Transition state stabilization by micelles: thiolysis of p-nitrophenyl alkanoates in cetyltrimethylammonium bromide micelles

2000 ◽  
Vol 78 (8) ◽  
pp. 1100-1108 ◽  
Author(s):  
Oswald S Tee ◽  
Ogaritte J Yazbeck
Keyword(s):  
Transition State ◽  
Cetyltrimethylammonium Bromide ◽  
Substrate Binding ◽  
Mercaptoacetic Acid ◽  
Passive Component ◽  
Dynamic Component ◽  
Micellar Phase ◽  
Hydrophobic Binding ◽  
Dynamic Components ◽  
Ester Chain

Thiolysis of p-nitrophenyl esters (acetate to decanoate) by the anion of 2-mercaptoethanol (ME) is catalyzed by micelles of cetyltrimethylammonium bromide (CTAB) in aqueous solution. At fixed [ME], the observed rate constants (kobs) show saturation with respect to added [CTAB], consistent with ester binding in the micelles. Plots of kobs vs. [ME] are linear in the absence and in the presence of the CTAB, and analysis of the slopes of the plots afford rates constants for thiolate ion attack on the esters in the aqueous phase (kN) and in the micellar phase (kcN). The strengths of substrate binding and transition state binding to the micelles are strongly correlated, with a slope of unity, because they have the same dependence on the ester chain. Consequently, the catalytic ratios (kcN/kN) are independent of the length of the ester. Similar behaviour is found for thiolysis by the dianions of mercaptoacetic acid, 3-mercaptopropionic acid, and cysteine, and also for ester cleavage by the anions of glycine and 2,2,2-trifluoroethanol, as earlier for cleavage by hydroxide ion. The results are consistent with Kirby's dissection of transition state binding into "passive" and "dynamic" components. The passive component involves hydrophobic binding of the ester chain which is more or less the same as in the substrate binding. The dynamic component is associated with reaction in the Stern layer of the micelle, and its magnitude varies with the nucleophiles because of differences in their ease of exchange between the aqueous medium and the Stern layer.Key words: catalysis, esters, thiolysis, micelles.

Transition state stabilization by micelles: the hydrolysis of p-nitrophenyl alkanoates in cetyltrimethylammonium bromide micelles

1997 ◽  
Vol 75 (10) ◽  
pp. 1434-1438 ◽  
Author(s):  
Oswald S. Tee ◽  
Alexei A. Fedortchenko
Keyword(s):  
Transition State ◽  
Chain Length ◽  
Cetyltrimethylammonium Bromide ◽  
Substrate Binding ◽  
Acyl Chain ◽  
Solution Rate ◽  
Reaction Centre ◽  
Acyl Chain Length ◽  
Transition State Stabilization ◽  
Hydrophobic Binding

The cleavage of p-nitrophenyl alkanoates (acetate to octanoate) at high pH is modestly catalyzed by micelles formed from cetyltrimethylammonium bromide (CTAB) in aqueous solution. Rate constants exhibit saturation behaviour with respect to [CTAB], consistent with substrate binding in the micelles. The strength of substrate binding and transition state binding to the micelles increases monotonically with the acyl chain length, and with exactly the same sensitivity. As a result, the extent of acceleration (or catalytic ratio) is independent of the ester chain. These and earlier results are consistent with the reaction centre being located in the Stern layer of the micelle, with the acyl chain of the ester being directed into the hydrophobic micellar interior. The chain length dependence of kinetic parameters found in this work is comparable to that found previously for ester cleavage by cyclodextrins and by various enzymes with hydrophobic binding sites, as well as to that observed for other phenomena involving hydrophobic effects. Keywords: catalysis, ester hydrolysis, micelles, transition state.

scholarly journals A crystallographic study of the Toho-1 β-lactamase acylation mechanism

2014 ◽  
Vol 70 (a1) ◽  
pp. C1207-C1207
Author(s):  
Leighton Coates
Keyword(s):  
Hydrogen Bonding ◽  
Transition State ◽  
Active Site ◽  
Catalytic Mechanism ◽  
Substrate Binding ◽  
Ligand Complex ◽  
Transition State Analog ◽  
Hydrogen Bonding Interactions ◽  
Active Site Region ◽  
Neutron Crystallography

