scholarly journals A computational and experimental study of O-glycosylation. Catalysis by human UDP-GalNAc polypeptide:GalNAc transferase-T2

2014 ◽  
Vol 12 (17) ◽  
pp. 2645-2655 ◽  
Author(s):  
Hansel Gómez ◽  
Raúl Rojas ◽  
Divya Patel ◽  
Lawrence A. Tabak ◽  
José M. Lluch ◽  
...  
Keyword(s):  
Experimental Study ◽  
Nucleophilic Attack ◽  
Front Side ◽  
Leaving Group

GalNAc-T2 catalyses GalNAc O-glycosylation via a front-side nucleophilic attack in which stabilization of the UDP leaving group is crucial.

ChemInform Abstract: An Experimental Study of Special Leaving Group Behavior in the Reaction of Arylidenebarbituric Acids with Carbon Nucleophiles.

ChemInform ◽  
2011 ◽  
Vol 42 (20) ◽  
pp. no-no
Author(s):  
Mohammad A. Bigdeli ◽  
Enayatollah Sheikhhosseini ◽  
Azizollah Habibi ◽  
Saeed Balalaie
Keyword(s):  
Experimental Study ◽  
Group Behavior ◽  
Carbon Nucleophiles ◽  
Leaving Group

The reactions of mercaptide anions with chlorinated sulfones

1977 ◽  
Vol 55 (12) ◽  
pp. 2316-2322 ◽  
Author(s):  
Richard F. Langler ◽  
James A. Pincock
Keyword(s):  
Substituent Effects ◽  
Nucleophilic Attack ◽  
Leaving Group

Mercaptide anions react with chlorinated sulfones in two modes, i.e. nucleophilic attack on carbon with chlorine as the leaving group and/or nucleophilic attack on chlorine with concomitant carbanion formation. Mercaptide anion pKb, degree of chlorination of the sulfone substrate, and substituent effects are qualitatively assessed in terms of the propensity for nucleophilic attack at carbon or chlorine.

Studies of azo and azoxy dyestuffs. Part 18. Kinetics and mechanism of the hydrolysis of 3- and 4-(p′-methoxyphenylazo)pyridines in aqueous sulfuric acid media

1986 ◽  
Vol 64 (11) ◽  
pp. 2115-2126 ◽  
Author(s):  
Erwin Buncel ◽  
Ikenna Onyido
Keyword(s):  
Sulfuric Acid ◽  
Transition States ◽  
Activation Parameters ◽  
Nucleophilic Attack ◽  
Acid Solutions ◽  
Acid Media ◽  
Charge Delocalization ◽  
Leaving Group ◽  
The Difference ◽  
Hydrolysis Of

The kinetics of hydrolysis of 4-(p′-methoxyphenylazo)pyridine, 1, and its 3-isomer, 2, have been studied in moderately concentrated sulfuric acid media at 25 °C. In all the acid solutions investigated, 1 reacted faster than 2; rate differences between the two compounds varied from ca. 1000-fold in the dilute region of acidity to ca. 250-fold in the more concentrated acid solutions. The observed first-order rate constants, kψ, for both substrates exhibit a maximum, at ca. 42% H2SO4 and 47% H2SO4 for 1 and 2 respectively. Activation parameters have also been determined. The pKa values for the second protonation equilibria of 1 and 2 have been evaluated and structures of the diprotonated species are discussed. Hydrolysis is shown to occur from the diprotonated substrates and two main mechanisms are operative. The first is an A-2 type mechanism, which involves rate-limiting attack of H2O on the aryl carbon center giving delocalized transition states and intermediates in which the pyridinium and azonium functions are involved in charge delocalization. Subsequent transfer of a proton and detachment of the leaving group are fast processes. In the second A-SE2 type mechanism, nucleophilic attack and transfer of the proton are fast steps preceding the slow general acid catalyzed separation of the leaving group. The difference in reactivity of the two compounds is attributed to differences in extent of charge delocalization in the transition states of the reactions: for 1 both the pyridinium and protonated azonium functions are involved whereas for 2 only the azonium function participates in charge delocalization.

