Changes In Activity Parameters Research Articles

ДЕЙСТВИЕ ГАММА-ОБЛУЧЕНИЯ НА ПАРАМЕТРЫ АКТИВНОСТИ МИКРОПОРОШКОВ АЛЮМИНИЯ

Актуальность исследования.Один из лидеров по производству алюминиевых порошков в России ООО «СУАЛПМ» перешел от выпуска грубодисперсных порошков к производству микронных порошков, имеющих более высокую реакционную способность. Микропорошки алюминия применяются во многих отраслях промышленности:порошковой металлургии, самораспространяющемся высокотемпературном синтезе новых материалов,водородной энергетике, пиротехнике иракетных топливах. Повышение реакционной способности порошков алюминия в различных процессах приводит к понижению энергозатрат и экономии ресурсов. Цель: получение и объяснение экспериментальных данных по изменению параметров активности микропорошков алюминия после их облучения в зависимости от дозыγ-облучения. Объекты: микропорошки алюминия АСД-6, АСД-6М. Методы:дифференциальный термический анализ, рентгенофазовый анализ, методика облучения микропорошков алюминия γ-излучением, методика расчета параметров активности порошков алюминия. Результаты. Получены количественные показатели реакционной способности микропорошков алюминия АСД-6 и АСД-6М до и после облучения γ-излучением изотопа Со60 с энергией 1,17 и 1,33 МэВ, т.е. энергией существенно ниже порога фотоядерных реакций. Дозы облучения образцов порошков составляли 1, 2, 4, 8 и 10 Мрад. После γ-облучения температура начала окисления микропорошков снизилась максимально на 90 и 85 °С; максимальная скорость окисления возросла на 83 и 36 %; степень окисленности (при нагревании до 1250 °С) повысилась на 5,1 %, и минимально понизилась на 1,3 % для микропорошков АСД-6 и АСД-6М, соответственно. Удельный тепловой эффект окисления после γ-облучения всеми дозами был больше, чем для необлученных порошков.Максимальные значения удельного теплового эффекта для АСД-6на 199,5 кДж/моль, а для АСД-6М – на 134,8 кДж/моль больше тепловых эффектов, чем для необлученных порошков, что существенно превышает стандартную теплоту плавления алюминия (10,8 кДж/моль). Следовательно, такое состояние системы «алюминий – оксид алюминия» характеризуется запасенной энергией в 9 раз больше стандартной теплоты плавления алюминия, что с позиции классической термодинамики невозможно. В то же время известно, что запасание энергии в нанопорошках происходит за счет формирования двойного электрического слоя в частицах алюминия.

Activation parameter changes as an additive tool for a mechanistic viewpoint in the aromatic nucleophilic substitution reactions in solution

A study was made of the influence of the substituent effects on the activation parameters changes in the SNAr reactions. It involved the analysis of both the free energy of activation ∆G ≠ from the reaction rate constants at single temperature and δ∆G ≠ reaction constants obtained by the Hammett-like equation. The analysis of the internal enthalpy constants δ∆H int ≠ and the Hammett ρ constants allowed one to prove the existence of dependence between the δ∆H int ≠ and ρ values in which the high magnitudes of these constants indicate a change in the mechanism from a polar SNAr pathway to a SET (single electron transfer).

Kinetics of n-propylmercaptan oxidation by cobalt(III) octahydroxyphthalocyanine in an alkali medium

The specific features of the association of cobalt octahydroxyphthalocyanine in aqueous alkaline solutions have been investigated by electron spectroscopy. The kinetic parameters of the oxidation of n-propylmercaptan by the phthalocyanine complex of cobalt in the absence of oxygen have been determined. The association leads to a decrease in the activity of the complex and to changes in activation parameters.

