A 11b and 13c nmr determination of the structures of borate complexes of pentoses and related sugars

Abstract

A 11B NMR study demonstrated that the four pentoses, like hexoses, form concurrently 1:1 and 1:2 borate complexes in aqueous solution, contrary to earlier reports. It also showed that in both species, the borate ion was bound to a vicinal diol site. The corresponding stability constants were determined by a potentiometric method. The structures of most 1:2 complexes were precised by 13C NMR spectroscopy, since the carbon atoms which bear the chelating diol group were clearly deshielded and had increased 1J CH values. The results were rationalised by grouping the sugars in series of related configurations. All sugars were complexed in furanose form. Those of the arabino - galacto - fructo series and of the xylo - gluco - sorbo series gave the same type of complex involving the anomeric hydroxyl group and the nearest ring CHOH. The very stable 1:2 complex of D-ribose was shown to be a mixture of two species in constant ratio, borate being bound either at C-1, C-2 or at C-2, C-3. The same result was obtained with the sugars of the lyxo - manno - tagato series, which also possessed two cis CHOH groups.