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PubMed2002

Gas-phase enzymatic esterification on immobilized lipases in MCM-41 molecular sieves.

Several lipolytic enzymes were immobilized in the pores of MCM-41 and Al-MCM-41 molecular sieves and used as catalysts in the gas-phase esterification of acetic acid with ethanol. The entrapment of enzymes depended on the molecular sieve and the type of enzyme used. The order of enzymatic activity for enzymes entrapped in the pores of MCM-41 and Al-MCM-41 in the esterification reaction was OF (Rhizopus niveus lipases) < FAP-15 (Rhizopus oryzae lipases) < LEX (Mucorjavanicus lipases) < PS (Pseudomonas cepacia lipases) < AK (Pseudomonas fluorescens lipases). Experiments carried out between 298 and 318 K showed no effect of temperature on catalyst yield, suggesting that the enzymes were appropriately immobilized in the pores of the molecular sieves, thus avoiding possible processes such as denaturing or autolysis.

Enzymes, Immobilized↗

PubMed2005

[Grafting reaction of tetrabutyltin on the MCM-41 surface and characterization of the grafted product].

MCM-41 modified with tetrabutyltin and Sn-O-MCM-41 obtained from calcination of the modified MCM-41 in pure oxygen were characterized by XRD, FTIR, UV-Vis DRS, and TG analysis. It was found that the hydrothermal stability, surface acidity, and coordinative states of tin atoms on both samples were evidently different. After calcination, the surface tin atoms of sample remained in the tetrahedronal structure, and the hydrothermal stability and surface acidity were improved. According to the results, the reason for the enhancement of catalytic activity of Sn-O-MCM-41 was discussed.

Catalysis↗

PubMedDate not supplied

Mcm-2 protein expression predicts prognosis better than Ki-67 antigen in oral cavity squamocellular carcinoma.

BACKGROUND: The present study aimed at evaluating the expression intensities of the Mcm-2 protein and Ki-67 antigen in squamocellular carcinomas of the oral cavity and comparing their prognostic value. MATERIALS AND METHODS: Forty-nine patients, operated on and treated with radiotherapy for carcinoma of the oral cavity floor and/or the mobile part of the tongue, were retrospectively analyzed. RESULTS: A significant positive correlation was noted between the expression of Mcm-2 protein and that of the Ki-67 antigen, as well as an absence of such correlations with the remaining examined factors. A significant correlation with worse disease-specific survival period (DSS) in the group of patients demonstrating Mcm-2 protein expression in over 10% of cancer cells was detected (5-year cumulative DSS 50% vs. 76%). CONCLUSION: The results suggested that the expression of Mcm-2 protein may be used as a prognostic factor in patients with squamocellular carcinoma of the oral cavity.

Carcinoma, Squamous Cell↗

PubMed2000

Laser-tissue interaction with a continuous wave 3-mcm fibre laser: preliminary studies with soft tissue.

BACKGROUND AND OBJECTIVE: Lasers operating at wavelengths in the mid-infrared region have become increasingly popular for applications in areas of surgery and medicine. Advances in fibre laser technology have introduced a highly efficient, compact, diode-pumped source operating at around the 3-mcm wavelength. This study examines the effects of this recently developed laser on soft biological tissue. STUDY DESIGN/MATERIALS AND METHODS: Chicken breast and liver tissue samples were exposed to 800 mW continuous wave laser power, at a wavelength of 2.71 mcm, with incident spot sizes of around 150 mcm. Samples were inspected grossly immediately after laser irradiation and also prepared for histologic processing. RESULTS: After irradiation, visual assessment of changes at sample surfaces indicated a region of thermally affected tissue surrounding the ablation crater. This region was observed to grow in size to around 1.0 mm in diameter after 3 seconds of laser exposure at 800 mW. An ablation velocity of 0.80 mm.s(-1) was determined in chicken breast for the same incident laser parameters. Analysis of samples irradiated at 800 mW and processed for histology revealed minimal damage at hole boundaries and no signs of char formation, providing incident exposure times were restricted to below around 0. 5 seconds. CONCLUSION: This fibre laser source has demonstrated its potential to fulfil medical applications, enabling accurate, precise tissue removal to proceed at a rapid ablation rate. The efficiency and small size of the laser are attractive features.

