243 research outputs found
Enhanced microwave-absorption performance of FeCoS/Polyimide-Graphene composite by electric field modulation
No full textGraphene was incorporated into polyimide resin to obtain Polyimide-Graphene polymer sheet via in-situ polymerization method. FeCoB nanoparticles, generated by a high energetic cluster deposition system, were then deposited on the Polyimide-Graphene sheets by electric field assisted deposition technology to form FeCoB/Polyimide-Graphene ternary composites. An electric field of about 5-30 kV was applied on the sample platform when the composites were manufactured. The strong magnetic properties of the composites were revealed by the measurement of hysteresis loops at room temperature. Then the laminated FeCoB/Polyimide(-Graphene) composites were used to perform reflection loss scan. It is found that the addition of a small amount of graphene promote the improvement of electromagnetic properties by increasing dielectric loss. And the cover of FeCoB films can dramatically enhance the microwave absorption capacity by enriching interfacial polarization effect and electromagnetic match. The results prove that the electric field assisted deposition technique is a very attractive avenue to enhance the microwave absorption performance of the ternary composite expressed by tuning their dielectric and magnetic properties. (C) 2017 Elsevier Ltd. All rights reserved
Mirror-Image Thymidine Discriminates against Incorporation of Deoxyribonucleotide Triphosphate into DNA and Repairs Itself by DNA Polymerases
No full textDNA polymerases are known to recognize preferably D-nucleotides over L-nucleotides during DNA synthesis. Here, we report that several general DNA polymerases catalyze polymerization reactions of nucleotides directed by the DNA template containing an L-thymidine (L-T). The results display that the 5'-3' primer extension of natural nucleotides get to the end at chiral modification site with Taq and Phanta Max DNA polymerases, but the primer extension proceeds to the end of the template catalyzed by Deep Vent (exo(-)), Vent (exo(-)), and Therminator DNA polymerases. Furthermore, templating L-nucleoside displays a lag in the deoxyribonucleotide triphosphate (dNTP) incorporation rates relative to natural template by kinetics analysis, and polymerase chain reactions were inhibited with the DNA template containing two or three consecutive L-Ts. Most interestingly, no single base mutation or mismatch mixture corresponding to the location of L-T in the template was found, which is physiologically significant because they provide a theoretical basis on the involvement of DNA polymerase in the effective repair of L-T that may lead to cytotoxicity.National Natural Science Foundation of China [21302008, 21272263]; State Key Laboratory of Natural and Biomimetic Drugs [K20140204, K20150204]; Foundation for University Young Teachers by Chinese Academy of Sciences [Y55103HY00]SCI(E)ARTICLE82125-21342
Synthetic protein assemblies for biocatalysis and biomedicine
No full textProteins, as biological building blocks, often assemble to form various sophisticated structures to play vital physiological roles. In this thesis, we focus on two primary application areas of protein assemblies, namely, biocatalysis and biomedicine. In nature, the majority of chemical and biological tasks in pathways are accomplished by highly organized enzymes called multienzyme complexes. These self-assembled metabolizers insulate toxic intermediates, increase the efficiency of intermediate transfer, minimize metabolic crosstalk, and improve catalytic yield. Biomedicine approaches, such as drug delivery, usually externally control physiological and pathological processes with the aid of natural organelles as carriers. These organelles can transport small molecules, nucleic acid, and protein drugs. Inspired by their artful structures and superior catalytic functions, we plan to design synthetic protein assemblies for the creation of cell-like structures that operate similarly to