Hydrogen isotope exchange measurements

The hydrogen-isotope exchange reactions have already been discussed in previous sections. For various aromatic and heteroaromatic substrates, standard reaction rates in protodedeuteration and in nitration have been obtained. A plot of these shows a wide scatter. This means that there is no simple relation between these two measures for the susceptibility to electrophilic attack. No single reactivity index can be used as a measure to derive a unique order of the susceptibility of individual ring positions towards electrophilic attack (75TL1395). 

A value of kA is available from measurements of hydrogen isotope exchange (kx) corrected for primary and secondary isotope effects. This is illustrated in Scheme 8 for the detritiation of tritiated benzene. In the lower part of the scheme, the reacting isotope of the benzenonium ion intermediate (9-t) is indicated as a superscript on the rate constant kv and a secondary isotope effect is neglected. 

It will be shown below that with the aid of liquid ammonia solutions of potassium amide, the acidity of hydrocarbons (including even saturated ones) cannot only be detected but also determined quantitatively from measurements of hydrogen isotope exchange rates. 

Experimental evidence for equation (55) was first reported by Kresge (1964) for proton (deuteron) addition to 1,3,5-trimethoxybenzene in H20-D20 the rate constants are obtainable from an analysis of kinetic measurements of hydrogen isotope exchange reactions. 

W.R. Wampler, C.W. Magee, Depth resolved measurements of hydrogen isotope exchange in carbon, J. Nucl. Mater. 103-104 (1981) 509... 

Ottesen, Martin, Methods for Measurement of Hydrogen Isotope Exchange in... 

Hydrogen Isotope Exchange in Globular Proteins, Methods for Measurement... 

CH bonds in alpha position to carbonyl, sulfonyl, or nitro groups may undergo slow hydrogen isotope exchange with a hydroxylic solvent in the presence of a strong base. In another method of measurement of the deprotonation rate, a reagent (bromine or iodine) is added which reacts... 

The rates of amine-hydrogen isotopic exchange in the [CoL(N4)] complex ions fL = CO3 C2O4 or malonate (N4) = tetramine, bis(en), or bis(trimethylene-diamine)] have been measured in DjO at 35°C. Three geometric isomers, equa-... 

A major contribution to the analysis of isotope effects associated with cases where there is no single rate-limiting step was made by the Streitwieser group in 1971 [4, 12]. Using single temperature rate constants for all three hydrogen isotopes and the Swain-Schaad relationship, they calculate an internal-return parameter, a = k i/k2, associated with the experimental rate constants for each of the isotopic exchange measurements. The rate constant for the actual hydron transfer step can now be calculated ... 

As seen in Chapter 11 (Hydrogen-Deuterium Exchange), infrared spectroscopy can be used conveniently for measuring the deuteration rate of biological polymers, e.g., polypeptides and proteins. Hydrogen isotopic exchange of DNA has been studied... 

Experimental Measurements of Amide Hydrogen Isotopic Exchange... 

Although the number of research papers reporting the use of NMR spectroscopy, especially by comparison to and is small the range of applications is extensive (and is described in more detail in the book by Evans et al.). For example, the measurement of specific activity, optical purity, stereochemistry, solution conformation, kinetic acidities, hydrogen isotope exchange, hydrogen bonding, fractionation factors, and radiolysis. 

Kinetic acidity is defined as the relative rate of proton abstraction by a base and is often measured by hydrogen isotope exchange. The linear free energy relationship often found between equilibrium and kinetic acidities for related groups of compounds is usually referred to as a Br nsted relationship (equation 4). 

R. B. Gregory and A. Rosenberg, Methods Enzymol, 131, 448 (1986). Protein Conformational Dynamics Measured by Hydrogen Isotope Exchange Techniques. 

The simplest reaction characteristic of compounds that are regarded as salts of carbanions is that of proton transfer (see Proton Transfers Involving Anions and Dianions) and a number of these have been studied. A typical example is the reaction of methane with lithium amide. This reaction is a model for many known hydrogen isotope exchange reactions of hydrocarbons with lithium and cesium cyclohexylamide whose kinetics have been measured in cyclohexylamine. The model reaction does involve a preliminary weakly bound complex that isomerizes via a transition structure to methyllithium coordinated to ammonia (Figure 3). 

This enzyme is of wide occurrence in bacteria where it is concerned with the reduction of nitrate and CO2 as well as sulphur. Methods for its estimation depend on measuring some activity of hydrogenase by (a) dye reduction (benzyl viologen or methylene blue), (b) isotopic exchange and (c) evolution of molecular hydrogen. Interpretation of quantitative results is difficult due to the complex relationship between the enzyme cell structure and the particular method selected. ... 

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