Animals↗

PubMed2001

Evaluation of Surface Enthalpy of Porous Aluminosilicates of the MCM-41 Type Using Immersional Calorimetry: Effect of the Pore Size and Framework Si:Al Ratio.

Surface properties of porous aluminosilicates of the MCM-41 type have been tested by immersional calorimetry. Two series of materials, referred to as SiAlxCn, where x is the Si : Al mole ratio and n the chain length of the surfactant template, having (1) x=32 and n=8, 12, 14, 16, 18 and (2) n=14 and x=8, 32, infinity, were used. The results of thermogravimetric analysis on these samples served to evaluate the surface density of hydroxyl groups. This parameter is rather sensitive to the pore size than to the aluminium content in the solid matrix. Based on the experimentally measured enthalpies of immersion in n-heptane, water, and formamide per unit BET specific surface area, estimates could be made of the apolar, Lewis acid, and Lewis base contributions to the total surface enthalpy of MCM-41 materials. The samples studied have a predominant surface acidic character, which is markedly enhanced by incorporating aluminium into the silica matrix. Surface acidity is also modified by changes in the porous structure, although the trends are less noticeable here. Nevertheless, the total surface enthalpy of MCM-41 aluminosilicates appears to be small in comparison with typical inorganic oxides, such as silica or alumina. Copyright 2001 Academic Press.

Journal Article↗

PubMed1996

cDNA cloning and characterisation of a maize homologue of the MCM proteins required for the initiation of DNA replication.

A central question in cell cycle regulation is how DNA replication is initiated and executed only once in each cell cycle. The cell cycle-regulated assembly of specific initiation protein complexes at chromosomal origins appears to specify the initial sites and timing of DNA replication, and to restrict this process to only one round in the somatic cell cycle. Among the enzymes involved in origin activation, the MCM proteins play a conserved key role. In particular, MCM3 homologues have been shown to be components of the DNA replication licensing activity in yeast and vertebrates. In spite of our detailed knowledge of the regulation of the initiation of DNA synthesis in yeast, there is virtually no information available on the molecules involved in origin activation in higher plants. We have isolated a cDNA from maize root apices, termed ROA (Replication Origin Activator), encoding a protein which shares a high degree of homology with the MCM3 subfamily of MCM proteins. Analysis of gene organisation by Southern blotting shows 2-4 copies per haploid genome of closely related ROA sequences and the presence of further less related sequences in a multigene family. The steady-state levels of ROA mRNA are under developmental control, being relatively high in proliferative tissues such as the root apex, the developing cob and the coleoptile, and are strongly correlated with that of the histone H4 transcript. In situ hybridisation analysis in the root apex reveals that ROA mRNA expression is limited to specific subpopulations of cycling cells, which is typical of cell cycle-regulated expression. The isolation of nearly identical sequences from barley and Arabidopsis by the polymerase chain reaction indicates that MCM-related proteins are conserved in higher plants.

Amino Acid Sequence↗

PubMed2005

Identification of a novel cell-cycle-induced MCM family protein MCM9.

The minichromosome maintenance (MCM) proteins are essential for DNA replication initiation and elongation in eukaryotes. In mammalian cells, MCM2-MCM7 complexes are believed to unwind DNA during chromosomal DNA replication. Here we identified a novel MCM family gene, MCM9, by using bioinformatics. Human, mouse, and rat MCM9 showed approximately 90-91% total-amino acid identity. MCM9 showed 24-31% total-amino acid identity with MCM2-MCM8 protein. Phylogenetic analysis on MCM family members revealed that MCM9 was most closely related to MCM8. Human, mouse, and rat MCM9 gene, consisting of 7, 8, and 7 exons, was mapped to 6q22.1-22.33, 10B3, and 20q11, respectively. We identified transcription factor E2F-binding motifs in the vicinity of the transcription start site among human, mouse, and rat MCM9 gene. MCM9 mRNA was upregulated by transcription factor E2E1 and serum stimulation in NIH3T3 cells.

