Partial Interconversion Research Articles

Tunable hybrid zeolites prepared by partial interconversion

Zeolite interconversion is a widely used strategy due to its unique advantages in the synthesis of some zeolites. By using a long-chain quaternary amine as both a structure-directing agent and porogen, we have produced superior catalysts, which we named Hybrid Zeolites, as their structures are made of building units of different zeolite types. The properties of these materials can be conveniently tuned, and their catalytic performance can be optimized simply by stopping the interconversion at different times. For cracking the 1,3,5-triisopropylbenzene, Hybrid Zeolites made of FAU and MFI units show a 5-fold increase in selectivity towards the desired product, that is, 1,3-diisopropylbenzene, compared to the commercial FAU, and a 7-fold increase in conversion at constant selectivity compared to MFI zeolite.

Mechanism for the synthesis of medium-chain 1-alkenes from fatty acids catalyzed by binuclear iron UndA decarboxylase

Terminal olefins synthesized by the decarboxylation of fatty acids are important platform chemicals. PfUndA is a nonheme di-iron oxidase capable of converting medium-chain (C10 ∼ C14) fatty acids into the corresponding 1-alkenes. Using density functional theory calculations, it is demonstrated that the PfUndA-catalyzed lauric acid decarboxylation is initiated by introducing one pair of H+/e−. The subsequent reaction mainly involves the O − O bond dissociation, the partial interconversion between active high-spin Fe(III)-O• and inactive medium-spin Fe(IV) = O, the formation of triple-hydroxyl intermediates and substrate Cβ radical resulted from successive hydrogen transfers, the stepwise decarboxylation of substrate radical, and the protonation of Fe-bound hydroxides with H+ and e− added. Other mechanistic possibilities have been excluded. It is further found that the low catalytic efficiency of UndA may be due to the energy penalty caused by the drop of active high-spin Fe(III)-O• to a deeper minimum (inert medium-spin Fe(IV) = O).

A "conversion-deterioration-double mutation" theory for the evolution and progression of colorectal cancer.

In this study, based on some clinical phenomena and recently published knowledge, we proposed our “conversion‐deterioration‐double mutation” theory, which provides a possible unifying explanation for the evolutionary process of colorectal cancer cells in the human body. In this theory, we proposed that there is a partial interconversion and a jump conversion relationship among normal colorectal epithelial cells, colorectal cancer cells, stem cells, and cancer stem cells (conversion). This conversion leads to tumor heterogeneity. We also proposed that well‐differentiated cancer cells converted from cancer stem cells have a more aggressive pattern than primary cancer cells (deterioration). The deterioration of primary cancer cells leads to differences in treatment responses and prognosis. Finally, we speculate a double mutation theory, indicating that for metastasis to occur, both mutations of cancer cells and mutations of target organs are needed and should match and meet. All these three points constitute the “conversion‐deterioration‐double mutation” theory.

ATR-SEIRAS study of CO adsorption and oxidation on Rh modified Au(111-25 nm) film electrodes in 0.1 M H2SO4

Rh modified Au(111-25nm) electrodes, prepared by electron beam evaporation and galvanostatic deposition, were employed to study adsorption and electro-oxidation of CO on Rh in 0.1M sulfuric acid solution by in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). The results of ATR-SEIRAS experiments were compared with those obtained by infrared reflection absorption spectroscopy on three low-index Rh single crystal surfaces. The Rh film deposited on Au(111-25nm) electrode consists of 3D clusters forming a highly stepped [n(111)×(111)]-like surface with narrow (111) terraces. When CO was dosed at the hydrogen adsorption potential region, CO adsorbed in both atop (COL) and bridge (COB) configurations, as well as coadsorbed water species, were detected on the Rh film electrode. A partial interconversion of spectroscopic bands due to the CO displacement from bridge to atop sites was found during the anodic potential scan, revealing that there is a potential-dependent preference of CO adsorption sites on Rh surfaces. Our data indicate that CO oxidation on Rh electrode surface in acidic media involves coadsorbed water and follows the nucleation and growth model of a Langmuir-Hinshelwood type reaction.

Photoswitchable Exciton Coupling in Merocyanine–Diarylethene Multi‐Chromophore Hydrogen‐Bonded Complexes

Light on: Photocontrol of J-type exciton interactions by using chromophores is reported (see picture). Hydrogen-bonded merocyanine dyes could be switched reversibly through photoinduced ring-closure/ring-opening reactions of diarylethene receptors. Addition of H-aggregation-inducing bismelamine receptors enabled the partial interconversion between J- and H-type exciton coupling.

