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At least 253 records · Page 14Linked to original sources
PubMed1970

Steroidal free radicals as possible intermediates in the biosynthesis of C19-delta 16-steroids.

Treatment of deoxycorticosterone with reagents that abstract a hydrogen atom from the C-21-hydroxyl group leads to the formation of androsta-4,16-dien-3-one. In vitro incubations of [1,2-(3)H]deoxycorticosterone with homogenates of boar testis tissue produce the tritiated dienone. This reaction can be formulated as an abstraction of hydrogen from the C-21-hydroxyl group to form a C-21-alkoxy radical, followed by a 1,5-H.shift from the C-16 atom and beta-scission of the C-16-alkoxy radical leading to the loss of a C(2) fragment (C-20 and C-21) with the formation of the double bond between C-16 and C-17. Both the chemical model and the in vitro experiment support further the thesis [Lieberman, S., L. Bandy, V. Lippman, and K. D. Roberts, Biochem. Biophys. Res. Commun., 34, 367 (1969)] that biosynthetic reactions in which molecular oxygen reacts with steroids (for example, side-chain cleavage or hydroxylation) involve steroidal free radicals.

Androstanes↗
PubMed1991

Nitric oxide in star-forming regions: further evidence for interstellar N-O bonds.

Nitric oxide has been newly detected towards several star-forming clouds, including Orion-KL, Sgr B2(N), W33A, W51M, and DR21(OH) via its J = 3/2 --> 1/2 transitions near 150 GHz, using the FCRAO 14 m telescope. Both lambda-doubling components of NO were observed towards all sources. Column densities derived for nitric oxide in these clouds are N approximately 10(15)-10(16) cm-2, corresponding to fractional abundances of f approximately 0.5-1.0 x 10(-8), relative to H2. Towards Orion-KL, the NO line profile suggests that the species arises primarily from hot, dense gas. Nitric oxide may arise from warm material toward the other clouds as well. Nitric oxide in star-forming regions could be synthesized by high-temperature reactions, although the observed abundances do not disagree with values predicted from low-temperature, ion-molecule chemistry by more than one order of magnitude. The abundance of NO, unlike other simple interstellar nitrogen compounds, does appear to be reproduced by chemical models, at least to a good approximation. Regardless of the nature of formation of NO, it appears to be a common constituent of warm, dense molecular clouds. N-O bonds may therefore be more prevalent than previously thought.

Extraterrestrial Environment↗
PubMed1997

First astronomical detection of the cumulene carbon chain molecule H2C6 in TMC-1.

The cumulene carbenes are important components of hydrocarbon chemistry in low-mass star-forming cores. Here we report the first astronomical detection of the long-chain cumulene carbene H2C6 in the interstellar cloud TMC-1, from observations of two of its rotational transitions: J(K,K') = 7(1,7) --> 6(1,6) at 18.8 GHz and 8(1,8) --> 7(1,7) at 21.5 GHz, using NASA's Deep Space Network 70 m antenna at Goldstone, California. In addition we also observed the shorter cumulene carbene H2C4 at the same position. The fractional abundance of H2C6 relative to H2 is about 4.7 x 10(-11) and that of H2C4 is about 4.1 x 10(-9). The abundance of H2C6 is in fairly good agreement with gas-phase chemical models for young molecular cloud cores, but the abundance of H2C4 is significantly larger than predicted.

Astronomy↗
PubMed2003

Taming of a poison: biosynthesis of the NiFe-hydrogenase cyanide ligands.

NiFe-hydrogenases have an Ni-Fe site in which the iron has one CO and two CN groups as ligands. Synthesis of the CN ligands requires the activity of two hydrogenase maturation proteins: HypF and HypE. HypF is a carbamoyltransferase that transfers the carbamoyl moiety of carbamoyladenylate to the COOH-terminal cysteine of HypE and thus forms an enzyme-thiocarbamate. HypE dehydrates the S-carbamoyl moiety in an adenosine triphosphate-dependent process to yield the enzyme thiocyanate. Chemical model reactions corroborate the feasibility of this unprecedented biosynthetic route and show that thiocyanates can donate CN to iron. This finding underscores a striking parallel between biochemistry and organometallic chemistry in the formation of an iron-cyano complex.

