Arabinose mutarotation

In the case of 3-L-arabinose the strongly positive rotation decreases and the conductivity increment likewise decreases. As A is rather large (for the equilibrium mixture, 119.7), furanoses or hydrates must be taken into account. However, both the constants of the mutarotation and of the change of A correspond very well, namely, k + k = 0.0582,... 

Simulation of the mutarotation of arabinose closely followed the work on galactose, except that four components plus a central intermediate were included, and the / -pyranose was the starting anomer. The components were the a-pyranose (kt, k2), / -pyranose (k3, k4), a-furanose (k5, k6), and / -furanose (k7j kg). Table III shows sets of values for the k s which yield the experimentally observed half-times for the approach of the components to their equilibrium levels. 

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... 

Many reactions can be induced by the presence of hydrogen ions as a catalyst, among them the saponification of esters, and the inversion and mutarotation of sugar. The opposite effect of retardation has sometimes been observed Lapworth and Hann 4 noticed it in connexion with the establishment of equilibrium between ketonic and enolic substances and Kerp and Baur5 noted the same phenomenon in regard to the formation of primary sulphite derivatives of arabinose, dextrose, and other substances. 

L-arabinose, and a monotrityl ether of D-xylose. The latter had the trityl group attached glycosidically at carbon atom 1, since it did not reduce Fehling solution and showed no mutarotation. 

The specific rotation of a-xylose is +79° and, after mutarotation, it falls to +19°. Draw the pyranose form that predominates at equilibrium. Similarly, the specific rotation of a-arabinose is +175° and it falls to +104° after mutarotation. Draw the pyranose form that predominates at equilibrium. 

In the ease of jq lose, the a/(3 ratio in the starting sugar was about 2 1 whereas the ratio of a/p octadienylethers was close to 1 1. The loss of the stereoehemistry of the starting g lopyranose could be due to a rapid eatalyzed mutarotation compared to the rate of the telomerization reaetion. Analogous results were observed starting from the L-arabinose derivative. 

A mechanism for the mutarotation of -D-arabinose oxime has been proposed, and involves cyclic N-arabinosyl hydroxylamlne inter-mediates. Separable mixtures of N-glycosylated L-a-amino-... 

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