Effects of substituents on activation parameter changes in the Michael-type reactions of nucleophilic addition to activated alkenes and alkynes in solution

Variation of the activation parameters in the Michael-type reactions (AdN reactions) offers an additive mechanistic tool for the studies of these reactions in solution. This approach uses the substituent effects on the aromatic rings to the variation of the activation parameters, ∆X ≠ (X = H, S, G) in the above reactions in the frameworks of the Hammett-like equations in order to evaluate the resultant δ∆X ≠ reaction constants. The single linear dependences of the internal enthalpy constants δ∆H ≠ on the δ∆G ≠ and the Hammett ρ constants show that the substituent effects in the substrates and nucleophiles on the mechanistic features in AdN reactions are governed by the magnitude of δ∆H ≠ when one of the steps of the process is the single rate-determining step.

Activation parameter changes as a mechanistic tool in SN2 reactions in solution

Recent applications of activation parameters variation approach to the elucidation of S N 2 reaction mechanisms have led to further clarifications of structures of transition states involved in the concerted reaction pathway. S N 2 reactions in solution are reviewed with special emphasis of activation parameter variation Δ X ≠ ( X = H , S and G ) with substituents in the nucleophile, leaving and nonleaving groups applying linear free energy relationships in order to evaluate the resultant δΔ X ≠ reaction constants. The use of internal enthalpy reaction constants δΔ H ≠ int as a mechanistic tool is stressed when the structure of transition state in S N 2 reaction is changed. Variations of the activation parameters in S N 2 reactions and their mechanisms were analyzed.

ChemInform Abstract: Activation Parameter Changes as a Mechanistic Tool in Acyl‐Transfer Reactions in Solution

AbstractReview: 129 refs.

Racemization of chiral Pd2Au36(SC2H4Ph)24: doping increases the flexibility of the cluster surface.

Pd2Au36(SC2H4Ph)24 clusters have been prepared, isolated and separated in their enantiomers. Compared to the parent Au38(SC2H4Ph)24 cluster, the doping leads to a significant change of the circular dichroism spectrum; however, the anisotropy factors are of similar magnitude in both cases. Isolation of the enantiomers allowed us to study the racemization of the chiral cluster, which reflects the flexibility of the ligand shell composed of staple motifs. The doping leads to a substantial lowering of the racemization temperature. The change in activation parameters due to the doping may be solely due to modification of the electronic structure.

Activation Parameter Changes as a Mechanistic Tool in Acyl-Transfer Reactions in Solution

Activation Parameter Changes as a Mechanistic Tool in Acyl-Transfer Reactions in Solution

Substituent Effects on the Activation Parameter Changes for the Aminolysis in the Bimolecular Nucleophilic Reactions in Solution

Variation of the activation parameters for the aminolysis in the SN2, acyl-transfer, SNAr and AdN reactions offers an additive mechanistic tool for the studies of these reactions in solution. This approach uses the substituent effects on the benzene and pyridine rings to the variation of the activation parameters, ΔX≠ (X = H, S, G), in the above reactions in the frameworks of the Hammett - like equations in order to evaluate the resultant δΔX≠ reaction constants. The single linear dependences of the internal enthalpy constants δΔH≠ int on the δΔG≠ and the Hammett ρ constants show that the substituent effects in the leaving and nonleaving groups and nucleophiles on the mechanistic features in aminolysis of bimolecular nucleophilic reactions are governed by the magnitude of δΔH≠ int when one of the steps of the process is the single rate-determining step.

Amination by nucleophilic substitution and addition in solution: Substituents effect on changes in activation parameters

Changes in the activation parameters of amination along mechanisms S N 2, S N Ar, Ad N and in reactions with acyl group transfer may serve as an additional investigation method for these reactions in solution. The approximation utilizes the substituents effect in benzene and pyridine derivatives on the changes in the activation parameters ΔX ≠ (X = H, S, G) in the framework of equations of Hammett’s type for the estimation of the resulting reaction constants δΔX ≠. The single linear dependences of the reaction constants of internal enthalpy δΔH int ≠ on the reaction constants δΔG ≠ and Hammett’s ρ show that the substituents effect in the leaving and non-leaving groups and in the nucleophiles on the amination mechanisms is governed by the δΔH int ≠ , when unique stage of the process determines its rate.