Glycosyl Group Transferases

2007 ◽  
Author(s):  
Perry A. Frey ◽  
Adrian D. Hegeman
Keyword(s):  
Great Majority ◽  
Nucleophilic Attack ◽  
Axial Position ◽  
Equatorial Position ◽  
Leaving Group ◽  
Dilute Aqueous Solutions ◽  
Anomeric Carbon ◽  
Basic Solutions ◽  
Hydrolysis Of ◽  
Acceptor Molecules

Glycosyl group transfer underlies the biosynthesis and breakdown of all nucleotides, polysaccharides, glycoproteins, glycolipids, and glycosylated nucleic acids, as well as certain DNA repair processes. Glycosyl transfer consists of the transfer of the anomeric carbon of a sugar derivative from one acceptor to another, as in, which describes the transfer of a generic pyranosyl ring between nucleophilic atoms :X and :Y of acceptor molecules. The stereochemistry at the anomeric carbon is not specified in eq. 12-1, but the leaving group occupies the axial position in an α-anomer or the equatorial position in a β-anomer. The overall transfer can proceed with either retention or inversion of configuration. In biochemistry, the acceptor atoms can be oxygen, nitrogen, sulfur, or in the biosynthesis of C-nucleosides even carbon. The great majority of biological glycosyl transfer reactions involve transfer between oxygen atoms of different acceptor molecules. Enzymes catalyzing glycosyl transfer are broadly grouped according to whether the acceptor :Y–R2 in is water or another molecule. In the actions of glycosidases, the acceptor is water, and glycosyl transfer results in hydrolysis of a glycoside, a practically irreversible process in dilute aqueous solutions. In the action of glycosyltransferases, the acceptors are molecules with hydroxyl, amide, amine, sulfhydryl, or phosphate groups. The simplest nonenzymatic glycosyl transfer reaction is the hydrolysis of a glycoside, and early studies revealed the fundamental fact that glycosides are much less reactive toward hydrolysis in basic solutions than in acidic solutions. This fact underlies much that is known about the mechanism of glycosyl transfer; that is, the anomeric carbon of a glycoside is remarkably unreactive toward direct nucleophilic attack, but it becomes reactive when one of the oxygens is protonated by an acid, as illustrated in fig. 12-1 for the acid-catalyzed hydrolysis of a generic glycoside. The reaction by both mechanisms in fig. 12-1 proceeds by pre-equilibrium protonation of the glycoside to form oxonium ion intermediates, which are subject to hydrolysis by water. The two mechanisms in fig. 12-1 are of interest. The mechanism proceeding through exocyclic cleavage of the glycoside has historically been regarded as the more likely, and for this reason, the route through endocyclic cleavage has received little consideration.

scholarly journals Expedient synthesis of 1,6-anhydro-α-D-galactofuranose, a useful intermediate for glycobiological tools

2014 ◽  
Vol 10 ◽  
pp. 1651-1656 ◽  
Author(s):  
Luciana Baldoni ◽  
Carla Marino
Keyword(s):  
Hydroxy Group ◽  
Building Blocks ◽  
Nucleophilic Attack ◽  
Step Procedure ◽  
Leaving Group ◽  
Anomeric Carbon ◽  
Good Precursor

A new and efficient three-step procedure for the synthesis of 1,6-anhydro-α-D-galactofuranose is described. The key step involves the formation of the galactofuranosyl iodide by treatment of per-O-TBS-D-Galf with TMSI, the selective 6-O-desilylation by an excess of TMSI, and the simultaneous nucleophilic attack of the 6-hydroxy group on the anomeric carbon, with the iodide as a good leaving group. This compound is a good precursor for building blocks for the construction of 1→6 linkages.

Impact to Composite Box Containing Water and Baffles

2016 ◽  
Vol 139 (3) ◽  
Author(s):  
Y. W. Kwon ◽  
T. J. South ◽  
K. J. Yun
Keyword(s):  
Experimental Study ◽  
Water Level ◽  
Computational Study ◽  
Structural Response ◽  
Experimental Tests ◽  
Low Velocity Impact ◽  
Front Side ◽  
Low Velocity ◽  
Structural Responses ◽  
The Impact

A series of experimental tests were conducted for low-velocity impact on a composite box containing water in order to study the fluid–structure interaction (FSI). Then, baffles were inserted in the box to examine their effect on the structural response of the composite box. Finally, a computational study was conducted to supplement the experimental study. The water level inside the composite box was varied incrementally from 0% (i.e., no water) to 100% (full water). The impact velocity was also changed. In the experimental study, strain gauges and the load cell were used to measure the strain responses at the front, side, and back surfaces as well as the impact force. The results showed that the FSI effect was significant to the structural responses depending on the water level. The effect of the baffle was different among the front, side, and back surfaces. Both experimental and numerical results agreed well.

scholarly journals Beacon-Based Individualized Hazard Alarm System for Construction Sites: An Experimental Study on Sensor Deployment