Adenosine Monophosphate↗
PubMed1970

Oncogenic purine derivatives: evidence for a possible proximate oncogen.

Two additional urinary metabolites of the chemical oncogen 3-hydroxyxanthine are now identified as 8-chloroxanthine and 8-methylmercaptoxanthine. Such products are thought to be derived from a reactive intermediate which can be tentatively considered to be a proximate oncogen. Since each of these 8-substituted xanthines has also been obtained in vitro by reactions of 3-acetoxyxanthine with chloride ion or methionine, their production in vivo can be explained as resulting through the metabolic formation of an activated ester with a reactivity similar to that of the chemical model.

Animals↗
PubMed2002

The simulated emergence of distributed environmental control in evolving microcosms.

This work continues investigation into Gaia theory [Lovelock, (1995) The ages of Gaia, Oxford University Press] from an artificial life perspective [Downing, (2000) in Proceedings of the 7th International Conference on Artificial Life, (pp. 90-99) MIT Press], with the aim of assessing the general compatibility of emergent distributed environmental control with conventional natural selection. Our earlier system, GUILD [Downing and Zvirinsky, (1999) Artificial Life, 5, 291-318], displayed emergent regulation of the chemical environment by a population of metabolizing agents, but the chemical model underlying those results was trivial, essentially admitting all possible reactions at a single energy cost. The new model, METAMIC, utilizes abstract chemistries that are both (a) constrained to a small set of legal reactions, and (b) grounded in basic fundamental relationships between energy, entropy, and biomass synthesis/breakdown. To explore the general phenomena of emergent homeostasis, we generate 100 different chemistries and use each as the basis for several METAMIC runs, as part of a Gaia hunt. This search discovers 20 chemistries that support microbial populations capable of regulating a physical environmental factor within their growth-optimal range, despite the extra metabolic cost. Case studies from the Gaia hunt illustrate a few simple mechanisms by which real biota might exploit the underlying chemistry to achieve some control over their physical environment. Although these results shed little light on the question of Gaia on Earth, they support the possibility of emergent environmental control at the microcosmic level.

Computer Simulation↗
PubMed2001

The stochastic evolution of catalysts in spatially resolved molecular systems.

A fully stochastic chemical modelling technique is derived which describes the influence of spatial separation and discrete population size on the evolutionary stability of coupled amplification in biopolymers. The model is analytically tractable for an infinite-dimensional space (simplex geometry), which also provides insight into evolution in normal Euclidean space. The results are compared with stochastic simulations describing the co-evolution of combinatorial families of molecular sequences both in the simplex geometry and in lower (one, two and three) space dimensions. They demonstrate analytically the generic limits which exploitation place on co-evolving multi-component amplification systems. In particular, there is an optimal diffusion (or migration) coefficient for cooperative amplification and minimal and maximal threshold values for stable cooperation. Over a bounded range of diffusion rates, the model also exhibits stable limit cycles. Furthermore, the co-operatively coupled system has a maximum tolerable error rate at intermediate rates of diffusion. A tractable model is thereby established which demonstrates that spatial effects can stabilize catalytic biological information. The analytic behaviour in infinite-dimensional simplex space is seen to provide a reasonable guide to the spatial dependence of the error threshold in physical space. Nanoscale possibilities for the evolution of catalysis on the basis of the model are outlined. We denote the modelling technique by PRESS, Probability Reduced Evolution of Spatially-discrete Species.

Catalysis↗
PubMed1997

Multicomponent spectroscopic investigations of salivary antioxidant consumption by an oral rinse preparation containing the stable free radical species chlorine dioxide (ClO2.).

A multicomponent evaluation of the oxidative consumption of salivary biomolecules by a commercially-available oral rinse preparation containing an admixture of the stable free radical species chlorine dioxide (ClO2.) with chlorite anion (ClO2-) has been investigated using high resolution 1H NMR spectroscopy. The results obtained demonstrated that ClO2. and/or ClO2- present in this preparation effected the oxidative decarboxylation of salivary pyruvate (to acetate and CO2). Experiments conducted on chemical model systems confirmed the oxidative decarboxylation of pyruvate by this oral rinse, and also demonstrated that urate, thiocyanate anion, and the amino acids cysteine and methionine (precursors to volatile sulphur compounds responsible for oral malodour), were oxidatively consumed. The biochemical, periodontal and therapeutic significance of the results are discussed.

