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At least 19 recordsLinked to original sources
PubMed2006

Gas adsorption in mesoporous micelle-templated silicas: MCM-41, MCM-48, and SBA-15.

This paper reports a molecular simulation and experimental study on the adsorption and condensation of simple fluids in mesoporous micelle-templated silicas MCM-41, MCM-48, and SBA-15. MCM-41 is described as a regular cylindrical silica nanopore, while SBA-15 is assumed to be made up of cylindrical nanopores that are connected through lateral channels. The 3D-connected topology of MCM-48 is described using a gyroid periodic minimal surface. Argon adsorption at 77 K is calculated for the three materials using Grand Canonical Monte Carlo simulations. Qualitative comparison with experiments for nitrogen adsorption in mesoporous micelle-templated silicas is made. The adsorption isotherm for SBA-15 resembles that for MCM-41. In particular, capillary condensation and evaporation are not affected by the presence of the connecting lateral channels. In contrast, the argon adsorption isotherm for MCM-48 departs from that for MCM-41 having the same pore size. While condensation in MCM-41 is a one-step process, filling of MCM-48 involves two successive jumps in the adsorbed amounts which correspond to condensation in different domains of the porosity. The condensation pressure for MCM-48 is larger than that for MCM-41. We attribute this result to the morphology of the MCM-48 surface (made up of both concave and convex regions) that differs from that for MCM-41 (concave only). Our results suggest that the pore connectivity affects pore filling when the size of the connections is comparable to that of the nanopores.

Adsorption↗
PubMed2003

CDK- and Cdc45-dependent priming of the MCM complex on chromatin during S-phase in Xenopus egg extracts: possible activation of MCM helicase by association with Cdc45.

BACKGROUND: MCM and Cdc45 are required for the initiation and elongation stages of eukaryotic DNA replication. Recent studies show that a purified Mcm4/6/7 complex has DNA helicase activity. However, the biochemical function of the MCM complex and Cdc45 bound to chromatin has not been elucidated. RESULTS: We have examined the biochemical properties of MCM proteins bound to chromatin fractions using Xenopus egg extracts. Immunoprecipitation of MCM proteins extracted under denaturing conditions reveals that all six subunits of MCM and Cdc45 form a tight complex following the initiation of DNA replication, and that both CDK activity and Cdc45 are essential for the complex formation. Chromatin immunoprecipitation of MCM proteins and Cdc45 shows that a complex containing MCM and Cdc45 has a DNA helicase activity which is dependent on CDK activity and Cdc45 in the extracts. Furthermore, both the complex and the helicase activity are resistant to treatment with phosphatase and high salt. CONCLUSIONS: Following the initiation of DNA replication, a tight MCM-Cdc45 complex is formed on chromatin and its formation is closely correlated with the DNA helicase activity of chromatin immunoprecipitates containing MCM and Cdc45. We propose that the tight MCM-Cdc45 complex functions as a replicative DNA helicase in vivo.

Animals↗
PubMed2005

Synthesis of hydrothermally stable and long-range ordered Ce-MCM-48 and Fe-MCM-48 materials.

The hydrothermally stable and long-range ordered Ce-MCM-48 and Fe-MCM-48 were conveniently synthesized at 393 K for 24 h by directly adding fluoride ions to the initial gel without posttreatment and pH adjustment. The Ce-MCM-48 with a Si/Ce of 200 and 100, which were prepared by directly adding fluoride ions, could still maintain their mesoporous structures after refluxing in boiling water for 3 days. The incorporation of Ce into MCM-48 could enhance the hydrothermal stability of MCM-48 in the absence of fluoride ions; however, the incorporation of Fe into MCM-48 materials barely improved the hydrothermal stability of MCM-48 materials. The effect of adding NaF was much more efficient in enhancement of hydrothermal stability than that of the incorporation of Ce. The addition of fluoride ions mainly improves the degree of polymerization of silicates. The Ce(4+) ions in Ce-MCM-48 appear to be present partly in tetrahedral coordination in the framework and partly as CeO(2) particles on the surface of framework. The Ce positioned on the surface of pore walls and in the framework both provide the protection against water attack.

Journal Article↗
PubMed2000

Formation Process of MCM-41 Precursor and Porous Texture of MCM-41.