Synthesis of PbS Nanorods and Other Ionic Nanocrystals of Complex Morphology by Sequential Cation Exchange Reactions

We show that nanocrystals (NCs) with well-established synthetic protocols for high shape and size monodispersity can be used as templates to independently control the NC composition through successive cation exchange reactions. Chemical transformations like cation exchange reactions overcome a limitation in traditional colloidal synthesis, where the NC shape often reflects the inherent symmetry of the underlying lattice. Specifically we show that full or partial interconversion between wurtzite CdS, chalcocite Cu(2)S, and rock salt PbS NCs can occur while preserving anisotropic shapes unique to the as-synthesized materials. The exchange reactions are driven by disparate solubilites between the two cations by using ligands that preferentially coordinate to either monovalent or divalent transition metals. Starting with CdS, highly anisotropic PbS nanorods are created, which serve as an important material for studying strong two-dimensional quantum confinement, as well as for optoelectronic applications. In NC heterostructures containing segments of different materials, the exchange reaction can be made highly selective for just one of the components of the heterostructure. Thus, through precise control over ion insertion and removal, we can obtain interesting CdS|PbS heterostructure nanorods, where the spatial arrangement of materials is controlled through an intermediate exchange reaction.

Comparative study of degenerate four‐wave mixing and cavity ringdown signal intensities of formaldehyde in a molecular beam

AbstractA comparison of degenerate four‐wave mixing (DFWM) and cavity ring‐down (CRD) spectroscopy is made on jet‐cooled formaldehyde under identical expansion conditions. We show that both methods are applicable to the low‐density environment of a molecular beam. However, a slightly superior signal‐to‐noise ratio for the background‐free DFWM method is observed for our experimental configurations. Absorption spectra simulated by applying an asymmetric rotor model are used to deduce Boltzmann temperatures of the rotational population distribution by a nonlinear least‐squares fit to the experimental data. In addition, partial interconversion of the nuclear spin upon supersonic expansion is observed. Consequently, a second fit parameter, describing the para/ortho concentration ratio in the molecular beam, is introduced. The resulting simulated spectra are in favorable agreement with the relative CRD intensities. A simple model for the reduction of the DFWM signal intensities to population is applied yielding absorption‐like spectra in accordance with both the simulations and the CRD measurements. We conclude that DFWM spectroscopy is applicable to deduce population distributions of an asymmetric rotor molecule under free‐jet conditions. Copyright © 2006 John Wiley & Sons, Ltd.

Exploring the (1 → 3)‐β‐D‐glucan conformational phase diagrams to optimize the linear to macrocycle conversion of the triple‐helical polysaccharide scleroglucan

The immunologically important (1 → 6) comb-like branched (1 → 3)-β-D-glucans scleroglucan, schizophyllan, lentinan, and others, exist mainly as linear triple-helical structures in aqueous solution. Partial interconversion from linear to circular topology has been reported to take place following conformational transition of the triple-helical structure and subsequent regeneration of the triplex conformation. We here report on experimental data indicating that complete strand separation of the triple-helical structure is required for this interconversion. NaOH or dimethylsulfoxide was used to induce dissociation of the triplex at combinations of concentrations and temperatures shown by calorimetry to yield a conformational transition of the triplex structures. For the alkaline treatment at 55°C, it is found that up to about 30% of the material readily can be converted to the cyclic topology. This fraction increased to about 60% when the subsequent annealing of the scleroglucan in aqueous solution at pH 7 was carried out at 100°C. Further increase of the annealing temperature yielded a smaller relative amount of cyclic species. The data indicate that the lower molecular weight fraction of the molecular weight distributions can be converted selectively to the macrocyclic topology by conditions that do not yield complete strand separation of the whole sample. These findings add to previous reports by providing more details about how the conditions required for the linear triplex to macrocycle interconversion relate to the conformational properties of the triple-helical structure. © 1999 John Wiley & Sons, Inc. Biopoly 50: 496–512, 1999

Exploring the (1 --> 3)-beta-D-glucan conformational phase diagrams to optimize the linear to macrocycle conversion of the triple-helical polysaccharide scleroglucan

