Abstract
Studies on the mutarotation of D-fructose show that whereas the furanose-pyranose transformation is rapid, the furanose-furanose transformation is very rapid. As fructose mutarotation is found to be multiphasic at ambient temperatures, estimates of the tautomer distribution and the specific rotation of the furanose forms, by application of the unimolecular equation to optical data, are in error. Calculations from fructose tautomer shifts with temperature, and also determinations made from the invertase hydrolysis of methyl -D-fructofuranoside indicate that the specific rotation of -D-fructofuranose is +78° and that for a-D-fructofuranose is -120°. The rules' of rotation and of anomeric specification do not therefore apply to the fructofuranoses. The saporous unit responsible for the sweet taste of -D-fructopyranose has been indentified as a tripartite grouping involving the anomeric hydroxyl group, the oxygen atom of the primary alcohol group and the ring methylene carbon atom. These represent, respectively, an appropriate proton donor (AH), and a proton acceptor (B) for an intermolecular hydrogen bonding system and also a hydrophobic bonding site (s). In the -D-fructopyranose structure they possess the proper steric disposition to elicit sweet taste. The tasteless furanose forms of fructose do not have the required steric relation between these functional groups.
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