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.

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.

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.

Hemi-alkoxides from reactions of trimethylaluminium with aldehydes or ketones

1970 ◽  
Vol 23 (4) ◽  
pp. 715 ◽  
Author(s):  
EA Jeffery ◽  
T Mole
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Proton Magnetic Resonance Spectroscopy ◽  
Alkoxy Group ◽  
Resonance Spectroscopy ◽  
Methyl Groups ◽  
Methyl Group ◽  
Hydrocarbon Solvents ◽  
Analogous Compound ◽  
New Compounds

One mole of an aldehyde or ketone reacts stoicheiometrically with one mole of trimethylaluminium dimer at ambient temperature in hydrocarbon solvents to form a "hemi-alkoxide" in which an alkoxy group and a methyl group bridge two aluminium atoms. Benzaldehyde, acetone, acetophenone, benzophenone, and cyclohexanone give p-alkoxy-p-methyl-tetramethyldialuminiums (Me2A1.OR.Me.AlMe2; R = CHPhMe, But, CphMea2 CPh2Me, and 1-methylcyclohexyl, respectively). Such compounds can also be prepared in low yields starting from alcohols, rather than aldehydes or ketones. The analogous compound, NezAl.N=CPhMe.Me.AlMez, is formed by reaction of benzonitrile with one mole of trimethylaluminium dimer in the absence of solvent at 100�. Exchange of methyl groups between bridge and terminal sites in these new compounds has been studied by proton magnetic resonance spectroscopy.

General-base-catalyzed Proton Exchange in Glycocyamidiniom Ions: A Comparison of Activation by Protonation and by Alkylation

1975 ◽  
Vol 53 (19) ◽  
pp. 2906-2910 ◽  
Author(s):  
Ross Stewart ◽  
R. Srinivasan
Keyword(s):  
Exchange Rate ◽  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Proton Magnetic Resonance Spectroscopy ◽  
Proton Exchange ◽  
Resonance Spectroscopy ◽  
Methyl Groups ◽  
General Base ◽  
Satisfactory Model ◽  
Conjugate Acid

The rates of exchange of methylene protons in seven glycocyamidinium ions in formate and chloroacetate buffers were measured in D2O by means of proton magnetic resonance spectroscopy. When protonation and methylation of glycocyamidines occur at the same site the methyated products react faster than the protonated products by factors of 1.8–1.9. Replacing ring protons in glycocyamidinium ions by methyl groups causes small increases in the méthylène exchange rate whereas corresponding substitution at the exocyclic nitrogen causes small rate decreases. It is concluded that in this reaction a quaternary ion ZR+ can serve as a satisfactory model for the analogous conjugate acid ZH+.

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