The immunologically important (1 --> 6) comb-like branched (1 --> 3)-beta-D-glucans scleroglucan, schizophyllan, lentinan, and others, exist mainly as linear triple-helical structures in aqueous solution. Partial interconversion from linear to circular topology has been reported to take place following conformational transition of the triple-helical structure and subsequent regeneration of the triplex conformation. We here report on experimental data indicating that complete strand separation of the triple-helical structure is required for this interconversion. NaOH or dimethylsulfoxide was used to induce dissociation of the triplex at combinations of concentrations and temperatures shown by calorimetry to yield a conformational transition of the triplex structures. For the alkaline treatment at 55 degrees C, it is found that up to about 30% of the material readily can be converted to the cyclic topology. This fraction increased to about 60% when the subsequent annealing of the scleroglucan in aqueous solution at pH 7 was carried out at 100 degrees C. Further increase of the annealing temperature yielded a smaller relative amount of cyclic species. The data indicate that the lower molecular weight fraction of the molecular weight distributions can be converted selectively to the macrocyclic topology by conditions that do not yield complete strand separation of the whole sample. These findings add to previous reports by providing more details about how the conditions required for the linear triplex to macrocycle interconversion relate to the conformational properties of the triple-helical structure. Copyright 1999 John Wiley & Sons, Inc.

Effect of direct suppression of thymidylate synthase at the 5,10-methylenetetrahydrofolate binding site on the interconversion of tetrahydrofolate cofactors to dihydrofolate by antifolates: Influence of degree of dihydrofolate reductase inhibition

An important unresolved issue in antifolate pharmacology is the basis for the observation that the major portion of cellular tetrahydrofolate cofactors is preserved after dihydrofolate reductase activity is abolished by antifolates despite the fact that tetrahydrofolate cofactor-dependent purine and pyrimidine biosynthesis ceases. This has been attributed to feedback inhibition of thymidylate synthase by dihydrofolate polyglutamates that accumulate in the presence of antifolates. This report combines network thermodynamic modeling and experimental observations to evaluate the effects of direct inhibition of thymidylate synthase at the 5,10-methylenetetrahydrofolate binding site with a potent lipophilic quinazoline antifolate PD130883 on folate oxidation in cells. Computer simulations predict and the data indicate that marked PD130883 suppression of thymidylate synthase only slows the rate but not the extent of tetrahydrofolate cofactor interconversion to dihydrofolate upon complete suppression of dihydrofolate reductase with trimetrexate. These observations are consistent with earlier studies from this laboratory with fluorodeoxyuridine inhibition at the deoxyuridylate binding site. Hence, the much weaker inhibition by dihydrofolate polyglutamates at the level of thymidylate synthase cannot account for the apparent preservation of tetrahydrofolate cofactor pools in cells and has virtually no pharmacologic significance under conditions in which antifolates completely suppress dihydrofolate reductase. The extent of interconversion of tetrahydrofolate cofactors to dihydrofolate is strongly influenced by residual dihydrofolate reductase catalytic activity. Exposure of cells to 0.1 microM trimetrexate results in only approximately 60% of maximum dihydrofolate levels achieved when dihydrofolate reductase activity is abolished. Network thermodynamic simulations predict, and experiments verify, that inhibition of thymidylate synthase at the 5,10-methylenetetrahydrofolate site by PD130883, when dihydrofolate reductase is only partially suppressed (approximately 85%) with 0.1 microM trimetrexate, substantially decreases (31-47%) the net level of interconversion of tetrahydrofolate cofactors to dihydrofolate. Further computer simulations predict that under conditions in which residual dihydrofolate reductase activity persists within the cells (more than about 5%), feedback inhibitory effects of dihydrofolate polyglutamates as well as other weak inhibitors of thymidylate synthase can significantly limit the extent of net interconversion of tetrahydrofolate cofactors to dihydrofolate and produce an apparent “compartmentation phenomenon” in which tetrahydrofolate cofactor pools are preserved within the cell in the presence of antifolates. Residual dihydrofolate reductase activity cannot, however, account for the partial interconversion of tetrahydrofolate cofactors to dihydrofolate after exposure to high trimetrexate or methotrexate levels. This is more likely attributed to tetrahydrofolate cofactors in a fraction of the cell population (i.e. in some cells out of S-phase) and a physical compartment (i.e. mitochondrial) and in a physical state within the cytosol which makes them unavailable for oxidation to dihydrofolate when dihydrofolate reductasies blocked by antifolates.