Investigating the origin of entropy-derived rate accelerations in ionic liquids

The effects on the rate and activation parameters of a series of Menschutkin processes on changing from a molecular solvent to an ionic liquid were investigated. The removal of delocalised pi-systems from the reagents does not affect the change in activation parameters on changing solvent. In each of the cases investigated, rate accelerations observed on moving to the ionic liquid could be attributed to an increase in reaction entropy. This suggests a specific interaction of the ionic liquid with the nucleophilic centre, rather than the delocalised pi-systems of either the electrophile or the nucleophile.

The influence of strain and activation on the locomotor function of rat ankle extensor muscles

The ankle extensor muscles of the rat have different mechanical and physiological properties, providing a means of studying how changes in locomotor demands influence muscle fascicle behaviour, force and mechanical power output in different populations of muscle fibre types. Muscle fascicle strain, strain rate and activation patterns in the soleus, plantaris and medial gastrocnemius muscles of the rat were quantified from sonomicrometric and myoelectric data, collected during treadmill locomotion under nine velocity/incline conditions. Significant differences in peak-to-peak muscle fascicle strains and strain rates were identified between the three muscles (P<0.001, all cases), with much smaller strains (<0.1) and strain rates (<0.5 s(-1)) occurring in soleus and plantaris compared with medial gastrocnemius (>0.2 and >1.0 s(-1), respectively). The proportion of stride duration that each muscle was active (duty cycle) differed between locomotor conditions as did the timing of the activation and deactivation phases. A simple Hill-based muscle model was used to determine the influence of muscle activation relative to maximum fascicle strain and duty cycle on total force production and mechanical power output, from a slow and a fast muscle fibre, simulated through two peak-to-peak strain cycles (0.1 and 0.3). The predictions of the model did not complement conclusions that may be drawn from the observation of myoelectric timing and fascicle strain trajectories in each of the muscles. The model predicted that changes in mechanical power output were more sensitive to changes in activation parameters than to changes in strain trajectories, with subtle changes in activation phase and duty cycle significantly affecting predicted mechanical power output.

Intensity of activation and timing of deactivation modulate elastic energy storage and release in a pennate muscle and account for gait-specific initiation of limb protraction in the horse

The equine biceps brachii (biceps) initiates rapid limb protraction through a catapult mechanism. Elastic strain energy is slowly stored in an internal tendon and is then rapidly released to protract the forelimb. The muscle fibres are short, have little scope for length change and can therefore only shorten slowly compared with the speed at which the whole muscle must shorten, which makes them poor candidates for driving rapid limb protraction. We suggest that the muscle fibres in the biceps act to modulate the elastic energy output of the muscle-tendon unit (MTU) to meet the demands of locomotion under different conditions. We hypothesise that more elastic strain energy is stored and released from the biceps MTU during higher speed locomotion to accommodate the increase in energy required to protract the limb and that this can be achieved by varying the length change and activation conditions of the muscle. We examined the work performed by the biceps during trot and canter using an inverse dynamics analysis (IDA). We then used excised biceps muscles to determine how much work could be performed by the muscle in active and passive stretch-shorten cycles. A muscle model was developed to investigate the influence of changes in activation parameters on energy storage and energy return from the biceps MTU. Increased biceps MTU length change and increased work performed by the biceps MTU were found at canter compared with at trot. More work was performed by the ex vivo biceps MTU following activation of the muscle and by increasing muscle length change. However, the ratio of active to passive work diminished with increasing length change. The muscle model demonstrated that duration and timing of activation during stretch-shorten cycles could modulate the elastic energy storage and return from the biceps. We conclude that the equine biceps MTU acts as a tuneable spring and the contractile component functions to modulate the energy required for rapid forelimb protraction at different speeds.

