The proton activating factor and its application to acid and base catalysis of aldehyde hydration

1984 ◽  
Vol 62 (5) ◽  
pp. 907-910 ◽  
Author(s):  
Ross Stewart
Keyword(s):  
Nucleophilic Addition ◽  
Base Catalysis ◽  
General Base ◽  
Uncatalyzed Reaction ◽  
General Acid ◽  
Acid And Base ◽  
Alternative Means ◽  
Acid Catalyzed ◽  
Substituted Benzaldehydes ◽  
Class E

The data of McClelland and Coe for the general acid and general base catalyzed hydration of a series of substituted benzaldehydes has been analyzed in terms of the proton activating factor, paf, and concepts related thereto. Although the latter were derived for use with prototropic systems, the present work shows that they have application to nucleophilic addition as well. Three conclusions emerge from the analysis. 1. Alternative means of calculating paf from buffer catalytic data that were previously derived on theoretical grounds can be used in practice to give essentially the same results. 2. The mechanism of the general acid catalyzed pathway for aldehyde hydration is that favoured by McClelland and Coe and by Jencks, to wit the class (e) mechanism, rather than the class (n) mechanism. 3. Electron-withdrawing groups in the benzaldehyde ring cause the disparity between the contributions of alternative, kinetically equivalent, pathways for the uncatalyzed reaction to become greater.

Deuterium exchange of C-methyl protons in lumazine derivatives

1970 ◽  
Vol 48 (2) ◽  
pp. 263-270 ◽  
Author(s):  
J. M. McAndless ◽  
Ross Stewart
Keyword(s):  
Proton Magnetic Resonance ◽  
Proton Magnetic Resonance Spectroscopy ◽  
Acid Catalysis ◽  
Deuterium Exchange ◽  
Base Catalysis ◽  
Resonance Spectroscopy ◽  
Methyl Group ◽  
General Base ◽  
General Acid ◽  
Acid Catalyzed

Proton magnetic resonance spectroscopy has been used to examine the deuterium exchange of the methyl protons in two lumazine derivatives. The exchange occurs at the C-7 methyl group in 6,7,8-trimethyllumazine (2) and at the C-6 methyl group in 1,7-dihydro-6,7,8-trimethyllumazine (3). The former reaction is subject to both general acid- and general base-catalysis but the latter only to general acid-catalysis. Plausible mechanisms for the reactions of both compounds are advanced, involving in the case of 3, acid-catalyzed addition of water across the C6—N5 double bond.

Kinetics of cyclization of S-ethoxycarbonylmethylisothiouronium chloride to 2-imino-4-thiazolidone

1979 ◽  
Vol 44 (3) ◽  
pp. 912-917 ◽  
Author(s):  
Vladimír Macháček ◽  
Said A. El-bahai ◽  
Vojeslav Štěrba
Keyword(s):  
Hydrochloric Acid ◽  
Dilute Hydrochloric Acid ◽  
Base Catalysis ◽  
General Base ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Kinetics Of Formation ◽  
Acid Catalyzed ◽  
Rate Limiting ◽  
Kinetics Of

Kinetics of formation of 2-imino-4-thiazolidone from S-ethoxycarbonylmethylisothiouronium chloride has been studied in aqueous buffers and dilute hydrochloric acid. The reaction is subject to general base catalysis, the β value being 0.65. Its rate limiting step consists in acid-catalyzed splitting off of ethoxide ion from dipolar tetrahedral intermediate. At pH < 2 formation of this intermediate becomes rate-limiting; rate constant of its formation is 2 . 104 s-1.

General Acid and Base Catalysis by Phosphate in Peroxyoxalate Chemiluminescence

2016 ◽  
Vol 1 (10) ◽  
pp. 2307-2315 ◽  
Author(s):  
Glalci A. Souza ◽  
Mônica M. M. Peixoto ◽  
Ana P. F. Santos ◽  
Wilhelm J. Baader
Keyword(s):  
Base Catalysis ◽  
Peroxyoxalate Chemiluminescence ◽  
General Acid ◽  
Acid And Base ◽  
Acid And Base Catalysis

Concurrent general-acid and general-base catalysis of esterification

1970 ◽  
Vol 92 (14) ◽  
pp. 4377-4382 ◽  
Author(s):  
Sheldon. Milstien ◽  
Louis Arthur. Cohen
Keyword(s):  
Base Catalysis ◽  
General Base ◽  
General Acid

