The 44Ti(alpha,p) reaction and its implication on the 44Ti yield in supernovae

Cross sections for the 44Ti(alpha,p)47V reaction which significantly affects the yield of 44Ti in supernovae were measured in the energy range 5.7 MeV相似文献   

Identifying Cyclic Patterns of Interaction to Study Individual and Collective Learning

Our analysis of a college level mathematics course for prospective secondary mathematics teachers revealed that each student developed, at least to some degree, a conceptual orientation for teaching mathematics (A. G. Thompson, Philipp, Thompson, & Boyd, 1994). This initial finding led to a more in-depth question: If we assume an emergent perspective (Cobb & Bauersfeld, 1995; Cobb & Yackel, 1996) in which the values, practices, and social motivations of the classroom are believed to play critical roles in students' conceptual development, what social aspects emerged that supported these individual constructions? To address this question, we documented the emergence of a collective conceptual orientation and then used this construct to explore the reflexivity between its emergence and individual students' development of conceptual orientations.     

Structure-Toxicity Relationships for Aliphatic Compounds Encompassing a Variety of Mechanisms of Toxic Action to Vibrio fischeri

Abstract

QSARs based upon the logarithm of the octanol-water partition coefficient, logP, and energy of the lowest unoccupied molecular orbital, E_LUMO were developed to model the toxicity of aliphatic compounds to the marine bacterium Vibrio fischeri. Statistically robust, hydrophobic-dependent QSARs were found for chloroalcohols and haloacetonitriles. Modelling of the toxicity of the haloesters and the diones required the use of terms to describe both hydrophobicity and electrophilicity. The differences in intercepts, slopes, and fit of these models suggest different electrophilic mechanisms occur between classes, as well as within the diones and haloesters. In order to model globally the toxicity of aliphatic compounds to V. fischeri, all the data determined in this study were combined with those determined previously for alkanones, alkanals, and alkenals. A highly predictive two-parameter QSAR [pT₁₅ = 0.760(log P) ?0.625(E _LUMO) ?0.466; n = 63, s = 0.462, r ² = 0.846, F = 171, Pr > F = 0.0001] was developed for the combined data that models across classes and is independent of mechanisms of action. The toxicity of these compounds to V. fischeri compares well to the toxicity (50% population growth inhibition) to the ciliate Tetrahymena pyriformis (r ² = 0.850).     

Duplex formation and the onset of helicity in poly d(CG)n oligonucleotides in a solvent-free environment

The gas-phase conformations of a series of cytosine/guanine DNA duplexes were examined by ion mobility and molecular dynamics methods. Deprotonated duplex ions were formed by electrospray ionization, and their collision cross sections measured in helium were compared to calculated cross sections of theoretical models generated by molecular dynamics. The 4-mer (dCGCG) and 6-mer (dCGCGCG) duplexes were found to have globular conformations. Globular and helical structures were observed for the 8-mer (dCGCGCGCG) duplex, with the globular form being the more favored conformer. For the 10-mer (dCGCGCGCGCG), 14-mer (dCGCGCGCGCGCGCG), and 18-mer (dCGCGCGCGCGCGCGCGCG) duplexes, only helical structures were observed in the ion mobility measurements. Theory predicts that the helical structures are less stable than the globular forms in the gas phase and should collapse into the globular form given enough time. However, molecular dynamics simulations at 300 K indicate the helical structures are stable in aqueous solution and will retain their conformations for a limited time in the gas phase. The presence of helical structures in the ion mobility experiments indicates that the duplexes retain "solution structures" in the gas phase on the millisecond time scale.     