β-lactam antibiotics have been used effectively over several decades against many types of highly virulent bacteria. The predominant cause of resistance to these antibiotics in Gram-negative bacterial pathogens is the production of serine β-lactamase enzymes. A key aspect of the class A serine β-lactamase mechanism that remains unresolved and controversial is the identity of the residue acting as the catalytic base during the acylation reaction. Multiple mechanisms have been proposed for the formation of the acyl-enzyme intermediate that are predicated on understanding the protonation states and hydrogen-bonding interactions among the important residues involved in substrate binding and catalysis of these enzymes. For resolving a controversy of this nature surrounding the catalytic mechanism, neutron crystallography is a powerful complement to X-ray crystallography that can explicitly determine the location of deuterium atoms in proteins, thereby directly revealing the hydrogen-bonding interactions of important amino acid residues. Neutron crystallography was used to unambiguously reveal the ground-state active site protonation states and the resulting hydrogen-bonding network in two ligand-free Toho-1 β-lactamase mutants which provided remarkably clear pictures of the active site region prior to substrate binding and subsequent acylation [1,2] and an acylation transition-state analog, benzothiophene-2-boronic acid (BZB), which was also isotopically enriched with 11B. The neutron structure revealed the locations of all deuterium atoms in the active site region and clearly indicated that Glu166 is protonated in the BZB transition-state analog complex. As a result, the complete hydrogen-bonding pathway throughout the active site region could then deduced for this protein-ligand complex that mimics the acylation tetrahedral intermediate [3].

scholarly journals Fatigue Cycles and Performance Evaluation of Accelerating Aging Heat Treated Aluminum Semi Solid Materials Designed for Automotive Dynamic Components

2020 ◽  
Vol 10 (9) ◽  
pp. 3008
Author(s):  
Mohamed Attia ◽  
Khaled Ahmed Ragab ◽  
Mohamed Bouazara ◽  
X.-Grant Chen
Keyword(s):  
Fatigue Life ◽  
Thermal Aging ◽  
Weight Ratio ◽  
Dynamic Component ◽  
Specific Strength ◽  
Suspension Control ◽  
Dynamic Components ◽  
Semi Solid ◽  
Automotive Suspension ◽  
Von Mises

The A357-type (Al-Si-Mg) aluminum semi solid casting materials are known for their excellent strength and good ductility, which make them materials of choice, preferable in the manufacturing of automotive dynamic mechanical components. Semi-solid casting is considered as an effective technique for the manufacturing of automotive mechanical dynamic components of superior quality performance and efficiency. The lower control arm in an automotive suspension system is the significant mechanical dynamic component responsible for linking the wheels of the vehicle to the chassis. A new trend is to manufacture this part from A357 aluminum alloy due to its lightweight, high specific strength, and better corrosion resistance than steel. This study proposes different designs of a suspension control arm developed, concerning its strength to weight ratio. Furthermore, this study aims to investigate the effect of accelerating thermal aging treatments on the fatigue life of bending fatigue specimens manufactured from alloy A357 using the Rheocasting semi-solid technology. The results revealed that the multiple aging cycles, of WC3, indicated superior fatigue life compared to standard thermal aging cycles. On the other hand, the proposed designs of automotive suspension control components showed higher strength-to-weight ratios, better stress distribution, and lower Von-Mises stresses compared to conventional designs.

The Relationship between Dynamic Components of Cutting Force and Adhesion of Tool-Chip Interface

2009 ◽  
Vol 407-408 ◽  
pp. 469-472 ◽  
Author(s):  
Ryo Tezuka ◽  
Katsuhiko Sekiya ◽  
Keiji Yamada ◽  
Yasuo Yamane
Keyword(s):  
Carbon Steel ◽  
Cutting Forces ◽  
Medium Carbon Steel ◽  
Dynamic Force ◽  
Rake Face ◽  
Stick Slip ◽  
Dynamic Component ◽  
Medium Carbon ◽  
Chip Flow ◽  
Dynamic Components

