Fast mutarotation

Exists as P-Pyranose form in cryst. state. In soln. rapidly equilibrates with 10-15% of a-P (v. fast mutarotation). 

In analyzing polysaccharide hydrolyzates, some workers have assumed that, because mutarotation in pyridine is slow and the tri-methylsilylation reaction is fast, no change in the composition occurs during trimethylsilylation. Others have equilibrated the monosaccharides in pyridine, before trimethylsilylation.172,255 In nonaqueous solvents, the mutarotation may be catalyzed by lithium perchlorate172 or, more conveniently, by 2-pyridinol,256,257 which is volatile, but does not interfere with subsequent analyses. 

For arabinose the half-times for all components are essentially the same. As long as this constraint is obeyed, the calculated optical rotation curves would be expected to show no rapid initial drop, and this is what was found. Therefore, we must postulate a fifth, as yet undetected, fast component in mutarotating arabinose solutions. Here the only reasonable... 

Mutarotation has limited, rather diagnostic, significance in food chemistry and technology. Practical use of this reaction is demonstrated in milk powder manufacture. Evaporation of milk at a rate lower than mutarotation of lactose yields a product with less a-lactose isomer, which crystallizes in prism- or pyramid-like form. Fast milk evaporation gives an amorphous mixture of a- and [3-lactose (5.7b). 

A particular problem is the demonstration of inversion with sialidases, since they act on ot-A-acetylneuraminides, but the mutarotational equilibrium is 10% ot 90% The chemical shifts of the two H3 protons have different chemical shifts in the ot- and p-forms of A-acetylneuraminic acid and its mutarotation is not inconveniently fast, so demonstrating retention is straightforward. However, to demonstrate inversion, it is necessary to match the rate of liberation of aglycone with that of a retaining sialidase and plot time courses for both enzymes, in order to distinguish inversion from uninformative conversion of substrate to the equilibrium mixture i.e. it has to be demonstrated that the chemical flux through the system is easily adequate to maintain non-equilibrium proportions of a- and p-A-acetylneuraminic acid. 

The first term in the denominator of Eq. (66) represents a non-competitive inhibition (compare to Eq. (31)) of the enzyme by the sum of concentrations of A, B and P. This non-specific inhibition could be correlated with an increased viscosity of the reaction medium in the presence of A, B and P147- mi. As the mutarotation of the carbohydrates is fast compared to the enzymatic reaction there was no need for a discrimination between the a,P-anomers or the open-chain form of the monosaccharides. A complete set of kinetic parameters was determined, summarized in Table 7-4. 

To measure rates of such slow processes as mutarotations, the change in intensity of a suitable spectral peak with time may be measured. Rates of such fast processes as conformational shifts are often indicated by changes in the spectrum with temperature ... 

Inversion at Co —SR R is fast (0.1-10s ) but diastereoisomeric systems have been detected by H and C NMR spectroscopy. Thus (305) shows clearly resolved signals of the tren ligand arising from the chiral thioether. " Also the related t-(N,S)[Co (R)cys (tren)] chelate shows immediate H-NMR spectral changes on deprotonation of the dangling equatorial carboxylate arising from rapid (< 5 s) mutarotation of the (/ )S(J )C, (5)S(Ji)C diastereoisomeric mixture. Thus inversion appears to be 10 -10 times more rapid than for uncoordinated sulfonium salts, a result in keeping with other M—SR R systems. ... 

Examples for reactions of fast protonations (Equation 2.29) are ester hydrolysis and alcoholysis, inversion of sucrose, and the hydrolysis of acetals. The mutarotation of glucose and the dehydration of acetaldehyde hydrate are examples of slow protonations described with Equation 2.30. 

Whilst human erythrocytes contain 7V-acetyl-neuraminic acid as the only sialic acid, BALB/c mouse erythrocytes contain 9-C7-acetyl-A-acetyl-neuraminic acid as their dominant sialic acid, which has not previously been encountered. TTie mutarotation of iV-acetyl-a-D-neuraminic acid was previously thought to be too fast to measure, but enzymic liberation of the free a-anomer in an n.m.r. tube has permitted this to be measured. Unexpectedly, at pH 5.4 mutarotation is quite slow, with a half-life of about 80 min for the a-anomer. MO calculations... 

The oxidation of /S-glucose by hypoiodous acid at pH 9.8 is initially at least 25 times as fast as that of the a-isomer 193). As the oxidation progresses, the simultaneous mutarotation tends to equalize the two rates. This difference in rates has been observed in the pH range of 7.6 to 11.8. 

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