The complexation of glucose and other monosaccharides with cyclodextrins

Equilibrium constants of complexes between β-cyclodextrin and glucose, galactose, or mannose have been determined by fluorometric competition titrations in water to be as low as K = 1 – 0.5 M−1, in sharp contrast to recently published values for glucose around K = 400 M–1. The pentoses ribose, xylose, and arabinose show association constants between 6.3 and 1.5 M−1, in agreement with published values. Preliminary computer-aided molecular modelling studies suggest that the small values observed for the hexoses are not due to steric hindrance of an intracavity inclusion but to a smaller number of intermolecular hydrogen bonds and to some strain as a result of cyclodextrin deformation. Calorimetric measurements of the glucose–β-cyclodextrin complex show, again in contradiction with the literature, an endothermic reaction, or a free enthalpy value close to zero. Key words: cyclodextrin complexes, sugar complexation, glucose complexation, calorimetry, computer-aided molecular modelling.