Absorption↗
PubMed2001

Developing a multimedia model of chemical dynamics in an urban area.

A multimedia model has been developed to account for the movement of semi-volatile organic compounds (SOCs) in an urban environment. The model, based on a Level III fugacity model of D. Mackay (Multimedia Environmental Models: The Fugacity Approach, Lewis Publishers, Boca Raton, FL, 1991), consists of six compartments: air, surface water, sediment, soil, vegetation, and an organic film that coats impervious surfaces. The latter is a newly identified compartment into which gas-phase SOCs partition and particle-phase SOCs are believed to be efficiently captured (M.L. Diamond, S.E. Gingrich, K. Fertuck, B.E. McCarry, G.A. Stern, B. Billeck, B. Grift, D. Brooker, T.D. Yager, Environ. Sci. Technol., 34 (2000a), 2900-2908). The model, parameterized for downtown Toronto, Ontario, Canada, and run with an illustrative emission rate for selected polycyclic aromatic hydrocarbons and homologues of polychlorinated dibenzodioxins, indicates that the film achieves the highest concentrations among media but that soils are the greatest sink for all but the least hydrophobic chemicals. The film "reflects" the more volatile chemicals into air, facilitates removal to surface waters by wash-off, and provides a surface on which photolytic degradation can occur. As such, the film is a transient sink that increases chemical mobility in urban areas by increasing air concentrations and the cycling of these compounds between air and urban surfaces and increasing water concentrations. Vegetation also accumulates SOCs, a portion of which is transferred to soil that reduces chemical mobility.

Air Movements↗
PubMed2007

The effect of tamoxifen on bladder functions and histology, and the role of estrogen receptor beta in a rat chemical cystitis model.

AIMS: The purpose of this study was to investigate the effect of tamoxifen citrate on bladder functions and histology, and also to investigate the role of estrogen receptor beta (ER beta) in a rat chemical cystitis model. METHODS: The study included 37 female Sprague-Dawley rats. Chemical cystitis was induced by intravesical instillation of hydrochloric acid in 32 rats, and the treatment group (n = 15) received daily 0.4 mg/kg of tamoxifen citrate with orogastric tube, and the control group (n = 17) received no treatment. The sham group consisted of five rats having no acid instillation and no treatment. Cystometric studies were performed in all rats at the beginning and end of the experiment. The rats were euthanized at 2 months. The bladders were removed and examined histologically for mast cells, inflammatory cells, and ER beta. RESULTS: The mean maximal bladder volume increased by 73.6% +/- 25.2 in the treatment group and decreased by 7.2% +/- 10.8 in the control group, revealing a significant difference (P = 0.007). The mean bladder compliance increased by 81.2% +/- 25.2 in the treatment group and decreased by 4.8% +/- 12.7 in the control group, revealing a significant difference between the two groups (P = 0.005). ER beta positive cells were significantly lower in the bladders with chronic cystitis than in the sham group (P = 0.038). CONCLUSIONS: Tamoxifen citrate may be an alternative choice, as easy, to other treatment options in the treatment of chronic inflammatory condition to improve deteriorated bladder function. In addition, ER beta may have a role on chronic bladder inflammation in a rat chemical cystitis model.

Animals↗
PubMed2006

Glass transition in 1,4-polybutadiene: Mode-coupling theory analysis of molecular dynamics simulations using a chemically realistic model.

We present molecular dynamics simulations of the glass transition in a chemically realistic model of 1,4-polybutadiene (PBD). Around 40 K above the calorimetric glass transition of this polymer the simulations reveal a well-developed two-stage relaxation of all correlation functions. We have analyzed the time-scale separation between vibrational degrees of freedom (subpicosecond dynamics) and the alpha relaxation behavior (nanosecond to microsecond dynamics) using the predictions of mode-coupling theory (MCT). Our value for the mode-coupling critical temperature Tc agrees perfectly with prior experimental estimates for PBD. The predictions of MCT for the scaling behavior of the so-called beta relaxation, the plateau regime separating vibrational dynamics and the alpha relaxation, are well fulfilled. Furthermore, we are able to derive a consistent set of MCT exponents, completely characterizing the scaling behavior of relaxation processes in the vicinity of Tc. For the temperature dependence of the alpha relaxation we find deviations from MCT predictions which we trace to the influence of torsional barriers on the atomic motions.