biological systems and explore their applications.In the first part of this thesis, we constructed multienzyme nanostructures based on synthetic protein scaffolds. The scaffolds were formed using the spontaneous protein reaction between SpyCatcher and SpyTag. Two types of protein scaffolds were developed: two skeleton proteins crosslinked and hierarchically assembled into heterogeneous nanostructures (the crosslinked scaffold), and the head-to-tail cyclization of a dual-reactive skeleton protein to introduce a homogeneous cyclic scaffold. Sequential enzymes from the menaquinone biosynthetic pathway were assembled on both scaffolds through docking domain interactions derived from polyketide synthases. Both scaffolded assemblies effectively increased the yield of the final product in the catalytic cascade reaction in menaquinone biosynthesis. Surprisingly, the rate enhancements were driven by different mechanisms: the crosslinked scaffold assembly streamlined the overall flow of the reactants, whereas the cyclic scaffold assembly accelerated the catalytic efficiency of the rate-limiting enzyme.In the second part of this thesis, we utilized heterologous caveolae (h-caveolae), a self-assembled protein assembly formed in Escherichia coli through the expression of the caveolin-1 protein, as the nanoscale scaffold for the multienzyme assembly. We engineered the surface of h-caveolae with two different types of protein linker pairs: Spy pairs and the RIDD-RIAD pair. SpyCatcher/SpyTag, and the similar orthogonal pair SnoopCatcher/SnoopTag, can covalently interact with other functional proteins without competitive effects. The RIAD tag can tightly and specifically interact with RIDD-tagged enzymes with high affinity. For both systems, sequential enzymes from isoprenoid biosynthesis were assembled on the h-caveolae scaffolds through these small interacting protein pairs. Interestingly, the overall catalytic efficiency of the cascade pathway was largely expedited both in vivo and in vitro, which may result from the local enrichment of the enzymes in the nanostructure provided by h-caveolae scaffolds.In the third part of this thesis, we extend the application of h-caveolae scaffolds by making use of their endocytic properties. Compared with traditional drug carriers, they are superior in several ways, such as in their biocompatibility, permeability, availability, and reduced toxicity. Synthetic h-caveolae scaffolds can successfully deliver both small molecules and a tumor-selective apoptotic protein, apoptin, into cells. This model application provides an ideal platform to achieve profound therapeutic efficacy and negligible side effects in cancer therapy. To the best of our knowledge, this is the first study to exploit h-caveolae formed inside of E. coli in both biocatalysis and biomedicine, which provides a basis for its future biotechnological applications.In summary, this thesis explored versatile strategies to derive more powerful biocatalysts and develop better drug loading carriers through synthetic protein assemblies. First, it showcased successful catalytic nanomachine designs with a superior catalytic function that mimic natural multienzyme complexes and applied them to different metabolic pathways both in vitro and in vivo. Second, this thesis explored the potential application value of the mysterious natural nanostructure of h-caveolae by simply engineering its surface with different binding modules. These studies provide us with new ideas to take advantage of protein-protein interactions.蛋白質是生物體的必要組成部分，它們通常會組裝成許多複雜的結構以便發揮重要生理作用。本文主要研究了蛋白質組裝結構在生物催化和生物醫學領域的應用。自然界中，大多數生物和化學的任務都是通過高度組織化的多酶複合物完成的。這些自組裝多酶複合物可以隔離有毒中間體，提高中間體轉化效率減少代謝途徑串擾，並提高代謝途徑的催化產率。生物醫學領域如藥物的體內遞送，通常會依賴天然細胞器作為載體從而實現控制生物體的生理和病理過程。這些細胞器可以轉運小分子，蛋白質和核酸藥物。受到這些組裝體出色的催化功能以及巧妙的結構啟發，我們計畫設計出一系列合成蛋白組裝體用於構建類似于天然生物系統的結構並探索其在生物催化和生物醫學領域的應用價值。本文的第一個部分我們構建了基於合成蛋白骨架的多酶自組裝納米結構。該蛋白组装体骨架結構由 SpyTag 和 SpyCatcher 的自發反應生成。通過設計的不同形成了兩種不同形狀的蛋白骨架結構：異質交聯骨架以及均質環形骨架。利用聚酮對接結構域（Docking domains）的正交反應我們巧妙地將生物合成途徑中的多種酶組裝在合成蛋白骨架上。該策略有效地提高了以甲基萘醌生物合成級聯反應為例的產物產率。