A new method for evaluation of adhesion in cutting is proposed. Adhesion of chip induces fluctuation in chip flow or stick-slip movement of chip, so that dynamic component of cutting forces depends on the cutting conditions and properties of the work materials. In this investigation continuous turning of a medium carbon steel was carried out and dynamic components of cutting forces are measured by piezoelectric dynamometer. Fluctuation in dynamic force on rake face of the tool is considered at a range of cutting speeds under dry condition and oil-mist lubrication. Surface profiles of machined surfaces and sticking of chip on tool face were also investigated. In cutting of medium carbon steel, the dynamic components below 500Hz increased under the condition of build-up edge (BUE) formation, and gradually decreased as increase of cutting speed. Tool-edge geometry well transferred onto the machined surface when the dynamic components were low level. Smoothness of chip flow on rake face is strongly associated with good surface finish.

scholarly journals Future precipitation changes in three key sub-regions of East Asia: the roles of thermodynamics and dynamics

2021 ◽  
Author(s):  
Jiao Li ◽  
Yang Zhao ◽  
Deliang Chen ◽  
Yanzhen Kang ◽  
Hui Wang
Keyword(s):  
East Asia ◽  
Climate Models ◽  
Coupled Model ◽  
Summer Precipitation ◽  
Tropical Region ◽  
Dynamic Component ◽  
Dynamic Components ◽  
Thermodynamics And Dynamics ◽  
Precipitation Changes

AbstractPrevious studies have projected an increase in future summer precipitation across East Asia (EA). This study investigates the relative contributions of thermodynamic and dynamic components to future precipitation changes in three key sub-regions of EA where the maximum centers of the historical precipitation are located (the tropical region, East China, and the Japan and Korea sector), and analyzes the causes of the changes in thermodynamic and dynamic components. Outputs from 30 climate models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are used. From these, the five best-performing models for historical summer precipitation climatology for EA are selected. The future summer precipitations in the three sub-regions over the near- to mid-term (2020–2069) and the long-term (2070–2095) are then examined using the multi-model ensemble mean of the five models selected (MMM05). The projections were driven by four combined scenarios of the Shared Socioeconomic Pathways (SSPs) and forcing levels of the Representative Concentration Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5). The results show that long-term precipitations under SSP5-8.5 are greater than those under the other scenarios across all sub-regions. After the 2070s under SSP5-8.5, a marked precipitation intensification is identified in all three sub-regions, but with different rates of increase. The projected precipitation increase is primarily attributed to the thermodynamic component, while the dynamic component related to circulation changes is relatively weak. Further analysis indicates that the pattern of the thermodynamic component in the three sub-regions is dominated by the climatological upward motion, mediated by an increase in moisture.

The dynamics of prey selection by the trap-building predator Gasteracantha hasselti

2020 ◽  
Vol 36 (3) ◽  
pp. 87-93
Author(s):  
Radek Michalko ◽  
Ondřej Košulič ◽  
Venus Saksongmuang ◽  
Prasit Wongprom ◽  
Prapinya Siripaiboon ◽  
...  
Keyword(s):  
Prey Availability ◽  
Prey Selection ◽  
Prey Type ◽  
Generalist Predators ◽  
Dynamic Component ◽  
Tropical Lowland ◽  
North Eastern ◽  
Lowland Forests ◽  
Dynamic Components ◽  
Relative Selectivity

AbstractPrey selection by generalist predators can be highly dynamic depending on the prey community structure. However, the dynamics of prey selection at the stage of prey entrapping are rarely investigated in trap-building predators, probably because their traps have been previously considered to intercept mobile prey proportionally to its availability in environment. Here we investigated the dynamics of prey selection by the orb-weaving spider Gasteracantha hasselti (Araneidae) depending on the composition of the available prey in tropical lowland forests located in north-eastern Thailand. We found that Gasteracantha captured a wide variety of prey but selected, on average, mostly Coleoptera and Diptera. The selectivity of Gasteracantha’s webs for Coleoptera was constant across the changes in overall prey availability and prey composition. The web selectivity for Hemiptera decreased rapidly with increasing relative densities of Hemiptera in the environment. The selectivity for Diptera and Hymenoptera increased and decreased, respectively, with their absolute densities in the environment. The relative selectivity of Gasteracantha’s traps for a particular prey type was driven by the presence and density of the highly selected prey rather than overall prey density. The results show that the selectivity of Gasteracantha’s traps for prey had both fixed and dynamic components and the dynamic component was determined by the relative as well as absolute densities of the particular prey types in the environment.