The formation process of the MCM-41 precursors (silicate/surfactant complex) was investigated on the basis of the pH titration curves of Na(4)SiO(4) in the presence of [C(16)H(33)N(CH(3))(3)]Cl. Measurements of the pH titration curves were carried out using the computer-controlled gravimetric titrator constructed in our laboratory. The white precipitate (MCM-41 precursor) was abruptly formed at pH 11.1 (298 K) and at pH 9.9 (343 K). Formation of the MCM-41 precursor can be explained by coagulation of the rod-like micelle colloids whose surface is covered by the condensed silicate anions of (HSiO(3))(n)(n-). The porous texture of the MCM-41 samples whose precursors were synthesized under different conditions was analyzed on the basis of the adsorption isotherms of nitrogen at 77 K. It has been shown that the MCM-41 sample whose precursor was prepared at pH 9.9 and 343 K shows one sharp peak (r(p) = 1.65 nm) in the pore size distribution curve, but the MCM-41 samples whose precursors were prepared at pH 6.5-5.0 and 343 K give two peaks (r(p) = 1.66 nm and r(p) = 2.12-2.36 nm). The appearance of the second peak (r(p) = 2.12-2.36 nm) has been considered to be in connection with the destruction of the MCM-41 precursor into small fragments in acidic medium. Copyright 2000 Academic Press.

Journal Article↗
PubMed2006

Evidence of nanoconfinement effects of MCM-41 on propylene polymerization catalyzed by MCM-41 supported metallocene catalyst in the presence and absence of beta-cyclodextrin.

Propylene polymerization was carried out with MCM-41 supported rac-Et(Ind)2ZrCl2 catalysts, in the presence and absence of beta-cyclodextrin. The resultant PP was studied by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and differential scanning calorimetry. Through comparison of the results, it was found that the channels of MCM-41 could act as a nanoreactor of propylene polymerization and the polypropylene (PP) contained in the channels had noncrystal structure. However, the PP could grow out of the channels and form some crystals after the active sites on the surface of MCM-41 were destroyed. This showed that the channels of MCM-41 had great confinement effects on propylene polymerization.

Journal Article↗
PubMed2001

Cationic host-guest polymerization of N-vinylcarbazole and vinyl ethers in MCM-41, MCM-48, and nanoporous glasses.

The synthesis of poly(vinyl ether)s or polyvinylcarbazole under the conditions of constricted geometry can be achieved by means of cationic host-guest polymerisation of the corresponding monomers in the pores of MCM-41 (pore diameter 3.6 nm), MCM-48 (pore diameter 2.4 nm) and in nanoporous glasses (Gelsil with a pore diameter of 5 nm) with bis(4-methoxyphenyl)methyl chloride (BMCC) or triphenylmethyl chloride as the internal surface initiator. The reaction products are new polymer/ MCM-41, polymer/MCM-48 etc., host-guest hybrid materials. The molecular mass of the enclosed polymer and the degree of loading of the host compounds can be adjusted within certain limits. The molecular dynamics were investigated by using broad-band dielectric spectroscopy. Under the conditions of constricted geometry, molecular fluctuation is observed as well as a secondary beta-relaxation, which is hardly affected (in comparison with the free melt) and which corresponds to the relaxation between structural substates (dynamic glass transition). This process is several orders of magnitude faster in its relaxation rate than in the free melt and thus follows a confinement effect. This is already well known in lower molecular weight systems with constricted geometry.

Journal Article↗
PubMed2002

Anchored Pd complex in MCM-41 and MCM-48: novel heterogeneous catalysts for hydrocarboxylation of aryl olefins and alcohols.

We report here, for the first time, synthesis of anchored Pd complexes in mesoporous supports such as MCM-41 and MCM-48 as true heterogeneous catalysts for hydrocarboxylation of aryl olefins and alcohols to give excellent conversion ( approximately 100%) and regioselectivity ( approximately 99%) for 2-arylpropionic acids. The catalysts were characterized by powder-XRD, 31P CP-MAS NMR, FT-IR, TEM, XPS and ICP-AES. Recycle studies with these anchored Pd mesoporous catalysts were performed to confirm true heterogeneity.

Alcohols↗
PubMed2002

Chiral ruthenium porphyrin encapsulated in ordered mesoporous molecular sieves (MCM-41 and MCM-48) as catalysts for asymmetric alkene epoxidation and cyclopropanation.

Encapsulation of chiral ruthenium porphyrin [RuII(D4-Por*)CO] in modified mesoporous silica supports such as MCM-41 and MCM-48 achieves active catalysts for asymmetric epoxidation of alkenes by 2,6-dichloropyridine N-oxide and intramolecular cyclopropanation, which is the first example of chiral metalloporphyrin supported on ordered molecular sieves.

Journal Article↗
PubMed2006

Heterogeneous photocatalytic degradation of methanol over uranyl-anchored nanoporous MCM-41 and MCM-48.

The vapor-phase photodegradation of methanol to carbon dioxide was carried out over uranyl-anchored nanoporous MCM-41 and MCM-48 hosts (designated as UO2(2+)/MCM-41 and UO2(2+)/MCM-48, respectively) under simulated light and ambient conditions. Preliminary results indicate that the photoactivity of the latter is considerably decreased as compared to the former due to the presence of a smaller fraction of photoactive uranyl (UO2(2+)) ions in UO2(2+)/MCM-48.