Amino Acid Sequence↗

PubMed2005

Application of zeolite MCM-22 for basic dye removal from wastewater.

MCM-22 was employed as an effective adsorbent for removal of basic dyes including methylene blue, crystal violet, and rhodamine B from aqueous solution. The adsorption kinetics and isotherms were investigated. The adsorption capacity of MCM-22 for three dyes follows an order of MB > CV approximately RB. Kinetic studies indicate that the adsorption follows the pseudo second-order kinetics and the adsorption is a two-step diffusion process with film diffusion dominating the process. The adsorption isotherm can be well fitted by both the Langmuir and the Freundlich models. Thermodynamic calculations suggest that the adsorption of basic dyes on MCM-22 is an endothermic reaction.

Adsorption↗

PubMed2005

Comparison of nitrogen adsorption at 77 K on non-porous silica and pore wall of MCM-41 materials by means of density functional theory.

Nitrogen adsorption on a surface of a non-porous reference material is widely used in the characterization. Traditionally, the enhancement of solid-fluid potential in a porous solid is accounted for by incorporating the surface curvature into the solid-fluid potential of the flat reference surface. However, this calculation procedure has not been justified experimentally. In this paper, we derive the solid-fluid potential of mesoporous MCM-41 solid by using solely the adsorption isotherm of that solid. This solid-fluid potential is then compared with that of the non-porous reference surface. In derivation of the solid-fluid potential for both reference surface and mesoporous MCM-41 silica (diameter ranging from 3 to 6.5 nm) we employ the nonlocal density functional theory developed for amorphous solids. It is found that, to our surprise, the solid-fluid potential of a porous solid is practically the same as that for the reference surface, indicating that there is no enhancement due to surface curvature. This requires further investigations to explain this unusual departure from our conventional wisdom of curvature-induced enhancement. Accepting the curvature-independent solid-fluid potential derived from the non-porous reference surface, we analyze the hysteresis features of a series of MCM-41 samples.

Journal Article↗

PubMed2005

Adsorption and diffusion of benzene in the nanoporous catalysts FAU, ZSM-5 and MCM-22: a molecular dynamics study.

Molecular dynamics (MD) simulations of benzene in siliceous zeolites (FAU, ZSM-5, and MCM-22) were performed at loadings of 1, 2, 4, 8, and 16 molecules per supercell. The potential energy functions for these simulations were constructed in a semi-empirical way from existing potentials and experimental energetic data. The MD simulations were employed to analyze the dynamic properties of the benzene-zeolite systems. The adsorption energies of benzene/siliceous zeolite complexes increase with increasing loading number, due to the intermolecular attraction between benzene molecules. The self-diffusion coefficient of benzene in siliceous zeolites decreases with increasing loading due to the steric hindrance between the sorbates passing each other. From the zeolite-benzene radial distribution functions it was found that the benzene molecules are relatively far from each other, about 5.2A for siliceous FAU, 5.2A for siliceous ZSM-5, and 4.8A for siliceous MCM-22. In the case of FAU, the benzene molecules prefer to be adsorbed parallel to the surface of the sodalite cage above the six-membered-ring. In ZSM-5, we found a T-structure of the benzene molecules at loadings 2, 4, and 8 molecules per supercell. At loadings of 16 molecules per supercell, the molecules are lined up along the straight channel and their movement is highly correlated. For MCM-22 we found adjacent benzene molecules at a loading of 4 molecules with an orientation similar to the stacked conformation of benzene dimer in the gas phase.

Adsorption↗

PubMed2006

Cdc7-Dbf4 phosphorylates MCM proteins via a docking site-mediated mechanism to promote S phase progression.

