemical Engineering  相似文献   
908.
909.
D. J. Hopkin 《火与材料》2013,37(4):311-327
The thermo‐physical response of timber structures in fire is complex. For this reason, debate still exists today as to the best approaches for simulating thermal response in fire using tools such as finite element analysis (FEA) modelling. Much of the debate is concerned with the thermal properties of timber, for example, conductivity, specific heat and density, at elevated temperature and how such properties should be implemented or interpreted in numerical calculations. For practitioners intending to use modelling as a fire design tool for timber buildings, guidance exists on the thermal properties of softwood in Annex B of EN 1995‐1‐2. These properties are limited for use under standard fire exposure conditions because of the way in which they were derived from calibration against focussed test data. As a result, they cannot be applied to non‐standard fires, which are more representative of real fires due to a combination of varying heating rates and the decay phase of fire development. The limitations of the standard fire test (and associated curve) are widely understood. As a result, much recent structures in fire research has focussed on the ‘performance based design’ of buildings subject to increasingly realistic fire conditions. Such an approach allows engineers to quantify the level of safety that can be achieved in a building should a fire occur. In addition, the design of buildings to withstand fires proportionate to the risks foreseen and also the geometry present results in better value buildings that are inherently more robust. For the same approaches and associated benefits to be realised for timber buildings, then a number of barriers must be overcome. The most obvious of these is engineers' ability to determine timber structure temperatures as a result of fires other than the standard fire curve. This however presents a number of challenges. Upon heating, the moisture bound within begins to evaporate, volatiles begin to flow from the heated surface and char forms. The rate of which these behaviours occur and the nature of the char that forms depends on a number of factors, but most notably the rate of heating. Upon cooling, the timber member continues to generate heat energy as the surface oxidises. As a result, any models intended to simulate temperature development must consider the relationship not only between temperature and thermo‐physical characteristics but also between heating rate and the process of heat generation. Many models have been developed for this purpose; however, they are extremely complex and are some way from being ready for implementation as design tools. This paper proposes implementing ‘heat of hydration’ routines, intended for the curing of concrete structures, to simulate the heating and cooling process in timber structures. Such routines are available in many commercial FEA software packages. The adoption of the hydration routines allows the heat generation process, as a result of oxidation, to be considered in parallel with solid phase heat transfer using apparent thermal properties. The approach is shown to be very effective in simulating temperature development in timber members subject to parametric design fires. The models developed are benchmarked against experiments conducted in the 1990s by SP Trätek. Predictably, a number of the heat generation parameters adopted are shown to depend on the fire dynamics considered. However, recommended parameters are given that provide an acceptable level of accuracy for most design purposes. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
910.
In this article, the development of a reaction engineering approach (REA) to modeling air drying of thin-layer materials or solids droplet/particles, a 10-year-old approach, is critically reviewed. This approach for drying data reduction has been seen to be successful in several applications (droplet drying and layer drying). The historical aspects and its possible relations to the characteristic drying rate curve (CDRC) model, one of the most well-known drying kinetics models, have been articulated. Possible improvements and future prospects are discussed.  相似文献   
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