A VIBRATIONAL MOLECULAR-FORCE FIELD OF MODEL COMPOUNDS WITH BIOLOGICAL INTEREST .4. PARAMETERS FOR THE DIFFERENT GLYCOSIDIC LINKAGES OF OLIGOSACCHARIDES

The force field previously obtained for both anomers of glucose has been applied to six disaccharides that are molecules of D-glucopyranosyl residues. These six disaccharides have different types of glycosidic linkages-that is, alpha,alpha trehalose dihydrate (1-1), sophorose monohydrate(beta, 1-2), laminarabiose(beta, 1-3), maltose monohydrate (alpha, 1-4) and cellobiose(beta, 1-4), and gentiobiose(beta, 1-6). From a careful analysis of the infrared and Raman spectra and from harmonic dynamics calculations in the crystalline state, the results show the reliability and the transferability of the set of parameters previously obtained for different carbohydrates. Below 1500 cm(-1), observed data and the corresponding calculated frequencies agreed within 5 cm(-1) for each of the six disaccharides. The vibrational density of states are well reproduced by these calculations for each molecule, particularly for the fingerprint regions. Moreover, as found by other workers who used sophisticated potential energy functions, no additional terms are needed to express the exoanomeric effect. Specific force constants characteristic of each glycosidic linkage have been derived, particularly for the glycosidic angle bending. More interesting are the values of the internal rotation barriers. It is shown that they are of the same size for both sides of the glycosidic linkage: VC1O1 = VO1Cx' = 3.29 kcal/mol for an alpha residue and 2.64 kcal/mol for a beta unit (x = 1-6 depends on the position of the glycosidic Linkages of the considered disaccharide). (C) 1995 by John Wiley and Sons, Inc.