Origins of DNA replication are licensed in G1 by recruiting the minichromosome maintenance (MCM) proteins to form a prereplicative complex (pre-RC). Prior to initiation of DNA synthesis from each origin, a preinitiation complex (pre-IC) containing Cdc45 and other proteins is formed. We report that Cdc7-Dbf4 protein kinase (DDK) promotes assembly of a stable Cdc45-MCM complex exclusively on chromatin in S phase. In this complex, Mcm4 is hyperphosphorylated. Studies in vitro using purified DDK and Mcm4 demonstrate that hyperphosphorylation occurs at the Mcm4 N terminus. However, the DDK substrate specificity is conferred by an adjacent DDK-docking domain (DDD), sufficient for facilitating efficient phosphorylation of artificial phosphoacceptors in cis. Genetic evidence suggests that phosphorylation of Mcm4 by DDK is important for timely S phase progression and for cell viability upon overproduction of Cdc45. We suggest that DDK docks on and phosphorylates MCM proteins at licensed origins to promote proper assembly of pre-IC.

Amino Acid Motifs↗

PubMed1997

Cloning of Drosophila MCM homologs and analysis of their requirement during embryogenesis.

MCM (minichromosome maintenance) gene family of Saccharomyces cerevisiae encodes essential DNA replication factors that participate in the initiation of DNA replication. In addition, their localization to the nucleus in a mitosis-dependent manner fueled the hypothesis that MCMs also act to couple DNA replication to mitosis. We report the identification of a Drosophila gene family with extensive sequence identity to the MCM genes. Results from antibody injection experiments suggest that MCMs play an essential role in DNA replication during embryogenesis. Evolutionary conservation of MCM sequences and function in Drosophila could potentially facilitate studies of how initiation of DNA replication is regulated and coupled to mitosis during metazoan development.

Amino Acid Sequence↗

PubMedDate not supplied

Biosorption of lead, cadmium, and zinc by Citrobacter strain MCM B-181: characterization studies.

The biosorption process for removal of lead, cadmium, and zinc by Citrobacter strain MCM B-181, a laboratory isolate, was characterized. Effects of environmental factors and growth conditions on metal uptake capacity were studied. Pretreatment of biomass with chemical agents increased cadmium sorption efficiency; however, there was no significant enhancement in lead and zinc sorption capacity. Metal sorption by Citrobacter strain MCM B-181 was found to be influenced by the pH of the solution, initial metal concentration, biomass concentration, and type of growth medium. The metal sorption process was not affected by the age of the culture or change in temperature. Equilibrium metal sorption was found to fit the Langmuir adsorption model. Kinetic studies showed that metal uptake by Citrobacter strain MCM B-181 was a fast process, requiring <20 min to achieve >90% adsorption efficiency. The presence of cations reduced lead, zinc, and cadmium sorption to the extent of 11. 8%, 84.3%, and 33.4%, respectively. When biomass was exposed to multimetal solutions, metals were adsorbed in the order Co2+ < Ni2+ < Cd2+ < Cu2+ < Zn2+ < Pb2+. Among various anions tested, only phosphate and citrate were found to hamper metal sorption capacity of cells. Biosorbent beads prepared by immobilizing the Citrobacter biomass in polysulfone matrix exhibited high metal loading capacities. A new mathematical model used for batch kinetic studies was found to be highly useful in prediction of experimentally obtained metal concentration profiles as a function of time. Metal desorption studies indicated that Citrobacter beads could, in principle, be regenerated and reused in adsorption-desorption cycles. In an expanded scale trial, biosorbent beads were found to be useful in removal/recovery of metals such as lead from industrial wastewaters.

Adsorption↗

PubMed2005

Properties of the silica layer during the formation of MCM-41 studied by EPR of a silica-bound spin probe.