Measurement of the alpha-secondary kinetic isotope effect for the reaction catalyzed by mammalian protein farnesyltransferase

Protein farnesytransferase (FTase) catalyzes the transfer of a 15-carbon prenyl group from farnesyl diphosphate (FPP) to the cysteine residue of target proteins and is a member of the newest class of zinc metalloenzymes that catalyze sulfur alkylation. Common substrates of FTase include oncogenic Ras proteins, and therefore inhibitors are under development for the treatment of various cancers. An increased understanding of the salient features of the chemical transition state of FTase may aid in the design of potent inhibitors and enhance our understanding of the mechanism of this class of zinc enzymes. To investigate the transition state of FTase we have used transient kinetics to measure the alpha-secondary 3H kinetic isotope effect at the sensitive C1 position of FPP. The isotope effect for the FTase single turnover reaction using a peptide substrate that is farnesylated rapidly is near unity, indicating that a conformational change, rather than farnesylation, is the rate-limiting step. To look at the chemical step, the kinetic isotope effect was measured as 1.154 +/- 0.006 for a peptide that is farnesylated slowly, and these data suggest that FTase proceeds via a concerted mechanism with dissociative character.     

Gas-phase conformations of deprotonated trinucleotides (dGTT-, dTGT-, and dTTG-): the question of zwitterion formation

The gas-phase conformations of a series of trinucleotides containing thymine (T) and guanine (G) bases were investigated for the possibility of zwitterion formation. Deprotonated dGTT-, dTGT-, and dTTG- ions were formed by MALDI and their collision cross-sections in helium measured by ion mobility based methods. dTGT- was theoretically modeled assuming a zwitterionic and non-zwitterionic structure while dGTT- and dTTG- were considered "control groups" and modeled only as non-zwitterions. In the zwitterion, G is protonated at the N7 site and the two neighboring phosphates are deprotonated. In the non-zwitterion, G is not protonated and only one phosphate group is deprotonated. Two conformers, whose cross-sections differ by 17 +/- 2 A2, are observed for dTGT- in the 80 K experiments. Multiple conformers are also observed for dGTT- and dTTG- at 80 K, though relative cross-section differences between the conformers could not be accurately obtained. At higher temperatures (>200 K), the conformers rapidly interconvert on the experimental time scale and a single "time-averaged" conformer is observed in the ion mobility data. Theory predicts only one low-energy conformation for the zwitterionic form of dTGT- with a cross-section 8% smaller than experimental values. Additionally, the extra H+ on G does not bridge both phosphates. Thus, dTGT- does not appear to be a stable zwitterion in the gas-phase. Theory does, however, predict two low-energy conformers for the non-zwitterionic form of dTGT- that differ in cross-section by 18 +/- 3 A2, in good agreement with the experiment. In the smaller cross-section form (folded conformer), G and one of the T bases are stacked while the other T folds towards the stacked pair and hydrogen bonds to G. In the larger cross-section form (open conformer), the unstacked T extends away from the T/G stacked pair. Similar folded and open conformers are predicted for all three trinucleotides, regardless of which phosphate is deprotonated.     

A stochastic cellular automata model of tautomer equilibria

ABSTRACT

Many chemical substances, including drugs and biomolecules, exist in solution not as a single species, but as a collection of tautomers and related species. Importantly, each of these species is an independent compoundwith its own specific biochemical and physicochemical properties. The species interconvert in a dynamic and often complicated manner, making modelling the overall species composition difficult. Agent-based cellular automata models are uniquely suited to meet this challenge, allowing the equilibria to be simulated using simple rulesand at the same time capturing the inherent stochasticity of the natural phenomenon. In the present example a stochastic cellular automata model is employed to simulate the tautomer equilibria of 9-anthrone and 9-anthrol in the presence of their common anion. The observed K_E of the 9-anthrone ? 9-anthrol tautomerisation along with the measured tautomer pK_a values were used to model the equilibria at pH values 4, 7 and 10. At pH 4 and 7, the anthrone comprises >99% of the total species population, while at pH 10the anthrone and the anion each represent just under half of the total population. The advantages of the cellular automata approach over the customary coupled differential equation approach are discussed.     

The Borsuk dimension of a graph and Borsuk's partition conjecture for finite sets

Borsuk's partition conjecture for finite subsets of euclidean space is placed in a graph theoretic setting and equivalent graph theoretic conjectures are raised. The Borsuk dimension of a graph is defined and the Borsuk dimensions of various graphs are tabulated.AMS Subject Classification (1985 Revision): 05C99, 05A17