Catalysis↗
PubMed2001

Redirect the assembly of hexagonal MCM-41 into cubic MCM-48 from sodium silicate without the use of an organic structure modifier.

In contrast to earlier methods requiring the presence of organic modifiers for the preparation of cubic MCM-48 mesostructures from low cost sodium silicate, we show that this three dimensional framework structure can be readily assembled in the absence of co-surfactants simply by using the overall SiO2/OH- ratio as means of controlling the surfactant packing parameter.

Journal Article↗
PubMed1996

Physical interactions among Mcm proteins and effects of Mcm dosage on DNA replication in Saccharomyces cerevisiae.

Mcm2, Mcm3, and Mcm5/Cdc46 are conserved proteins essential for the initiation of DNA synthesis at replication origins in Saccharomyces cerevisiae. The accumulation of these proteins in the nucleus before the onset of DNA synthesis suggests that they play a role in restricting DNA synthesis to once per cell cycle. In this work, we show that Mcm2, Mcm3, and Mcm5 self-interact and interact with one another to form complexes. Mcm2 and Mcm3 are abundant proteins, present in approximately 4 X 10(4) and 2 X 10(5) copies per cell, respectively. Reducing the dosage of Mcm2 by half results in diminished usage of specific replication origins. These results together suggest that a significant molar excess of Mcm proteins relative to replication origins is required for the proper initiation of all replication origins.

Cell Cycle Proteins↗
PubMed2005

Synthesis and characterization of highly ordered Ni-MCM-41 mesoporous molecular sieves.

Highly ordered Ni-MCM-41 samples with nearly atomically dispersed nickel ions were prepared reproducibly and characterized. Similar to the Co-MCM-41 samples, the pore diameter and porosity can be precisely controlled by changing the synthesis surfactant chain length. Nickel was incorporated by isomorphous substitution of silicon in the MCM-41 silica framework, which makes the Ni-MCM-41 a physically stable catalyst in harsh reaction conditions such as CO disproportionation to single wall carbon nanotubes or CO2 methanation. X-ray absorption spectroscopy results indicate that the overall local environment of nickel in Ni-MCM-41 was a tetrahedral or distorted tetrahedral coordination with surrounding oxygen anions. Hydrogen TPR revealed that our Ni-MCM-41 samples have high stability against reduction; however, compared to Co-MCM-41, the Ni-MCM-41 has a lower reduction temperature, and both the H2-TPR and in situ XANES TPR reveal that the reducibility of nickel is not clearly correlated with the pore radius of curvature, as in the case of Co-MCM-41. This is probably a result of nickel being thermodynamically more easily reduced than cobalt. The stability of the structural order of Ni-MCM-41 has been investigated under SWNT synthesis and CO2 methanation reaction conditions as both require catalyst exposure to reducing environments leading to formation of metallic Ni clusters. Nitrogen physisorption and XRD results show that structural order was maintained under both SWNT synthesis and CO2 methanation reaction conditions. EXAFS results demonstrate that the nickel particle size can be controlled by different prereduction temperatures but not by the pore radius of curvature as in the case of Co-MCM-41.

Journal Article↗
PubMed2006

Adsorption of basic dyes onto MCM-41.

The adsorption of two basic dyes, Basic Green 5 (BG5) and Basic Violet 10 (BV10), onto MCM-41 was studied to examine the possible effect of interactions between large adsorbates and MCM-41 on the pore structure stability of MCM-41 and the potential of MCM-41 for the removal of basic dyes from wastewater. The revolutions of surface characteristics and pore structure of MCM-41 induced by dyes adsorption were characterized based on the analyses of the nitrogen isotherms, the XRD patterns, and the FTIR spectra. It was experimentally concluded that when the effect of interactions between large dyes (such as BV10) and MCM-41 on the pore structure stability of MCM-41 was insignificant, MCM-41 might be a good adsorbent for the removal of basic dyes from wastewater. The adsorption of BV10 on MCM-41 with respect to contact time, pH, and temperature was then measured to provide more information about the adsorption characteristics of MCM-41. Both Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms and the pseudo-second-order kinetic model was used to describe the kinetic data, from which some adsorption thermodynamic parameters were also evaluated.

Adsorption↗
PubMed1997

A globular complex formation by Nda1 and the other five members of the MCM protein family in fission yeast.