The properties of the silica layer during the formation of the mesoporous material MCM-41 were investigated by electron paramagnetic resonance (EPR) experiments carried out on a specifically designed, organo(trialkoxy)silane spin probe, SL1SiEt. Minute amounts of the spin probe were co-condensed with the silica source, tetraethyl orthosilicate (TEOS), in the synthesis of MCM-41 with cetyltrimethylammonium bromide (CTAB) under basic conditions. The mobility and location of the spin probe were followed in the CTAB micellar solution before the reaction, in the reaction mixture and in the final ordered material. It was found that the EPR spectra of hydrolyzed SL1SiEt throughout the room temperature part of the reaction are characteristic of a fast tumbling species, indicating that the silica is highly fluid prior to drying. After filtering, a slow motion type spectrum was observed, showing that the spin-label experiences considerable motional hindrance. The liquidlike behavior could be restored upon stirring the material in water. When the reaction is performed with a hydrothermal stage, the spectrum of SL1SiEt in the final product is the same as that of the room temperature synthesized material, but the addition of water did not restore the high mobility, due to a higher degree of silica cross-linking. The location of SL1SiEt throughout the formation process was obtained from electron spin-echo envelope modulation (ESEEM) measurements on MCM-41 prepared with CTAB deuterated either at the N-methyl or the alpha position and in a reaction carried out in D2O. Comparing the deuterium modulation depth, k(2H), induced by CTAB-alpha-d2, CTAB-d9, or D2O in CTAB micellar solutions of a number of reference spin probes with those of SL1SiEt revealed that the hydrolyzed SL1SiEt is located near the polar heads of the surfactant in the absence of base and TEOS. This supports the postulation of charge matching at the interface as a driving force for the formation of the mesostructure. Similar experiments carried out on reaction mixtures containing SL1SiEt showed a decrease of k(2H) from CTAB-alpha-d2 and CTAB-d9 compared to the micellar solution, exhibiting practically no time dependence. This indicates that the spin probe is pulled away from the micelle-water interface into the loosely linked, forming silica network. After drying, the modulation depth induced by CTAB-alpha-d2 and CTAB-d9 increases, showing that, once the water is removed, the silica walls contract around the micelles, pushing the silica-linked spin probe into the organic phase within the mesopores.

Electron Spin Resonance Spectroscopy↗

PubMed2005

Spectroscopic characterization of polyaniline formed by using copper(II) in homogeneous and MCM-41 molecular sieve media.

This work emphasizes the important role of the synthetic parameters in the structure of the polymeric material obtained in the aniline polymerization. The polymers formed by the oxidative polymerization of aniline by copper(II) ions in acidic aqueous solution, acetonitrile/water medium, and also copper(II) acetate complex encapsulated into MCM-41 molecular sieve were characterized by resonance Raman spectroscopy using three exciting laser lines and other techniques such as UV-vis, FTIR, and XANES (Nitrogen K edge). Additionally the products were investigated by thermogravimetric analysis and powder X-ray diffraction. When Cu(II) ions in acidic aqueous medium are used, emeraldine salt (ES-PANI) is formed through the usual head-to-tail polymerization mechanism, while in acetonitrile/water medium a polymer is observed having mainly phenazine-like rings, quinonediimine, and/or phenylenediamine segments in the chains, suggesting that a distinct mechanism is operating. The average molecular weights of the free polymers synthesized in water and in acetronile/water were, respectively, ca. 37 300 and 16 900 Da. The encapsulated polymer synthesized in Cu(II)-MCM-41 is a polymeric mixture of (i) ES-PANI and (ii) the polymer obtained when this metal cation was used as oxidant in acetonitrile/water medium. All the characterization data were compared to those ones obtained for standard free polyaniline and also for the encapsulated polymer into mesoporous MCM-41 formed by using persulfate in acidic aqueous medium as oxidant.

Acetonitriles↗

PubMed2005

A model for the structure of MCM-41 incorporating surface roughness.