Journal Article↗
PubMedDate not supplied

Acetic acid, a potent agent of tumor progression in the multistage mouse skin model for chemical carcinogenesis.

Acetic acid, a very weak tumor promoter in the multistage mouse skin model for chemical carcinogenesis, was found to be very effective at enhancing cancer development, when applied during the progression phase of the model. Papilloma-bearing mice when repeatedly treated with acetic acid had a greater carcinoma incidence and a greater conversion of papillomas to carcinomas than vehicle treated mice. Selective cytotoxicity is discussed as a possible mechanism.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1989

Population dynamics in a spin-glass model of chemical evolution.

We introduce a simple model describing the evolution of a population of information-carrying macromolecules. We discuss the asymptotic dependence of the variability of the population on different parameters, representing the severity or the fluctuations of the environment. We show the existence of a transition separating a neutralist evolutionary regime from a trapped one. We investigate the dependence of the evolutionary behavior of the population on the correlation properties of the fitness landscape.

Biological Evolution↗
PubMed2006

Chemically induced model of hypermethioninemia in rats.

In the present study, we developed a chronic chemically induced model of hypermethioninemia in rats. We induced elevated concentrations of methionine in the blood by injecting subcutaneously methionine (1.34-2.68 micromol/g of body weight) to developing animals of various ages. Brain methionine concentrations were approximately 1.25 micromol/g wet tissue ( approximately 1.0mM). We then injected the same doses of methionine to young rats twice a day at 8h intervals from the 6(th) to the 28(th) postpartum day. Controls received saline in the same volumes. The body, brain and hippocampus of rats were weighed after treatment and showed that hypermethioninemic animals had no differences in these parameters, when compared to the control group, suggesting that methionine did not cause malnutrition in the rats. Considering that experimental animal models are useful to understand the pathophysiology of human disease, the present model of hypermethioninemia may contribute to the investigation of the mechanisms of brain damage caused by high tissue methionine levels.

Age Factors↗
PubMed1990

Theoretical nuclear magnetic resonance-lineshape study of a two-site chemical exchange model of sodium ions (I = 3/2) in an intracellular environment.

We present a theoretical calculation of the lineshape function based on the solution of the semiclassical Liouville equation, of a two-site chemical exchange model of biological relevance. The bound site is allowed to be in the slow region regime that is the inverse quadrupole interaction of one bound site is in the same range as the reorientational correlation time. We compare different chemical exchange models, and several different physical situations are investigated. The variation of the width at half height (WHH) and the relative intensity (l/lo) is shown to be important, experimentally accessible quantities that are useful in order to discriminate between different model systems.

Body Fluids↗
PubMed2006

Chemical oscillations and Turing patterns in a generalized two-variable model of chemical self-replication.

Chemical self-replication of oligonucleotides and helical peptides show the so-called square root rate law. Based on this rate we extend our previous work on ideal replicators to include the square root rate and other possible nonlinearities, which we couple with an enzimatic sink. Although the nonlinearity is necessary for complex dynamics, the nature of the sink is the essential feature in the mechanism that allows temporal and spatial patterns. We obtain exact general relations for the Poincare-Adronov-Hopf and Turing bifurcations, and our generalized results include the Higgins, autocatalator, and templator models as specific cases.

Journal Article↗
PubMed2001

How persistent is cyclopropyl upon nucleophilic substitution, and is frontside displacement possible? A model study.

Quantum chemical model calculations (MP2/6-31G(d,p)) demonstrate that frontside nucleophilic substitution is not possible in the reaction between water and protonated cyclopropanol. Instead, ring opening occurs, in accordance with a well-known disrotary ring-opening mechanism. When the cyclopropane ring is embedded in a stabilizing bicyclic structure, as in protonated bicyclo[3.1.0]hexanol, the mechanistic landscape changes. In this case frontside nucleophilic substitution occurs, and has a potential energy barrier which is lower than that of the corresponding backside substitution, which implies that the stereochemical outcome of this gas-phase nucleophilic substitution reaction is uncoupled from its kinetic order. This and similar results challenge the traditional view that nucleophilic substitution reactions should be categorized as being either S(N)1 or S(N)2.

Journal Article↗