scholarly journals Chaperonins

1998 ◽  
Vol 333 (2) ◽  
pp. 233-242 ◽  
Author(s):  
Neil A. RANSON ◽  
Helen E. WHITE ◽  
Helen R. SAIBIL
Keyword(s):  
Conformational Changes ◽  
Substrate Binding ◽  
Atp Binding ◽  
Dependent Manner ◽  
Oligomeric Proteins ◽  
Double Ring ◽  
Substrate Protein ◽  
Hydrophobic Binding ◽  
Substrate Binding Sites

The molecular chaperones are a diverse set of protein families required for the correct folding, transport and degradation of other proteins in vivo. There has been great progress in understanding the structure and mechanism of action of the chaperonin family, exemplified by Escherichia coli GroEL. The chaperonins are large, double-ring oligomeric proteins that act as containers for the folding of other protein subunits. Together with its co-protein GroES, GroEL binds non-native polypeptides and facilitates their refolding in an ATP-dependent manner. The action of the ATPase cycle causes the substrate-binding surface of GroEL to alternate in character between hydrophobic (binding/unfolding) and hydrophilic (release/folding). ATP binding initiates a series of dramatic conformational changes that bury the substrate-binding sites, lowering the affinity for non-native polypeptide. In the presence of ATP, GroES binds to GroEL, forming a large chamber that encapsulates substrate proteins for folding. For proteins whose folding is absolutely dependent on the full GroE system, ATP binding (but not hydrolysis) in the encapsulating ring is needed to initiate protein folding. Similarly, ATP binding, but not hydrolysis, in the opposite GroEL ring is needed to release GroES, thus opening the chamber. If the released substrate protein is still not correctly folded, it will go through another round of interaction with GroEL.

Ester Cleavage by Cyclodextrins in Aqueous Dimethyl Sulfoxide Mixtures. Substrate Binding versus Transition State Binding

1994 ◽  
Vol 59 (25) ◽  
pp. 7602-7608 ◽  
Author(s):  
Oswald S. Tee ◽  
Charles Mazza ◽  
Rafael Lozano-Hemmer ◽  
Javier B. Giorgi
Keyword(s):  
Transition State ◽  
Dimethyl Sulfoxide ◽  
Substrate Binding ◽  
Versus Transition

scholarly journals Dynamics of cetyltrimethylammonium bromide mediated reaction of phenylsulfinylacetic acid with Cr(VI): Treatment of pseudo-phase models

2015 ◽  
Vol 80 (8) ◽  
pp. 1019-1034 ◽  
Author(s):  
Perumal Subramaniam ◽  
Thamil Selvi
Keyword(s):  
Cetyltrimethylammonium Bromide ◽  
Initial Rate ◽  
Intermediate Formation ◽  
Oxidative Decarboxylation ◽  
Micellar Phase ◽  
Cooperative Model ◽  
Rate Acceleration ◽  
Rate Retardation ◽  
Phase Models ◽  
Rate Profile

micellar effect % Hammett correlation % Piszkiewicz cooperative model KR nema The influence of cetyltrimethylammonium bromide, CTon the oxidative decarboxylation of phenylsulfinylacetic acid, PSAA and several meta- and para-substituted PSAAs by Cr(VI) was investigated in 95 % H2O-5 % CH3CN medium. The rate profile displayed a peculiar trend with an initial rate increase at low [CTAB] followed by sharp rate inhibition at higher [CTAB]. The initial rate acceleration can be explained by strong binding of SO42- on the positively charged micellar surface. The specific partitioning of PSAA in micellar phase by hydrophobic interaction and the oxidizing species, HCrO3+ in aqueous phase by electrostatic repulsion accounted the rate retardation at higher [CTAB]. The Hammett plot with different substituted PSAAs showed an excellent correlation affording negative ? value which supports the proposed mechanism involving a sulfonium cation intermediate formation. The obtained ? value in CTmedium is found to be slightly lower than that in aqueous medium. The quantitative analysis of rate data for the inhibition shown by CTwas performed using Menger-Portnoy and Piszkiewicz pseudo-phase models. The binding constant for PSAA with micelle was evaluated from Piszkiewicz cooperative model.

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