Rate and extent of interconversion of tetrahydrofolate cofactors to dihydrofolate after cessation of dihydrofolate reductase activity in stationary versus log phase L1210 leukemia cells.

Previous studies from this laboratory established that the rapid but partial interconversion of tetrahydrofolate cofactors to dihydrofolate after exposure of L1210 leukemia cells to antifolates cannot be due to direct feedback inhibition of thymidylate synthase by dihydrofolate or any other endogenous folylpolyglutamates when dihydrofolate reductase activity is abolished by antifolates. Rather, the data suggested this preservation of tetrahydrofolate cofactor pools is likely due to a fraction of cellular folates unavailable for oxidation to dihydrofolate. This paper explores the role of cell cycle phase in L1210 leukemia cells in logarithmic versus stationary phase growth as a factor in the rate and extent of tetrahydrofolate cofactor interconversion to dihydrofolate after exposure of cells to the dihydrofolate reductase inhibitor trimetrexate. The S phase fraction was reduced by inoculating L1210 leukemia cells at high density to achieve a stationary state. Flow cytometric analysis of DNA content indicated that log phase cultures were 53.0% S phase; this decreased to 42.1% at 24 h and 24.1% at 48 h in stationary phase cultures. 5-Bromo-2'-deoxyuridine incorporation into DNA decreased 80 and 96%, while [3H]dUrd incorporation into DNA declined 70 and 95% for stationary cultures at 24 and 48 h, respectively, as compared with the log phase rates. Log phase cells interconverted 28.0% of the total pool of radiolabeled folates to dihydrofolate with a half-time of approximately 30 s. Stationary cells at 24 h interconverted 20.4% of the total folate pool with a t1/2 of approximately 3 min, and at 48 h, net interconversion to dihydrofolate decreased further to 12.1% with a t1/2 of approximately 6 min. The decrease in the extent of tetrahydrofolate cofactor interconversion to dihydrofolate in stationary phase cells was directly proportional to the decrease in the S phase fraction determined by total DNA content. This suggests that tetrahydrofolate cofactor depletion occurs only in S phase cells. The much larger drop in [3H]dUrd and 5-bromo-2'-deoxyuridine incorporation into DNA in comparison with the decline in the S phase fraction measured by DNA content along with the reduced rate of tetrahydrofolate cofactor interconversion to dihydrofolate indicates that the rate of DNA synthesis is decreased in S phase cells in stationary cultures. Network thermodynamic simulations suggest that a reduction in the number of S phase cells and their thymidylate synthase catalytic activity would account for the observed decrease in the rate and extent of interconversion of tetrahydrofolate cofactors to dihydrofolate after trimetrexate in stationary phase cultures.(ABSTRACT TRUNCATED AT 400 WORDS)

Folate-Pool Interconversions and Inhibition of Biosynthetic Processes after Exposure of L1210 Leukemia Cells to Antifolates: Experimental and network thermodynamic analyses of the role of dihydrofolate polyglutamylates in antifolate action in cells

Folate analogs that inhibit dihydrofolate reductase result in only partial interconversion of tetrahydrofolate cofactors to dihydrofolate with preservation of the major portion of reduced cellular folate cofactors in L1210 leukemia cells. One possible explanation for this phenomenon is that low levels of dihydrofolate polyglutamates that accumulate in the presence of antifolates block thymidylate synthase to prevent depletion of reduced folate pools. This paper correlates biochemical analyses of rapid interconversions of radiolabeled folates and changes in purine and pyrimidine biosynthesis in L1210 murine leukemia cells exposed to antifolates with network thermodynamic computer modeling to assess this hypothesis. When cells are exposed to 1 microM trimetrexate there is an almost instantaneous inhibition of [3H] deoxyuridine or [14C]formate incorporation into nucleotides which is maximal within 5 min. This is associated with a rapid rise in cellular dihydrofolate (t1/2 approximately 1.5 min), which reaches a steady state that represents only 27.9% of the total folate pool. Pretreatment of cells with fluorodeoxyuridine, to inhibit thymidylate synthase by about 95% followed by trimetrexate only slows the rate of folate interconversion (t1/2 approximately 25 min) but not the final dihydrofolate level achieved. This is consistent with computer simulations which predict that direct inhibition of thymidylate synthase by 97, 98, and 99% should increase the half-time of dihydrofolate rise after trimetrexate to 40, 60, and 124 min, respectively, but the final level achieved is always the same as in cells with normal thymidylate synthase activity. The data reflect the high degree of catalytic activity of thymidylate synthase relative to tetrahydrofolate cofactor pools in the cells and the enormous extent of inhibition of this enzyme that is necessary to slow the rate of folate interconversions after addition of antifolates. The model predicts, and the data demonstrate, that virtually any residual thymidylate synthase activity will permit the interconversion of all tetrahydrofolate cofactors available for oxidation to dihydrofolate when dihydrofolate reductase activity is abolished, but the rate of interconversion will be slowed. Additional simulations indicate that the time course of cessation of tetrahydrofolate-dependent purine and pyrimidine biosynthesis after antifolates in these cells can be accounted for solely on the basis of tetrahydrofolate cofactor depletion alone. These data exclude the possibility that direct inhibition of thymidylate synthase by dihydrofolate polyglutamates, or any other intracellular folates that accumulate in cells after antifolates, can account for the rapid but partial interconversion of reduced folate cofactors to dihydrofolate.(ABSTRACT TRUNCATED AT 400 WORDS)