2021 ◽  
Vol 11 (24) ◽  
pp. 11654
Author(s):  
Fansheng Kong ◽  
Seungjun Ahn ◽  
JoonOh Seo ◽  
Tae Wan Kim ◽  
Ying Huang
Keyword(s):  
Experimental Study ◽  
Concrete Structure ◽  
Target Location ◽  
Side Wall ◽  
Access Point ◽  
Alarm System ◽  
Front Side ◽  
Internal Wall ◽  
Study Approach ◽  
The Impact

Researchers have proposed several forms of beacon sensor-based hazard alarm systems for increasing construction workers’ awareness of site hazards, but research on how to deploy beacon sensors so that the system is adequate for achieving timely individualized hazard alarms is scarce. Against this background, this research investigates the impact of different beacon sensor locations in a construction site on how quickly a worker can receive the individualized hazard alarms. This research took an experimental study approach to address this objective. After a prototype of a beacon-based hazard alarm system was developed, the system was tested in a concrete structure building under construction. In the experiment, the locations where the experimenter received the first hazard alarm were recorded in repetitive trials while the beacon sensor was located in four different locations, such as (1) at the entrance of the room, (2) behind the front side wall, (3) on the internal wall facing the access point, and (4) on the internal wall not facing the access point and in a partially enclosed room in the concrete structure. The rate of successful alarm notification (i.e., the rate that the person received the hazard alarm before arriving at the target location) was 89%, 68%, 48%, and 19%, respectively, for the four locations of the beacon sensor. Meanwhile, the heat maps indicating where the hazard alarm notification was received show that the “behind the front side wall” setting yielded the most desired pattern of notification reception, wherein the person received the hazard alarm just before arriving at the room. These results show that the hazard alarm function of the system could be severely affected by the beacon sensor’s location and implies that the locations of beacon sensors should be decided carefully based on the type of hazard involved and the workers targeted for receiving the alarms.

A mechanistic study of the [La2(OCH3)2]4+- and [(1,5,9-triazacyclodo-decane):Zn:(OCH3)]+-catalyzed methanolysis of carbonates: possible application for the recycling of bisphenol A polycarbonates

2013 ◽  
Vol 91 (11) ◽  
pp. 1139-1146 ◽  
Author(s):  
Alexei A. Neverov ◽  
Leanne D. Chen ◽  
Sean George ◽  
David Simon ◽  
Christopher I. Maxwell ◽  
...  
Keyword(s):  
Bisphenol A ◽  
Metal Ion ◽  
Catalytic Decomposition ◽  
Nucleophilic Attack ◽  
Alkyl Derivative ◽  
Mechanistic Study ◽  
Tetrahedral Intermediate ◽  
Rate Limiting Step ◽  
Leaving Group ◽  
Rate Limiting

The kinetics of the methanolysis of seven methyl aryl carbonates (3) and two methyl alkyl carbonates (4) promoted by [12[ane]N3:Zn:(OCH3)]+ and [La2(OCH3)2]4+ catalysts (1 and 2, respectively) have been studied at 25.0 °C. Brønsted plots of the [Formula: see text] values for methanolysis versus aryloxy and alkoxy leaving group (LG) [Formula: see text] or [Formula: see text] values (the pKa values of the parent ArOH or ROH in methanol) for substrates 3 and 4 show an apparent downward break at [Formula: see text] ∼16.6 and 15.2 with [12[ane]N3:Zn:(OCH3)]+ and [La2(OCH3)2]4+, respectively. The breakpoint is not due to a change in rate-limiting step in a two-step process involving metal ion delivery of a coordinated methoxide to a transiently associated substrate and the subsequent breakdown of a tetrahedral intermediate to form product. The more satisfactory explanation is that the break arises when one correlates the rate constants for two dissimilar sets of substrates, namely aryloxy- and alkoxy-substituted 3 and 4. DFT calculations for the 1-promoted reactions of methyl 4-nitrophenyl carbonate (3b), which has a good aryloxy leaving group, and methyl isopropyl carbonate (4b), which has a relatively poor alkyl one, indicate that the catalyzed processes involve two steps. Accordingly, the methanolysis of all 3 having [Formula: see text] values for the parent phenols ≤15.3 involves rate-limiting nucleophilic attack and fast breakdown. For the isopropyl alkyl derivative (4b) having a [Formula: see text] > 18.13, the rate-liming step is the metal ion promoted breakdown of a tetrahedral intermediate. The catalytic system employing 2 has utility for the catalytic decomposition of poly(bisphenol A carbonate). In a semi-optimized system where 1000 mg of poly(bisphenol A carbonate), treated at 100 °C for 30 min in 2 mL of 60:40 chloroform−methanol containing La(OTf)3:NaOMe (5:7.5 mmol L−1), the reaction gave an 84% yield of bisphenol A, corresponding to >300 turnovers per catalyst.

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