PHYSICAL ACTIVITY PATTERNS IN THE NATIONAL WEIGHT CONTROL REGISTRY

Objective: The National Weight Control Registry (NWCR) was established in 1993 to examine the characteristics of those who are successful at weight loss: individuals maintaining a 13.6-kg weight loss for >1 year. The size of the registry has increased substantially since the early descriptions of this group a decade ago. The purpose of this study was to describe in detail the weekly physical activity habits of NWCR members, to examine the relationship between amount of activity and demographic characteristics, and to determine if changes in activity parameters have occurred over time. Methods and Procedures: Participants were 887 men and 2,796 women who enrolled in the NWCR between 1993 and 2004. Physical activity was evaluated at registry entry using the Paffenbarger Physical Activity Questionnaire. Results: NWCR entrants report an average of 2,621 ± 2,252 kcal/week in physical activity. There is considerable variability in the amount of activity reported: 25.3% report 3,000 kcal/week. Activity level on registry entry is related to the magnitude but not the duration of weight loss. The amount of activity reported by men has decreased over time while no significant change was observed in women. Changes in the types of activities most frequently reported were also observed. Discussion: Overall, NWCR participants are an extremely physically active group. However, the amount of activity reported is highly variable, making it difficult to develop a single recommendation for the optimum amount of physical activity for weight loss maintenance. A better understanding of individual-specific determinants of how much activity is required for weight loss maintenance ought to be a high research priority.

Escherichia coli RNase P RNA: Substrate Ribose Modifications at G+1, but Not Nucleotide − 1/+73 Base Pairing, Affect the Transition State for Cleavage Chemistry

The temperature dependence of processing of precursor tRNA Gly (ptRNA Gly) variants carrying a single 2′-OCH 3 or locked nucleic acid (LNA) modification at G+1 by Escherichia coli endoribonuclease P RNA was studied at rate-limiting chemistry. We show, for the first time, that these ribose modifications at nucleotide + 1 increase the activation energy and alter the activation parameters for the transition state of hydrolysis at the canonical (c 0) cleavage site (between nucleotides − 1 and + 1). The modified substrates, particularly the one with LNA at G+1, caused an increase in the activation enthalpy Δ H ‡, which was partly compensated for by a simultaneous increase in the activation entropy Δ S ‡. NMR imino proton spectra of model acceptor stems derived from the same ptRNA variants unveiled that a riboT or U at − 1 forms two hydrogen bonds with U+73, thus extending the acceptor stem by 1 bp. The non-canonical base pair is substantially stabilized by LNA substitution at nucleotides − 1 or + 1. To address if the activation energy increase owing to LNA at G+1 stems from dissociation of the U(− 1)–U(+ 73) base pair as a prerequisite for interaction of U(+ 73) with U294 in endoribonuclease P RNA, we tested a ptRNA Gly variant that is capable of forming an extra C(− 1)–G(+ 73) Watson–Crick base pair. However, compared with a control ptRNA (C at − 1, U at + 73), no significant change in activation parameters was observed for this ptRNA. Thus, our results argue against the possibility that breaking of an additional base pair at the end of the acceptor stem may present an energetic barrier for reaching the transition state of the chemical step for cleavage at the canonical (c 0) phosphodiester.

Physical activity patterns in the National Weight Control Registry.

The National Weight Control Registry (NWCR) was established in 1993 to examine the characteristics of those who are successful at weight loss: individuals maintaining a 13.6-kg weight loss for >1 year. The size of the registry has increased substantially since the early descriptions of this group a decade ago. The purpose of this study was to describe in detail the weekly physical activity habits of NWCR members, to examine the relationship between amount of activity and demographic characteristics, and to determine if changes in activity parameters have occurred over time. Participants were 887 men and 2,796 women who enrolled in the NWCR between 1993 and 2004. Physical activity was evaluated at registry entry using the Paffenbarger Physical Activity Questionnaire. NWCR entrants report an average of 2,621 +/- 2,252 kcal/week in physical activity. There is considerable variability in the amount of activity reported: 25.3% report <1,000 kcal/week and 34.9% report >3,000 kcal/week. Activity level on registry entry is related to the magnitude but not the duration of weight loss. The amount of activity reported by men has decreased over time while no significant change was observed in women. Changes in the types of activities most frequently reported were also observed. Overall, NWCR participants are an extremely physically active group. However, the amount of activity reported is highly variable, making it difficult to develop a single recommendation for the optimum amount of physical activity for weight loss maintenance. A better understanding of individual-specific determinants of how much activity is required for weight loss maintenance ought to be a high research priority.