BACKGROUND: In the fission yeast Schizosaccharomyces pombe, Nda1, Nda4, Mis5 and Cdc21 proteins belong to the MCM (minichromosome maintenance) protein family which is thought to have six members. Each MCM member is required for the early stages of DNA replication, and has a well-conserved central 200-amino acid domain containing a putative ATP binding motif. However, the precise molecular functions of MCM proteins are not yet clear. RESULTS: We investigated the physical interaction of Nda1 protein with the other fission yeast MCM proteins using specific antibodies. Immunoprecipitation of Nda1 protein leads to the co-precipitation of all the other members of the fission yeast MCM protein family. We purified the MCM protein complex by a combination of column chromatography. The native molecular weight of the MCM complex was estimated by gel filtration to be 560 kDa. The purified fraction contained nearly equal quantities of the six MCM proteins. Electron microscope observation showed that the MCM complex has a globular shape with a central cavity. CONCLUSIONS: We have developed a procedure to purify fission yeast MCM proteins in a native hetero-oligomeric complex form for the first time, which opens an avenue to further biochemical analysis.

Amino Acid Sequence↗
PubMed2004

Pairwise interactions of the six human MCM protein subunits.

The eukaryotic minichromosome maintenance (MCM) proteins have six subunits, Mcm2 to 7p. Together they play essential roles in the initiation and elongation of DNA replication, and the human MCM proteins present attractive targets for potential anticancer drugs. The six MCM subunits interact and form a ring-shaped heterohexameric complex containing one of each subunit in a variety of eukaryotes, and subcomplexes have also been observed. However, the architecture of the human MCM heterohexameric complex is still unknown. We systematically studied pairwise interactions of individual human MCM subunits by using the yeast two-hybrid system and in vivo protein-protein crosslinking with a non-cleavable crosslinker in human cells followed by co-immunoprecipitation. In the yeast two-hybrid assays, we revealed multiple binary interactions among the six human MCM proteins, and a subset of these interactions was also detected as direct interactions in human cells. Based on our results, we propose a model for the architecture of the human MCM protein heterohexameric complex. We also propose models for the structures of subcomplexes. Thus, this study may serve as a foundation for understanding the overall architecture and function of eukaryotic MCM protein complexes and as clues for developing anticancer drugs targeted to the human MCM proteins.

Cell Cycle Proteins↗
PubMed2005

Postsynthesis vapor-phase functionalization of MCM-48 with hexamethyldisilazane and 3-aminopropyldimethylethoxylsilane for bioseparation applications.

MCM-48 was surface modified via vapor-phase reactions with hexamethyldisilazane (CH(3)-MCM-48) and 3-aminopropyldimethylethoxysilane (NH(2)-MCM-48). (29)Si NMR confirmed that the resulting materials contained covalently attached trimethylsilane and 3-aminopropyldimethylsilane moieties, both important functionalities for bioseparation applications. The surface coverage was approximately 1.8 and 0.9 groups per nm(2), respectively. The X-ray diffraction patterns and the narrow pore size distributions obtained from the gas sorption isotherms showed that the modified materials retained the characteristic pore structure of the underlying MCM-48 material. CH(3)-MCM-48 exhibited significantly improved hydrolytic stability over the unmodified MCM-48 under the aqueous conditions tested, whereas NH(2)-MCM-48 appeared to be less stable than the unmodified MCM-48. The decrease in stability is most likely due to the nature of the attachment of the 3-aminopropyldimethylsilane moiety, where the conversion of surface silanol groups is limited by H bonding with the amino end, leading to a 50% lower surface concentration and resulting in an increased likelihood of nucleophilic attack on the silica surface, enhancing the rate of hydrolysis. Hexamethyldisilazane thus appears to be a superior functional group for modifying the MCM-48 surface.

Indicators and Reagents↗
PubMed2005

Synthesis, structure, and acidic properties of MCM-41 functionalized with phosphate and titanium phosphate groups.

Mesoporous molecular sieves Si-MCM-41 (purely siliceous) and Ti-MCM-41 (partly covered with a surface layer of TiO2) were functionalized with phosphate groups by treatment with POCl3 (denoted -MCM-41(P)and Ti-MCM-41(P), respectively). With the use of TEM, X-ray diffraction, and N2 adsorption, it was shown that the initial hexagonal structure, the high specific surface area, and porosity are retained in the functionalized materials but are not as good as in the starting materials. 1H MAS NMR and 31P MAS NMR revealed that the surface of Si-MCM-41(P) consists of silicon phosphate and pyrophosphate species. That of Ti-MCM-41(P) additionally contains titanium dihydro-, hydro-, and pyrophosphate species, the latter being predominant. TPD of adsorbed ammonia for Si-MCM-41(P) and Ti-MCM-41(P) showed that functionalization leads to the creation of moderate and strong acid sites. A combination of mesoporous structure with acidic properties makes the MCM-41 functionalized with phosphate groups promising for use as solid acid catalysts.

Journal Article↗