Enediolates, formation

FIGURE 19.6 The phosphoglncoisomerase mechanism involves opening of the pyranose ring (Step A), proton abstraction leading to enediol formation (Step B), and proton addition to the double bond, followed by ring closure (Step C). 

Enediol formation is also rate limiting in the oxidation of aldoses and ketoses by alkaline ferricyanide , the rate expression being... 

Thr90 sam 268 Base catalyzed cis-enediol formation by HjO interaction b-439 e=421... 

OTHER EXAMPLES OF KINETIC ISOTOPE EFFECTS. The power of kinetic isotope effects in enzymol-ogy is well illustrated in the work of Rose ° and Knowles deahng with hydrogen effects in proton transfer to and from carbon. Abstraction of a proton from a tetrahedral carbon is a fundamental step in many enzyme-catalyzed reactions. Intramolecular proton transfer as well as partial loss (wash-out) migrating protons have provided important clues in mechanistic investigations. Enol and enediolate formation constitute several... 

The importance of 2,3-enediol as compared to 1,2-enediol formation from a ketose presents a problem that has not yet been quantitatively investigated. Lemieux30 has pointed out that the large, non-... 

Fig. 2-33. Alkaline peeling reaction of cellulose (R = cellulose chain). 1 — 2, Isomerization 2 — 3, 2,3-enediol formation 3 — 4, j3-alkoxy elimination 4 —> 5, tautomerization 5 — 6, benzilic acid rearrangement leading to glucoisosaccharinic acid.

Fig. 2-34. Stopping reaction. 1 —>2, 1,2-Enediol formation 2 — 3, j3-hydroxy elimination 3 —> 4, tautomerization 4 —> 5, benzilic acid rearrangement leading to a glucometasaccharinic acid end group, (cf. Fig. 2-33.)...

Reviews devoted to various aspects of the transformation have already been published in this Series, and are very informative to the reader who desires more experimental evidence. However, explanation of these transformations and rearrangement reactions proved extremely difficult, and, to the 1,2-enediol, long regarded as the only intermediate, had to be added new enediols and dismutation reactions. Ionization of the enediol was proposed for the sake of clarity and to reach more confident conclusions. Enediol formation and the benzilic acid type of rearrangement, as well as fragmentation and disproportionation of the molecule, are found to occur, and must be regarded as basic reactions in carbohydrate chemistry. Various combinations of these four mechanisms are responsible for the several reaction products found. 

Enediol formation (Section 23.22) Enolization of an aldose or a ketose gives an enediol. Enediols can revert to aldoses with loss of stereochemical integrity at the a-carbon atom or isomerize to ketoses. 

The similarities between glycoxidation and glucose autoxidation (Figs. 2 and 3) are the generation of oxidants at an early stage of exposure of protein to glucose the requirement for transition metals dependence upon enediol formation, which is rate-limiting for oxidant formation and the formation of dicarbonyl products (Hunt et aL, 1993). 

Epimerization Enediol formation provides a pathiway for the interconversion of the C-2 epimers. The stereochemistry at C-2 is iost on enolization reversion to the aldehyde can give either the fl or S configuration at C-2. 

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