Thermodynamic Characteristics of Plasminogen Activation by Indirect Activators

Several indirect plasminogen (Pg) activators are known including streptokinase and the monoclonal antibody IV-Ic, whose mechanism of activation is well studied. To characterize thermodynamically the activation of Pg by streptokinase (SK) and the monoclonal antibody (mAB) IV-Ic, the activation energies were calculated for various reaction stages. Activation energy of 7.4 kcal/mol was determined for the interaction of the chromogenic substrate S-2251 with plasmin (Pm) and activated equimolar complexes Pm-SK and Pg*SK at the steady-state reaction stage, and 18.7 kcal/mol with the complexes Pg*IV-Ic. A 2.5-fold increase in the energy of activation for the Pg*IV-Ic complex suggests a more intricate mechanism of its interaction with the substrate. At the stage of increasing active center concentrations and the formation of activated complexes Pg*SK and Pg*mAB IV-Ic, the activation energy was found to be 10.5 and 38 kcal/mol, respectively. At this reaction stage the conformational rearrangement of Pg molecule with the formation of active center is the limiting stage determining the reaction rate. Unexpectedly high energy of activation at the second stage of interaction between mAB IV-Ic and Pg suggests several simultaneous reactions and complexity of conformation rearrangement in the Pg molecule in activated complexes, thus requiring large energy expense. Formation of the active center is probably accompanied by its transition within a narrow temperature range into another conformation state with the change in activation parameters of the reaction. Quantitative evaluation of the studied reactions from the perspective of thermodynamics of the enzymatic reactions gives more comprehensive characteristics of the activation mechanism.

The effect of polymer chain structure on the catalytic activity of polymer-copper complexes

Polymer chain effects on catalytic activity were investigated for binuclear coordination complexes of copper ions with polymeric tertiary amines like dimethylaminomethylated polystyrene, copolymers of styrene with 4-vinylpyridine or N-vinylimidazole, and some N-alkylated polyimines. Oxidative coupling of phenols by oxygen in the presence of these copper complexes was chosen as a model reaction. Two polymer effects will be discussed: (i) the increase of catalytic activity with increasing degree of loading of the polymer chains with catalytic centers. This effect will be analyzed in terms of changes in activation parameters and can be related to an increase in conformational freedom of the intermediate chain segments during deformation of the catalyst-substrate complex prior to electron transfer from substrate to copper ions; (ii) restrictions in the above enhancement of catalytic activity at still higher degrees of loading because the intermediate chain segments become too short for linking neighbouring ligands to the same complex.

Influence of ligand-water interactions on the aquation of pentacyano(saturated amine)ferrate(II) ions

Rate constants and activation parameters for the aquation of 12 saturated amines from pentacyanoamineferrate(I1) in 1 M pyridine solutions are reported. While AG* values do not correlate with AGIO (for the ionization of the conjugate acid) or with u* and E, Taft's constants, an isokinetic relationship is found. For the series (CH3)xSH3_, and RSH2, AP correlates with A,' in water and each series shows an independent correlation of AS* with B,. These results are interpreted in terms of an adapted Caldin and Benetto's model for exchange reactions, the main contribution to the changes in activation parameters arising seemingly from the energetics of transfer of the released ligand to bulk water. This small contribution to the activation process in water is rendered observable by the remarkable insensitivity of the rate process to inner bonding effects.

Enthalpies of activation in solvolysis reactions

New data for the rate of solvolysis of PhCH2Cl at 0 and 4 °C fit on the same Arrhenius plot as data at higher temperatures contradicting the sharp change in activation parameters reported by Hills and Viana; the solvent isotope effect at 4 °C is normal and changes in the aqueous solvent between 0 and 4 °C produce no marked effect on the reaction rate of either PhCH2Cl or CMe3Cl.