Specific rotation of monosaccharides: a global property bringing local information

Carbohydrates generally occur in several conformations that may differ among themselves by energy values that are smaller than the accuracy of the most sophisticated theoretical methods used to determine them. In addition, the preferential orientations of the hydroxyl groups of these molecules cannot be identified by any experimental technique. Therefore, a method that is able to validate the absolute conformations (i.e., consisting of the orientations of the hydroxyl groups) of carbohydrates would be helpful to improve our knowledge about monosaccharides. SR has been used for this purpose, and here, we present a test to measure the specific rotation (SR) ability of a molecule that possesses not only many conformations, but also four adjacent chiral centers. The results show that the final SR value is a weighted average of a global property (obtained for each conformation), and the latter by its turn is influenced by each chiral center in a multi chiral system. By comparing the SR values calculated for the most abundant anomers of xylopyranose with those of the corresponding monochiral analogs obtained by saturation of three different chiral centers each time, the influence of each center on the global property is confirmed.