Esterification isotope labels

Since the imidazolide method proceeds almost quantitatively, it has been used for the synthesis of isotopically labeled esters (see also Section 3.2), and it is always useful for the esterification of sensitive carboxylic acids, alcohols, and phenols under mild conditions. This advantage has been utilized in biochemistry for the study of transacylating enzymes. A number of enzymatic transacylations (e.g., those catalyzed by oc-chymo-trypsin) have been shown to proceed in two steps an acyl group is first transferred from the substrate to the enzyme to form an acyl enzyme, which is then deacylated in a second step. In this context it has been shown[21] that oc-chymotrypsin is rapidly and quantitatively acylated by Af-fraw.s-cinnamoylimidazole to give /ra/w-cinnamoyl-a-chymotrypsin, which can be isolated in preparative quantities and retains its enzymatic activity (see also Chapter 6). 

The mechanism of the Fischer esterification was controversial until 1938, when Irving Roberts and Harold Urey of Columbia University used isotopic labeling to follow the alcohol oxygen atom through the reaction. A catalytic amount of sulfuric acid was added to a mixture of 1 mole of acetic acid and 1 mole of special methanol containing the heavy lsO isotope of oxygen. After a short period, the acid was neutralized to stop the reaction, and the components of the mixture were separated. 

This question was resolved using isotopic labeling. For example, Fischer esterification of benzoic acid with methanol that had been enriched with the isotope of oxygen gave labeled methyl benzoate. ... 

Isotopic-labeling experiments and evidence of a dimeric intermediate in the transition-metal-free aerobic oxidative C-C bond cleavage of a-hydroxy ketones and their esterification, on reaction with O2 as the ideal oxidant in the presence of K2CO3, have disclosed the reaction mechanism. ... 

The data obtained from isotopic labeling studies and several other observations have clearly established the mechanism of the acid-catalyzed esterification of carboxylic acids. The reaction occurs in four steps. 

As already indicated, a special problem with esters is their preparation from two natural precursor molecules by a chemical ester synthesis. Such products have to be labelled nature-identical. For an interesting positional H-NMR study on ethyl butyrate from enzymatic esterification of beet ethanol with butyric acid from milk see [317]. Another chance to detect a corresponding adulteration would be a positional carbon and oxygen isotope analysis of the ester components. Isotope effects on the esterification reaction in question seem to influence characteristically the 8-values of the atoms involved, and hence form a basis for the origin assignment of these compounds (for further details see 6.2.2.4.4). 

Isotopes of elements are useful in chemistry for studies of the mechanisms of chemical reactions. A standard technique is to label one of the atoms in a molecule by using an isotope of the element. It is then possible to trace the way in which this atom behaves throughout the course of the reaction. For example, in the esterification reaction ... 

Isotopes of elements are useful in chemistry for studies of the mechanisms of chemical reactions. A standard technique is to label one of the atoms in a molecule by using an isotope of the element. It is then possible to trace the way in which this atom behaves throughout the course of the reaction. For example, in the esterification reaction ROH + R COOH H2O + R COOR it is possible to find which bonds are broken by using a labeled oxygen atom. If the reaction is performed using in the alcohol it is found that this nuclide appears in the ester, showing that the C-OH bond of the acid is broken in the reaction. In labeling, radioisotopes are detected by coun-... 

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