209 research outputs found
Cryo-EM structure of a helicase loading intermediate containing ORC-Cdc6-Cdt1-MCM2-7 bound to DNA
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A chromatinized origin reduces the mobility of ORC and MCM through interactions and spatial constraint
No full textChromatin replication involves the assembly and activity of the replisome within the nucleosomal landscape. At the core of the replisome is the Mcm2-7 complex (MCM), which is loaded onto DNA after binding to the Origin Recognition Complex (ORC). In yeast, ORC is a dynamic protein that diffuses rapidly along DNA, unless halted by origin recognition sequences. However, less is known about the dynamics of ORC proteins in the presence of nucleosomes and attendant consequences for MCM loading. To address this, we harnessed an in vitro single-molecule approach to interrogate a chromatinized origin of replication. We find that ORC binds the origin of replication with similar efficiency independently of whether the origin is chromatinized, despite ORC mobility being reduced by the presence of nucleosomes. Recruitment of MCM also proceeds efficiently on a chromatinized origin, but subsequent movement of MCM away from the origin is severely constrained. These findings suggest that chromatinized origins in yeast are essential for the local retention of MCM, which may facilitate subsequent assembly of the replisome.BN/Nynke Dekker La
One-Pot Room-Temperature Synthesis of Mg Containing MCM-41 Mesoporous Silica for Aldol Reactions
No full textOne-pot synthesis of Mg containing MCM-41 (Mg-MCM-41) materials was respectively carried out by a room temperature (RT) method and a hydrothermal (HT) method for aldol condensation of 4-nitrobenzaldehyde and acetone and self-condensation of acetone. The RT method can prepare MCM-41 materials containing large amounts of Mg while maintaining the structural characteristics of MCM-41 even at very low Si/Mg ratios (large Mg loadings), but the HT method cannot. The RT method can also give more active catalysts than the HT method, because the catalysts prepared by the RT method are more basic than those prepared by the HT one. The characterization indicates that Mg atoms in the Mg-MCM-41 prepared by the RT method exist as MgO disperses well on the wall surface of pores, while those in Mg-MCM-41 prepared by the HT method are included in the bulk with a smectite-like structure. (C) 2018 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology
Synthesis and Characterization of MCM-41 spheres inside bioactive glass-ceramic scaffold
No full textComposite systems have been prepared, combining the drug up-take and release properties of MCM-41 silica sub-micron spheres with those of a bioactive glass-ceramic macroporous scaffold belonging to the SiO2-CaO-K2O (SCK) system (potentially able to promote successful osteointegration in tissue engineering). The systems are prepared by dipping the glass-ceramic scaffold into the MCM-41 synthesis solution. To this purpose, the pH of the MCM-41 synthesis solution has been lowered with respect to the usual literature procedure to minimize the detrimental effects of the impregnation treatment on glass-ceramic scaffolds that were observed in a previous work by the author. MCM-41-SCK composite scaffolds have been characterized by means of XRD, N2 adsorption, thermogravimetry and Scanning Electron Microscopy (coupled with EDS analysis). The adsorption capacity toward ibuprofen, tested as model drug, is three times higher than that of the MCM-41-free scaffold, because of the presence of the ordered mesoporous silica. Also the release behaviour in SBF at 37◦C is strongly affected by the presence of MCM-41 inside the scaffold macropore
The design of a high efficiency RF power amplifier for an MCM process
No full textThesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 91-92).In this thesis, I addressed issues arising in the design of a high efficiency RF power amplifier for the Draper Laboratory multi-chip module (MCM) process. A design for a 2.3 GHz PCB amplifier using an enhancement-mode pHEMT device that achieves 68.9% PAE at 30 dBm output power is presented. Analysis of heat management, die connection parasitics, and transmission line structures in the context of the MCM process is performed to show that a similar design could realistically be adapted to the MCM process with possible performance enhancement.by James Noonan.M.Eng
Grafting luminescent metal-organic species into mesoporous MCM-41 silica from europium(III)tetramethylheptanedionate, Eu(thd)3