並且我們驚喜地發現兩種蛋白組裝骨架提高催化速率的途徑是由不同機制驅動的異質交聯骨架增加了催化酶的局部濃度提高了底物與活性位點的撞擊機會而均質環形骨架加速了催化途徑中限速酶的催化效率。論文的第二部分我們利用了異源的小窩結構（h-caveolae）作為蛋白組裝體的支架該結構是一種通過在大腸桿菌中表達caveolin-1蛋白形成的纳米自组装结构蛋白。我們通過兩種不同類型的蛋白反應對改造了小窩支架表面一種是共價反應對標籤蛋白和捕手蛋白另一種為非共價對： RIDD 和 RIAD 。這兩種反應對各具優點 SpyCatcher/SpyTag 和與之相似的正交反應對 SnoopCatcher/SnoopTag 可以使得小窩支架共價結合上兩種功能蛋白而無相互競爭效應。而另一反應對 RIAD 標籤可以與 RIDD 結構域緊密且特異性地結合在一起該反應具有更多的靈活性。經過這兩種反應對改造的合成異源小窩支架都可以成功組裝以類異戊二烯生物合成級聯酶为模型的多酶代谢途径。有趣的是無論在體內或體內,基于h-caveolae組裝策略都大大提高了級聯反應的催化效率。我們推測原因可能是異源小窩支架提供了一種納米催化反應結構該結構提高了酶的局部濃度提高了底物與活性位點的撞擊機會從而導致整體催化效率增長。論文的第三部分我們利用小窩結構的本身的內吞屬性擴展了h-caveolae支架的應用範圍。與傳統的藥物載體相比它們具有更好的優勢如良好的生物相容性通透性可利用性和低毒特性。改造的異源小窩支架可以成功地將小分子和一種腫瘤選擇性凋亡蛋白凋亡素傳遞到細胞中。該模型的應用為我們在癌症治療中同時實現長期的治療功效與可忽略的副作用。據我們所知這也是首次將大腸桿菌內部形成的小窩結構同時應用在生物催化和生物醫學領域。該研究也為研究該未知結構奠定了良好生物技術應用基礎。綜合起來，本文探索了多種策略去基於合成蛋白組裝體而構建和開發功能更強大的生物催化劑和載藥載體。一方面我們向大家展示了怎樣利用天然蛋白單體以及天然蛋白複合物結構設計具有卓越催化功能的合成蛋白納米結構器，這些結構類比了天然多酶複合物的相互作用可同時應用在體內和體外催化反應上。另一方面我們通過不同蛋白反應對的相互作用對異源小窩結構進行表面改造探索了這種僅自主存在於高級生物體內的未知結構的更多潛在應用價值。這些研究為我們提供了利用蛋白質相互作用的新思路。Liu, Zhenjun."October 2020."Ph.D. Chinese University of Hong Kong 2021.Includes bibliographical references.Abstracts also in Chinese.Title from PDF title page (viewed on November 04, 2022)
Studies on the preferred uracil-adenine base pair at the cleavage site of 10-23 DNAzyme by functional group modifications on adenine
No full text10-23 DNAzyme is capable of catalytically cleaving RNA substrates with the preferred cleavage sites rAU and rGU, in which the common base pair U-dA(0) forms between the substrate and the DNAzyme in the cleavage reaction. Here its conservation was studied with base modifications on dA and extra functional groups introduced. The nitrogen atom at 7- or 8-position of adenine was demonstrated to be equally important for the cleavage reaction, although it is not related to the thermal stability of the base pair. Deletion of 6-amino group led to decreased stability of the base pair and a slight slower reaction rate. Extra functional groups through 6-amino group were not favorably accommodated in the cleavage site. From these modifications at the level of functional groups, it demonstrated that the base pair U-dA(0) not only contributes to the recognition and binding stability, but also it is involved in the active catalytic center by its functional groups and base stacking. This kind of chemical modifications with 7-substituted 8-aza-7-deaza-2'-deoxyadenosine at dA(0) is favorable for the introduction of signal molecules for mechanistic studies and biological applications, without significant loss of the catalytic function and structural destruction. (C) 2015 Elsevier Ltd. All rights reserved.State Key Laboratory of Natural and Biomimetic Drugs, Peking University; National Natural Science Foundation of China [21072229]SCI(E)[email protected]; [email protected]
Thermal and Mechanical Properties of Geopolymers Exposed to High Temperature: A Literature Review
No full textGeopolymers are prepared by alkali solution-activated natural minerals or industrial waste materials, which have been widely used as new sustainable building and construction materials for their excellent thermal and mechanical properties. The thermal and mechanical properties of geopolymers at high temperature have attracted great attention from many researchers. However, there are few systematic works concerning these two issues. Therefore, this work reviewed the thermal and mechanical behaviors of geopolymers at high temperature. Firstly, the thermal properties of geopolymers in terms of mass loss, thermal expansion, and thermal conductivity after high temperature were explained. Secondly, the mechanical properties of residual compressive strength and stress-strain relationship of fly ash geopolymers and metakaolin geopolymers after high temperature were analyzed. Finally, the microstructure and mineralogical characteristics of geopolymers upon heating were interpreted according to the changes of microstructures and compositions. The results show that the thermal properties of geopolymers are superior to cement concrete. The