Palisadins A,B and related monocyclofarnesol-derived sesquiterpenoids from the red marine alga [formula omitted] cf. [formula omitted

Five new monocyclofarnesol-derived sesquiterpenoids, along with the previously reported lactone aplysistatin, have been isolated from the tropical red alga Laurencia cf. palisada . The structures of these new compounds were defined by spectral and chemical analyses, including their partial interconversion with aplysistatin.

Changes in cell wall polysaccharides and monosaccharides during development and ripening of Japanese pear fruit1

Changes in polysaccharide and monosaccharide components in the cell wall were studied during cell division, cell enlargmement and softening in Japanese pear fruit. Wall polysaccharides were fractionated into water soluble carbohydrate, NaClO2 soluble carbohydrate, EDTA soluble carbohydrate, acid soluble hemicellulose, alkali soluble hemicellulose and cellulose. These polysaccharides were composed of glucose, uronic acid, xylose, arabinose, galactose, rhamnose, mannose and fucose.The total polysaccharide content of the cell wall per cell (DNA content basis) remained constant during the cell division period (S1). But during the pre-enlargement period (S2) it began to increase rapidly in spite of the slightness of cell enlargement. Thereafter, during the enlargement period (S3) the polysaccharides remained almost constant although the fruits enlarged dramatically, and the polysaccharides increased somewhat with ripening. The quality of the polysaccharides, however, seemed to change actively at each stage. This suggested that the extensive fruit enlargement did not require an increase in polysaccharide content, and was rather accompanied by the partial breakdown or partial interconversion of polysaccharide components already present.The loss of arabinose and galactose in acid soluble hemicellulose was prominant in fruit softening occurring in the ripening stage. The cellulose component decreased with overripening. Water soluble pectin increased parallel to the increase in total pectin with ripening. On the other hand, xylose and non-cellulosic glucose residues did not alter with ripening or overripening. Non-cellulosic glucose continued to accumulate during cell enlargement. Changes in polysaccharide and monosaccharide components in the cell wall were studied during cell division, cell enlargmement and softening in Japanese pear fruit. Wall polysaccharides were fractionated into water soluble carbohydrate, NaClO2 soluble carbohydrate, EDTA soluble carbohydrate, acid soluble hemicellulose, alkali soluble hemicellulose and cellulose. These polysaccharides were composed of glucose, uronic acid, xylose, arabinose, galactose, rhamnose, mannose and fucose. The total polysaccharide content of the cell wall per cell (DNA content basis) remained constant during the cell division period (S1). But during the pre-enlargement period (S2) it began to increase rapidly in spite of the slightness of cell enlargement. Thereafter, during the enlargement period (S3) the polysaccharides remained almost constant although the fruits enlarged dramatically, and the polysaccharides increased somewhat with ripening. The quality of the polysaccharides, however, seemed to change actively at each stage. This suggested that the extensive fruit enlargement did not require an increase in polysaccharide content, and was rather accompanied by the partial breakdown or partial interconversion of polysaccharide components already present. The loss of arabinose and galactose in acid soluble hemicellulose was prominant in fruit softening occurring in the ripening stage. The cellulose component decreased with overripening. Water soluble pectin increased parallel to the increase in total pectin with ripening. On the other hand, xylose and non-cellulosic glucose residues did not alter with ripening or overripening. Non-cellulosic glucose continued to accumulate during cell enlargement.