The surface area of MCM-41 mesoporous silica, estimated by several models in the literature, is significantly less than the value derived from BET analysis of nitrogen adsorption at 77.4 K. In the past, the difference has been attributed to several reasons including the errors involved in the BET analysis of the multilayer-capillary condensation region and the heterogeneity of the walls. In the present work, we present an alternate model of MCM-41 based on molecular simulations that gives surface area values that are in closer agreement to those determined by experiment. The model incorporates bulk heterogeneity of the material, surface hydroxyls, and most importantly, physical deformations or indentations of the pore surface. The model predicts small-angle X-ray diffraction (XRD) and wide-angle X-ray scattering (WAXS) results that are consistent with experimental data as well as surface areas and pore volumes that compare favorably with published experimental results. The simulation results are consistent with the hypothesis that the interstitial space in MCM-41 is relatively amorphous despite the regular arrangement of the mesopores. The surface roughness associated with the amorphous structure increases the surface area beyond the nominal value produced by assuming smooth cylindrical pores.

Journal Article↗

PubMed2005

Low temperature properties of acetonitrile confined in MCM-41.

The effect of confinement on the phase changes and dynamics of acetonitrile in mesoporous MCM-41 was studied by use of adsorption, FT-IR, DSC, and quasi-elastic neutron scattering (QENS) measurements. Acetonitrile molecules in a monolayer interact strongly with surface hydroxyls to be registered and perturb the triple bond in the C[triple bond]N group. Adsorbed molecules above the monolayer through to the central part of the cylindrical pores are capillary condensed molecules (cc-acetonitrile), but they do not show the hysteresis loop in adsorption-desorption isotherms, i.e., second order capillary condensation. FT-IR measurements indicated that the condensed phase is very similar to the bulk liquid. The cc-acetonitrile freezes at temperatures that depend on the pore size of the MCM-41 down to 29.1 A (C14), below which it is not frozen. In addition, phase changes between alpha-type and beta-type acetonitriles were observed below the melting points. Application of the Gibbs-Thomson equation, assuming the unfrozen layer thickness to be 0.7 nm, gave the interface free energy differences between the interfaces, i.e., Deltagamma(l/alpha) = 22.4 mJ m(-2) for the liquid/pore surface (ps) and alpha-type/ps, and Deltagamma(alpha/beta) = 3.17 mJ m(-2) for alpha-type/ps and beta-type/ps, respectively. QENS experiments substantiate the differing behaviors of monolayer acetonitrile and cc-acetonitrile. The monolayer acetonitrile molecules are anchored so as not to translate. The two Lorentzian analysis of QENS spectra for cc-acetonitriles showed translational motion but markedly slowed. However, the activation energy for cc-acetonitrile in MCM-41 (C18) is 7.0 kJ mol(-1) compared to the bulk value of 12.7 kJ mol(-1). The relaxation times for tumbling rotational diffusion of cc-acetonitrile are similar to bulk values.

Journal Article↗

PubMed2005

Combined DFT theoretical calculation and solid-state NMR studies of Al substitution and acid sites in zeolite MCM-22.

The distributions of Brönsted acidic protons and their acid strengths in zeolite H-MCM-22 have been characterized by density functional theory (DFT) calculations as well as magic angle spinning (MAS) NMR experiments. The embedded scheme (ONIOM) that combines the quantum mechanical (QM) description of active sites and semiempirical AM1 treatment of the neighboring environment was applied to predict the aluminum substitution mechanism and proton affinity (PA), as well as adsorption behaviors of acetone and trimethylphosphine oxide (TMPO) onto the zeolite. Our theoretical results indicate that the Al substitution takes place in the order of Al1-OH-Si2 > Al8-OH-Si8 > Al5-OH-Si7. The DFT theoretical and NMR experimental results suggest that the acid strength of the three Brönsted acid sites in H-MCM-22 zeolite is slightly lower than that of H-ZSM-5 zeolite and the accessible Brönsted acidic protons most likely reside in both the supercages (at the Al8-OH-Si8 and Al1-OH-Si2 sites) and external surface pocket (at the Al8-OH-Si8 site) rather than in the sinusoidal channels (Al5-OH-Si7), with the Al1-OH-Si2 site having the strongest acid strength (as probed by TMPO). This may partially explain the special selectivity of acid-catalyzed reactions occurring inside the channels of H-MCM-22 zeolite.

Journal Article↗