geopolymers possess few mass loss and a low expansion ratio and thermal conductivity at high temperature. The thermal and mechanical properties of the geopolymers are usually closely related to the raw materials and the constituents of the geopolymers. Preparation and testing conditions can affect the mechanical properties of the geopolymers. The stress-strain curves of geopolymer are changed by the composition of geopolymers and the high temperature. The silicon-type fillers not only improve the thermal expansion of the geopolymers but also enhance mechanical properties of the geopolymers. But, they do not contribute to reducing the thermal conductivity. the different raw materials, aluminosilicate precursor and reinforcement materials, result in different geopolymer damage during the heating. However, phase transitions can occur during the process of heating regardless of the raw materials. The additional performance enhancements can be achieved by optimizing the paste formulation, adjusting the inner structure, changing the alkali type, and incorporating reinforcements
Shear transfer across a crack in recycled aggregate concrete
No full textIn this paper, 32 pre-cracked recycled aggregate concrete (RAC) push-off specimens made from 10 mix designs were tested to study the shear transfer performance across cracks. The effects of recycled coarse aggregate (RCA) replacement ratio, the water to cement ratio, the concrete strength, and the lateral constraint on the shear transfer performance were carefully investigated. The experimental results and data analysis show that the shear transfer mechanism and process across cracks in RAC is largely the same as that in natural aggregate concrete (NAC). Both the lateral constraint and the concrete compressive strength positively affect the shear transfer strength of the RAC. However, the RCA replacement ratio has adverse effects on the shear transfer strength when it is over 30%. It is also found that the design equations for NAC in ACI and PCI codes may be used to predict the shear transfer strength of RAC. © 2012 Elsevier Ltd
Research of Optimal Experiment on Bridge Pier Types for Reducing Backwater
No full textThere are many kinds of commonly-used bridge pier types. This paper researches the selection principle and optimal type of the bridge pier types from the perspective of reducing backwater. This paper also tests and analyzes the high resistance water for different types of bridge pier under different flow velocities and conditions of water resistance ratio through establishment of a physical model of wide water channel. The result shows that, the bridge pier has an optimal profile curve, and the characteristic parameter (b’/L) is from 0.071 to 0.083; three kinds of commonly-used pier types – square pier, streamline pier and bicircular pier have different strength angles. If the angle between the axis of bridge pier and the water flow direction is less than 36°, it should give priority to the streamline pier; if the angle is greater than 36°, then it should give priority to the bicircular pier
Response of Fresh Water Distributions on Abrupt Changes of Topography in the Pearl River Networks of China
No full textA 2-D numerical model was used to study the response of fresh water transports and distributions on the abrupt changes of topography in the Pearl River Networks (RNPRD). The results indicate that both the tidal forces in Jiaomen and Humen and the runoff power in Modaomen are intensified, which leads to a fresh water movement from the northeast to the southwest side of the West and North River Delta Networks. However, the water distributions in the East River Delta Networks remain almost the same. The residual currents in the RNPRD decreased dramatically in the West and North River Delta Networks due to the increasing volume of the river channels. This decreasing trend was intensified in the North River Main Channel due to the annual water discharge redistribution in the RNPRD
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