The log kexovs. log kendo correlation as a mechanistic criterion; on the mechanisms of solvolysis of norbornyl derivatives and base-catalyzed isotope exchange of norbornanones

1976 ◽  
Vol 54 (5) ◽  
pp. 678-684 ◽  
Author(s):  
Sujit Banerjee ◽  
Nick Henry Werstiuk
Keyword(s):  
Transition State ◽  
Exchange Rates ◽  
Isotope Exchange ◽  
Transition States ◽  
Substituent Effects ◽  
Proton Exchange ◽  
Hydrogen Deuterium Exchange ◽  
Rate Data ◽  
Hydrogen Deuterium ◽  
The Difference

Rate data for the acetolysis of exo-norbornyl-sulfonates have been correlated with those for the corresponding endo isomers. It is shown that the slopes of the log kexovs. log kendo plots reflect the difference in delocalization between the transition states derived from the exo and endo isomers, respectively. The log kexovs. log kendo plot, which is comprised of the parent norbornyl sufonate and its derivatives substituted at the 5, 6, and 7 positions, has a slope of 1.11 ± 0.08, which establishes that σ bridging is absent in the transition state obtained from the exo isomer. A similar analysis of base-catalyzed hydrogen–deuterium exchange rates of norbornanones reveals that exo proton exchange is more sensitive to substituent effects than the corresponding endo process.

scholarly journals The stereospecificity and catalytic efficiency of the tryptophan synthase-catalysed exchange of the α-protons of amino acids

2004 ◽  
Vol 381 (3) ◽  
pp. 847-852 ◽  
Author(s):  
Máire E. NíBEILLIÚ ◽  
J. Paul G. MALTHOUSE
Keyword(s):  
Amino Acids ◽  
Exchange Rates ◽  
Catalytic Efficiency ◽  
Deuterium Exchange ◽  
Proton Exchange ◽  
Tryptophan Synthase ◽  
Hydrogen Deuterium Exchange ◽  
Pka Values ◽  
First Order ◽  
Hydrogen Deuterium

13C-NMR has been used to follow the tryptophan synthase (EC 4.2.1.20) catalysed hydrogen–deuterium exchange of the pro-2R and pro-2S protons of [2-13C]glycine at pH 7.8. 1H-NMR has also been used to follow the tryptophan-synthase-catalysed hydrogen–deuterium exchange of the α-protons of a range of L- and D-amino acids at pH 7.8. The pKa values of the α-protons of these amino acids have been estimated and we have determined whether or not their exchange rates can be predicted from their pKa values. With the exception of tryptophan and norleucine, the stereospecificities of the first-order α-proton exchange rates are independent of the size and electronegativity of the amino acid R-group. Similar results are obtained with the second-order α-proton exchange rates, except that both L-tryptophan and L-serine have much higher stereospecificities than all the other amino acids studied.

scholarly journals NMR measurements of proton exchange between solvent and peptides and proteins.

1999 ◽  
Vol 46 (3) ◽  
pp. 651-663 ◽  
Author(s):  
J Wójcik ◽  
K Ruszczyńska ◽  
I Zhukov ◽  
A Ejchart
Keyword(s):  
Exchange Rates ◽  
Magnetization Transfer ◽  
Deuterium Exchange ◽  
Proton Exchange ◽  
Hydrogen Deuterium Exchange ◽  
Hydrogen Deuterium ◽  
Transfer Method ◽  
Amide Protons

Scope and limitations of the NMR based methods, equilibration and magnetization transfer, for measuring proton exchange rates of amide protons in peptides and proteins with water protons are discussed. Equilibration is applied to very slow processes detected by hydrogen-deuterium exchange after a solute is dissolved in D2O. Magnetization transfer allows to study moderately rapid processes in H2O. A number of precautions should be undertaken in order to avoid systemic errors inherent in the magnetization transfer method.