Ketimine-enamine tautomerism of 2-ethoxycarbonylmethylene-4-methyl-3-oxo-1,2,3,4-tetrahydroquinoxaline: A kinetic study

1981 ◽  
Vol 46 (9) ◽  
pp. 2104-2109
Author(s):  
Jaromír Toman ◽  
Vojeslav Štěrba ◽  
Jiří Klicnar
Keyword(s):  
Carbon Atom ◽  
Solvent Effects ◽  
Title Compound ◽  
Base Catalysis ◽  
Methine Carbon ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Acid And Base ◽  
Acid Catalyzed ◽  
Rate Limiting

Tautomerism of the title compound in methanol in the presence of buffers is subject to general acid and base catalysis. The rate-limiting step of the acid-catalyzed reaction consists in addition of the proton to the methine carbon atom of the enamine form, whereas that of the base catalyzed reaction consists in protonation of the formed conjugated base of the enamine. Solvent effects on the equilibrium constant of the isomerization have been measured.

scholarly journals pH-dependence and structure–activity relationships in the papain-catalysed hydrolysis of anilides

1971 ◽  
Vol 124 (1) ◽  
pp. 117-122 ◽  
Author(s):  
G. Lowe ◽  
Y. Yuthavong
Keyword(s):  
Acid Catalysis ◽  
Ph Dependence ◽  
Base Catalysis ◽  
General Base ◽  
Structure Activity ◽  
Equilibrium Binding ◽  
General Acid ◽  
Leaving Group ◽  
Hydrolysis Of ◽  
Equilibrium Binding Constant

The pH-dependence of the Michaelis–Menten parameters for the papain-catalysed hydrolysis of N-acetyl-l-phenylalanylglycine p-nitroanilide was determined. The equilibrium binding constant, Ks, is independent of pH between 3.7 and 9.3, whereas the acylation constant, k+2, shows bell-shaped pH-dependence with apparent pKa values of 4.2 and 8.2. The effect of substituents in the leaving group on the acylation constant of the papain-catalysed hydrolysis of hippuryl anilides and N-acetyl-l-phenylalanylglycine anilides gives rise in both series to a Hammett ρ value of -1.04. This indicates that the enzyme provides electrophilic, probably general-acid, catalysis, as well as the nucleophilic or general-base catalysis previously found. A mechanism involving a tetrahedral intermediate whose formation is general-base-catalysed and whose breakdown is general-acid-catalysed seems most likely. The similarity of the Hammett ρ values appears to exclude facilitated proton transfer as a means through which the specificity of papain is expressed.

scholarly journals Mechanisms of RNA catalysis

2011 ◽  
Vol 366 (1580) ◽  
pp. 2910-2917 ◽  
Author(s):  
David M. J. Lilley
Keyword(s):  
Distinct Species ◽  
Base Catalysis ◽  
Nucleophilic Attack ◽  
Phosphoryl Transfer ◽  
Rna Catalysis ◽  
Rna Molecules ◽  
General Base ◽  
General Acid ◽  
Guanine And Adenine ◽  
Self Splicing

Ribozymes are RNA molecules that act as chemical catalysts. In contemporary cells, most known ribozymes carry out phosphoryl transfer reactions. The nucleolytic ribozymes comprise a class of five structurally-distinct species that bring about site-specific cleavage by nucleophilic attack of the 2′-O on the adjacent 3′-P to form a cyclic 2′,3′-phosphate. In general, they will also catalyse the reverse reaction. As a class, all these ribozymes appear to use general acid–base catalysis to accelerate these reactions by about a million-fold. In the Varkud satellite ribozyme, we have shown that the cleavage reaction is catalysed by guanine and adenine nucleobases acting as general base and acid, respectively. The hairpin ribozyme most probably uses a closely similar mechanism. Guanine nucleobases appear to be a common choice of general base, but the general acid is more variable. By contrast, the larger ribozymes such as the self-splicing introns and RNase P act as metalloenzymes.

Studies in general acid and general base catalysis. Rate–acidity profiles for prototropic processes

1981 ◽  
Vol 59 (24) ◽  
pp. 3318-3318
Author(s):  
Ross Stewart ◽  
R. Srinivasan
Keyword(s):  
Base Catalysis ◽  
General Base ◽  
General Acid

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