Granular media filtration in water and wastewater treatment: Part 2∗

Jackson, Greg; Letterman, Raymond D.

doi:10.1080/10643388009381684

Cited by 17 publications

(8 citation statements)

References 65 publications

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“…These approaches range from ab initio quantum-mechanical calculations 8,23,24 to those based on empirical interatomic potentials. [25][26][27][28][29][30][31] Teter et al have correctly predicted the structures of ␣-quartz, ␣-cristobalite, and stishovite based on firstprinciples calculations. 23 The structural change of ␣-quartz under pressure and the structural properties of ␤-cristobalite have also been investigated using a similar approach.…”

Section: Introductionmentioning

confidence: 99%

Molecular dynamics study of cristobalite silica using a charge transfer three-body potential: Phase transformation and structural disorder

Huang

Kieffer

2003

The Journal of Chemical Physics

101

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Structural and dynamic properties of cristobalite silica have been studied using molecular dynamics simulations based on a charge transfer three-body potential model. In this potential model, the directional covalent bonding of SiO 2 is characterized by a charge transfer function of the interatomic distance between Si and O atoms, and in the form of Si-O-Si and O-Si-O three-body interactions. The dynamic properties such as infrared spectra and density of states at room and elevated temperatures are in excellent agreement with experiments, and are also consistent with the recently proposed rigid unit modes model. The ␣and ␤-cristobalite crystallographic structures are well reproduced in this model, and the transition between these modifications occurs reversibly and reproducibly in simulations, both as a result of changes in pressure and temperature. The thermally induced transition results in a significantly more disordered ␤-cristobalite than the pressure-induced ␤-cristobalite at room temperature. While simulated ␣-cristobalite exhibits a positive thermal expansion coefficient, it is almost zero for ␤-cristobalite up to 2000 K and slightly negative at higher temperatures, confirming results from recent x-ray diffraction experiments and other simulations with potential models based on ab initio calculations.

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Section: Introductionmentioning

confidence: 99%

Molecular dynamics study of cristobalite silica using a charge transfer three-body potential: Phase transformation and structural disorder

Huang

Kieffer

2003

The Journal of Chemical Physics

101

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“…Furthermore, there were no significant changes in nucleoside triphosphate pools in NAS-treated cells (data not shown). However, as shown for a number of cell lines, the toxicity of agents that interfere with thymidine synthesis and the reversal of the toxic effects by exogenous thymidine administration are highly dependent on the timing and sequence of the treatments (28).…”

mentioning

confidence: 99%

Tetrahydrobiopterin, the cofactor for aromatic amino acid hydroxylases, is synthesized by and regulates proliferation of erythroid cells.

Tanaka

Kaufman

Milstien

1989

Proc. Natl. Acad. Sci. U.S.A.

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The only known role for 6(R)-5,6,7,8-tetrahydrobiopterin (BH4) is as the cofactor for the aromatic amino acid hydroxylases. However, BH4 has been shown to be synthesized by cells that do not contain any hydroxylase activity, suggesting that it may have still undiscovered functions. Our fmding of much higher levels of BH4 and GTP by the sequential reductions of sepiapterin reductase and dihydrofolate reductase. Treatment of MEL cells with hexamethylene bisacetamide, an inducer of differentiation, results in a decrease in BH4 synthesis accompanied by a cessation of growth and concomitant hemoglobin synthesis. The inhibition of proliferation induced by hexamethylene bisacetamide can be reversed by maintaining high intracellular levels of BH4, which also decreases the amount of hemoglobin. The mechanism of the BH4 effect has not yet been elucidated, but it appears as though BH4 synthesis is more intimately linked with cell proliferation than with the differentiation process.

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“…In a series of papers, Jackson (15)(16)(17)(18)(19)(20) showed that mathematical models can be used to develop a better understanding of folate metabolism. His studies inspired others to develop mathematical models to understand the effects of anti-folate drugs (21)(22)(23)(24).…”

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confidence: 99%

A Mathematical Model of the Folate Cycle

Nijhout

Reed

Budu

et al. 2004

Journal of Biological Chemistry

177

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A mathematical model is developed for the folate cycle based on standard biochemical kinetics. We use the model to provide new insights into several different mechanisms of folate homeostasis. The model reproduces the known pool sizes of folate substrates and the fluxes through each of the loops of the folate cycle and has the qualitative behavior observed in a variety of experimental studies. Vitamin B 12 deficiency, modeled as a reduction in the V max of the methionine synthase reaction, results in a secondary folate deficiency via the accumulation of folate as 5-methyltetrahydrofolate (the "methyl trap"). One form of homeostasis is revealed by the fact that a 100-fold up-regulation of thymidylate synthase and dihydrofolate reductase (known to occur at the G 1 /S transition) dramatically increases pyrimidine production without affecting the other reactions of the folate cycle. The model also predicts that an almost total inhibition of dihydrofolate reductase is required to significantly inhibit the thymidylate synthase reaction, consistent with experimental and clinical studies on the effects of methotrexate. Sensitivity to variation in enzymatic parameters tends to be local in the cycle and inversely proportional to the number of reactions that interconvert two folate substrates. Another form of homeostasis is a consequence of the nonenzymatic binding of folate substrates to folate enzymes. Without folate binding, the velocities of the reactions decrease approximately linearly as total folate is decreased. In the presence of folate binding and allosteric inhibition, the velocities show a remarkable constancy as total folate is decreased.The folate cycle plays a central role in cell metabolism. Among its important functions are the delivery of one-carbon units to the methionine cycle, for use in methylation reactions, and the synthesis of pyrimidines and purines. Dietary folate deficiency or genetic polymorphisms in folate-metabolizing enzymes are associated with megaloblastic anemia, developmental abnormalities including neural tube defects and Down's syndrome, and various types of cancer, especially those of the gastrointestinal tract and leukemias (1-8). Elevated homocysteine concentrations, a biomarker of a low folate status, have been implicated in cardiovascular diseases and Alzheimer's disease (6, 9, 10). Furthermore, several enzymes in the cycle are the targets of cancer chemotherapeutic agents (11, 12). Because of its importance in human health, folate metabolism has long been the focus of clinical, nutritional, and biochemical investigations. Biochemical studies have provided extensive and detailed information about each of the enzymes and metabolites of the folate cycle. As a consequence, the components and the reaction diagram of the folate cycle are well understood (13).The structure of the folate cycle is relatively complex (Fig. 1) and consists of several interacting loops. Most of the reactions depend in a nonlinear way on the concentrations of their substrates. Therefore, the behavior of the f...

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Granular media filtration in water and wastewater treatment: Part 2∗

Cited by 17 publications

References 65 publications

Molecular dynamics study of cristobalite silica using a charge transfer three-body potential: Phase transformation and structural disorder

Molecular dynamics study of cristobalite silica using a charge transfer three-body potential: Phase transformation and structural disorder

Tetrahydrobiopterin, the cofactor for aromatic amino acid hydroxylases, is synthesized by and regulates proliferation of erythroid cells.

A Mathematical Model of the Folate Cycle

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