ACYCLIC MOLYBDATE COMPLEXES OF ALDOSES IN THE ARABINO AND XYLO SERIESAND THEIR APPLICATION TO THE DETERMINATION OF THE PROPORTION OF ACYCLIC FORMS IN AQUEOUS-SOLUTION

Aldoses of the arabino and xylo series with molybdate ions in aqueous acidic medium. form tetradentate acyclic complexes which are much weak er than those of the related alditols. The formation constants (K(f)) of these complexes were obtained by two independent methods, potentiom etry and UV spectrophotometry, which gave values in good agreement. Th e structures of the complexes with aldoses were shown by C-13 NMR spec troscopy to involve the ligand in its acyclic form. A study of the com plexes of carbohydrate derivatives proved that the magnitude of K(f) w as mainly dependent on the configuration of the site of chelation. The lower stabilities of the aldose complexes were due to the endergonic opening of the pyranose heterocycle in the first step of the complex f ormation. An application to the determination of the equilibrium const ant for the ring-opening reaction of aldopyranoses is described. It ul timately allowed the calculation of the proportion of acyclic forms (a ldehyde and hydrate) in aqueous solutions of the aldoses in the arabin o and xylo series.