No full textMixed systems with Eu(III) β-diketonates as optically active guest species, and mesoporous silicas MCM-41 as a host matrix have been investigated. The grafting of europium(III) onto the inner walls of unmodified MCM-41 has been achieved starting from Eu(thd)3 (thd = 2,2,6,6- tetramethyl-3,5-heptanedionate), using two routes: wet impregnation (WI) at room temperature,and chemical vapour infiltration (CVI) at 185 °C. In received hybrids, denoted Eu(thd)x@MCM- 41, the same maximum yield [Eu]/[Si] = 8.2 at% on average has been achieved with either methods. The molar ratio x = [thd]/[Eu] is 0.6 on average for WI samples, and 1.5 for CVI samples. In the latter, higher contents in thd compensate lower contents in silanols with respect to the former. Rationalizing the possible bonds exchanged at the silica surface leads to a great diversity of possible co-ordination schemes according to the expression Σ[Si(OH)nx (O)xEu(thd)3-x] (where Σ means that surface species are considered). Chromophore neutral ligands phenanthroline (phen) or bipyridine (bipy) have been added to induce efficient Eu3+ luminescence under 270–280 nm excitation, via the antenna effect. For the most favourable case, (phen)yEu(thd)x@MCM-41, the emission intensity at 612 nm under excitation at 270 nm is 2/3 that for the genuine heteroleptic complex Eu(thd)3(phen). Moreover the hybrid material is stable up to 440 °C
Hydrodeoxygenation of Guaiacol over Al-MCM-41 Supported Metal Catalysts: A Comparative Study of Co and Ni
No full textAbstractThe hydrodeoxygenation (HDO) of guaiacol, a model compound of pyrolysis oil, was catalyzed by Al-MCM-41 supported Co and Ni catalysts in a fixed-bed continuous reactor at ambient pressure. The effects of contact time (W/F) and reaction temperature on HDO of guaiacol over the Co and Ni catalyst were studied. Cobalt showed higher activity than nickel in HDO via C−O hydrogenolysis pathway. The conversion of guaiacol over Ni catalyst principally occurred via C−C hydrogenolysis pathway, resulting in methane formation. Besides, the acidic Al-MCM-41 support catalyzed the methyl transfer reaction to form the methylated products. Additionally, increasing reaction temperature improved the HDO and suppressed the hydrogenation but promoted the methanization activities
Open-ringed structure of the Cdt1-Mcm2-7 complex as a precursor of the MCM double hexamer
No full textThe minichromosome maintenance complex (MCM) hexameric complex (Mcm2-7) forms the core of the eukaryotic replicative helicase. During G1 phase, two Cdt1-Mcm2-7 heptamers are loaded onto each replication origin by the origin-recognition complex (ORC) and Cdc6 to form an inactive MCM double hexamer (DH), but the detailed loading mechanism remains unclear. Here we examine the structures of the yeast MCM hexamer and Cdt1-MCM heptamer from Saccharomyces cerevisiae. Both complexes form left-handed coil structures with a 10-15-angstrom gap between Mcm5 and Mcm2, and a central channel that is occluded by the C-terminal domain winged-helix motif of Mcm5. Cdt1 wraps around the N-terminal regions of Mcm2, Mcm6 and Mcm4 to stabilize the whole complex. The intrinsic coiled structures of the precursors provide insights into the DH formation, and suggest a spring-action model for the MCM during the initial origin melting and the subsequent DNA unwinding.Ministry of Science and Technology of China [2013CB910404, 2016YFA0500700]; National Natural Science Foundation of China [31422016, 31470722, 31630087]; Research Grants Council of Hong Kong [GRF16138716, GRF664013, HKUST12/CRF/13G, GRF16104115, GRF16143016, IGN15SC02]; Peking-Tsinghua Center for Life SciencesSCI(E)ARTICLE3300-+2
Selecive Catalytic Reduction of NOx in Lean Burn Engine Exhaust by Highly Active Pt Supported on V-impregnated MCM-41
No full textOn the Pt/V/MCM-41 catalyst with 1wt% Pt and 4wt% V, the maximum conversion of NO reduction into N2+N2O by C3H6 was about 73%, which was 10% higher than on 1wt% Pt/silica, and this maximum conversion was maintained over a temperature range of 70.DEG.C. between 270 and 340.DEG.C.. (author abst.
Comprehensive characterization of iron oxide containing mesoporous molecular sieve MCM-41
No full textWe have synthesized iron oxide-containing mesoporous silica MCM-41 by a direct route. This material has a significant microporousity, which might be related with the loading of iron oxide in the MCM-41 mesopores, though no crystalline phases of iron oxide were detected by XRD. Intensity deviations and extra bright intensity within the pores, as well as local superstructural phases within a large area, were observed in the TEM images. The position and area of such a crystalline phase could change during the successive electron beam exposure and resulted in a formation of a cycle. These phenomena imply the possible existence of small Fe2O3 nanocrystals loaded in the mesoporous silica materials with regular and oriented arrangement. EELS spectrum of the sample proves the existence of iron, but the chemical environment of iron in the MCM-41 silica should be different from that of bulk amorphous porous iron oxide. The small iron oxide crystals in the MCM-41 sample might be linked with silica wall through oxygen. Another possibility is that iron oxide reacts with silica during synthesis, causing the partial crystallization of the MCM-41 framework. In other words, microcrystals of iron oxide may exist in the silica matrix.Chemistry, PhysicalSCI(E)CPCI-S(ISTP)
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