Mannose mutarotation

It shows the usual carbohydrate reactions and most resembles mannose in behaviour it exists in OL and fi forms which exhibit mutarotation. 

Maltose, l- 4- -link in, 998 molecular model of, 998 mutarotation of, 998 structure of, 998 Manicone, synthesis of. 805 Mannich reaction. 915 Mannose, biosynthesis of, 1011 chair conformation of, 126 configuration of, 982 molecular model of, 126 Margarine, manufacture of, 1063 Markovnikov. Vladimir Vassilyevich. 192... 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

In contrast to other 2,5-anhydroaldoses (which exhibit mutarota-tion, possibly due to the formation of hemiacetals28), 2,5-anhydro-D-glucose does not show any mutarotation.27 The importance of this compound as a potentially useful precursor to C-nucleosides warrants a reinvestigation of the deamination reaction, and the definitive proof of the structure of the compound. The readily accessible 2,5-anhydro-D-mannose (11) does not possess the cis-disposed side-chains at C-2 and C-5 that would be required of a synthetic precursor to the naturally occurring C-nucleosides, with the exception of a-pyrazomycin (8). The possibility of an inversion of the orientation of the aldehyde group in 11 by equilibration under basic conditions could be considered. 

Hexokinase is of great biological interest since it would appear that not only in yeast cells but in most, if not all, plant and animal cells phosphorylation at C6 of the common hexoses D-glucose, D-fructose and D-mannose initiates sugar utilization. Since on solution in water the crystalline hexoses quickly undergo mutarotation, resulting in an equilibrium mixture of various tautomeric modifications, the fermentability... 

To accommodate these facts, the earliest mechanisms proposed for degradation of D-fructose assumed that it was present in the furanose form, and that the ring remained intact. It was assumed that the initial reaction was the elimination of water, to form the 1,2-enolic form of 2,5-anhydro-D-mannose, and that further dehydration resulted in 2-furaldehyde. The necessity for D-glucose to isomerize to D-fructose was assumed to account for the much lower reaction-rate of D-glucose. This mechanism does not account for the observation that 2,5-anhydro-D-mannose is less reactive than D-fructose, nor is there any evidence that 2,5-anhydro-D-mannose is present in reacting D-fructose solutions. Nevertheless, similar mechanisms have since been proposed.13-16 Because of the ease of mutarotation of D-fructose... 

Grant74 conducted a detailed study of the properties of 2,5-anhydro-D-mannose (chitose), and prepared several new derivatives that confirmed the anhydrohexose structure. The mutarotation of 2,5-anhydro-D-mannose, its conversion under mild conditions into a... 

The reported lack of mutarotation for 2,5-anhydro-D-glucose, which was tentatively identified as the product of the reaction of 2-amino-2-deoxy-D-mannose with mercuric oxide, has not been confirmed.77,78... 

For some monosaccharides, the rate of mutarotation (K = k 3-kf) is found to obey a simple first-order rate law in which — d[a]/dt = A [a] — A2[P] (Scheme 1.4). Glucose, mannose, lyxose and xylose exhibit... 

Anomeric ediyl hemiacetals of the methyl ester of aldehydo-D-galacturonic acid tetraacetate were prepared by Dimler and Link. In chloroform, these isomers mutarotated to an intermediate value over a period of several hours, and the mutarotation was complex. Solvent-free oldehydo-D-galactose pentaacetate was obtained in crystalline form, and did not show mutarotation in 1,1,2,2-tetrachloro-ethane. " In the presence of water, the corresponding crystalline aldehydrols of aZde/tydo-D-galactose pentaacetate and aldehydo-T>-mannose pentaacetate were obtained. 

Aldose-1-epimerase), quite widespread in animal tissue and bacteria, which catalyses mutarotation. The Escherichia coli enzyme has a maximum activity close to neutral pH. The activation energy 4G =11.9 kcal mol" is greatly lowered, as usual with respect to that of the non-enzymically catalysed reaction, close to 17 kcal mol". o-Glucose, D-galactose, and D-fucose are substrates but not D-mannose (Hucho and Wallenfels 1971). 

Fig. 7.—Upper Curve Mutarotation of n-Mannose Phenylhydrazone [c 0.9, pyridine-ethanol (9 1 by vol.)] Lower Curve n-Mannose Diphenylformazan Formation.

Ea was called the apparent activation energy, and the true activation energy ( ) was calculated from the relationship EA = E +JT. For the mutarotations of D-xylose, D-mannose, D-glucose, and lactose, values of EA were found to be 16,245, 16,375, 16,945, and 17,225 cal. mol-1, respectively. Later, Dyas and Hill, and coworkers289,290 pointed out that Ea is the same as AGf. They calculated AGt and ASf from the reaction constant (k) by die Eyring equation ... 

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