2,4-Diazapentadienes. II. A Carbanion Cyclization

1972 ◽  
Vol 50 (5) ◽  
pp. 678-689 ◽  
Author(s):  
D. H. Hunter ◽  
S. K. Sim
Keyword(s):  
Isotope Effect ◽  
Solvent Effects ◽  
Isotope Effects ◽  
Substituent Effects ◽  
Deuterium Exchange ◽  
Hydrogen Deuterium Exchange ◽  
Deuterium Isotope Effect ◽  
Potassium Methoxide ◽  
Proton Shift ◽  
Hydrogen Deuterium

The mechanism of the cyclization and 1,3-proton shift of 1,3,5-triaryl-2,4-diaza-1,3-pentadienes (1) catalyzed by phenyllithium and by potassium methoxide–methanol has been studied. On the basis of substituent effects, hydrogen–deuterium exchange, isotope effects, and solvent effects, it was deduced that both the cyclization and prototropy involve a common W-shaped carbanion which rapidly cyclizes. A kinetic deuterium isotope effect of 2 was calculated for protonation of this intermediate carbanion in methanol.

scholarly journals Protein-like proton exchange in a synthetic host cavity

2015 ◽  
Vol 112 (50) ◽  
pp. 15303-15307 ◽  
Author(s):  
William M. Hart-Cooper ◽  
Carmelo Sgarlata ◽  
Charles L. Perrin ◽  
F. Dean Toste ◽  
Robert G. Bergman ◽  
...  
Keyword(s):  
Active Site ◽  
Proton Exchange ◽  
Hydrogen Deuterium Exchange ◽  
Amide Hydrogen ◽  
Acid Base ◽  
Enzyme Active Site ◽  
Hydrogen Deuterium ◽  
Water Rate ◽  
Exchange Kinetics ◽  
Metal Ligand

The mechanism of proton exchange in a metal–ligand enzyme active site mimic (compound 1) is described through amide hydrogen–deuterium exchange kinetics. The type and ratio of cationic guest to host in solution affect the rate of isotope exchange, suggesting that the rate of exchange is driven by a host whose cavity is occupied by water. Rate constants for acid-, base-, and water-mediated proton exchange vary by orders of magnitude depending on the guest, and differ by up to 200 million-fold relative to an alanine polypeptide. These results suggest that the unusual microenvironment of the cavity of 1 can dramatically alter the reactivity of associated water by magnitudes comparable to that of enzymes.

THE REACTIVITY OF CELLULOSE: II.WATER SORPTION, HEATS OF WETTING, AND THE REACTIONS WITH THALLOUS ETHYLATE IN ETHER, NITRATION MIXTURES, AND HEAVY WATER OF COTTON LINTERS ALTERNATELY WETTED WITH WATER AND DRIED

1953 ◽  
Vol 31 (6) ◽  
pp. 550-563 ◽  
Author(s):  
W. James Brickman ◽  
H. Brian Dunford ◽  
Elmer M. Tory ◽  
John L. Morrison ◽  
Robert K. Brown
Keyword(s):  
Heavy Water ◽  
Water Sorption ◽  
Marked Reduction ◽  
Chemical Nature ◽  
Reaction Times ◽  
Hydrogen Deuterium Exchange ◽  
Wetting And Drying ◽  
Cotton Linters ◽  
Hydrogen Deuterium ◽  
The Difference

Cotton linters dewaxed with benzene and alcohol possess a slightly expanded structure attributed to the swelling effect of the alcohol. Storage causes a partial collapse of the linters especially so when moisture is present. Wetting with water followed by rigorous drying produces a marked reduction in accessibility, but with each additional wetting–drying cycle accessibility of the dried linters increases slightly as measured by reaction with thallous ethylate in ether, a nitration mixture, and in hydrogen–deuterium exchange. Differences in nitration found for limited reaction times are obliterated when these reaction times are extended. Increasing accessibility due to repeated wetting and drying is accompanied by lower water sorption and smaller heats of wetting. This anomaly is due to the fact that cellulose samples obtained by alternately wetting and drying dewaxed linters, when stored with a desiccant, compete for the limited amount of water present and adsorb moisture in proportion to their accessibility. Upon further exposure to water the sample of least accessibility, having adsorbed less water, can now adsorb to a greater extent than do the linters of somewhat greater accessibility.The evidence indicates that the difference in accessibility occasioned by repeated wetting and drying is of a physical rather than chemical nature.

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