STEREOCHEMICAL ASPECTS OF HYDRATION OF CARBOHYDRATES IN AQUEOUS-SOLUTIONS .2. KINETIC MEDIUM EFFECTS

Rate constants for the hydrolysis of 1-benzoyl-3-phenyl-1,2,4-triazole in aqueous solutions of carbohydrates have been measured as a function of molality and nature of added mono- and disaccharides. The kinetic medium effects induced by the carbohydrates originate from hydration sphere overlap effects. The results are analyzed using the additivity principle and reveal specificity in the stereochemical aspects of hydration. The major effect determining the hydration of a monosaccharide appears to be the position of the OH(4) group in conjunction with OH(2). The position of the carbonyl function and the number of equatorial groups present in the molecule are of minor importance. The experimentally obtained G(C) values, which are representative of the interaction between the carbohydrate and the initial state and activated complex for the hydrolysis reaction, show that the hydration of the carbohydrates is mainly determined by the methine moieties. The G(CHOH,endo) values obtained for the dominant conformers in solution point to a similar conclusion. With an increase in compatibility of the carbohydrate molecule with the three-dimensional hydrogen-bond structure of water, the hydroxy groups become less important in determining carbohydrate-solute interactions. This might be important in molecular recognition, since under these conditions the carbohydrates are recognized as hydrophobic moieties. For disaccharides the medium effects are larger than expected on the basis of the medium effect of two monosaccharide subunits. We suggest that this is caused by a cooperativity effect, which makes the methine moieties even more dominant in governing the hydration characteristics. The G(C) values reveal that the type of linkage in the disaccharide molecule hardly influences the kinetic medium effect. Only when one of the monosaccharide subunits has an axial OH(4) or when there is a 1-3 type of linkage between the moieties is a significantly different G(C) found. It is suggested that the compatibility of the carbohydrates with the three-dimensional hydrogen-bond structure of water largely depends on the compatibility of the next nearest neighbor oxygens of the carbohydrate molecule with the nearest or next nearest neighbor oxygens of liquid water.