FORCE-FIELD AND VIBRATIONAL-SPECTRA OF OLIGOSACCHARIDES WITH DIFFERENT GLYCOSIDIC LINKAGES .1. TREHALOSE DIHYDRATE, SOPHOROSE MONOHYDRATE AND LAMINARIBIOSE

The vibrational spectra of the disaccharides, trehalose dihydrate, sophorose monohydrate and laminaribiose, have been recorded in the crystalline state in the 4000-100 cm-1 spectral region for the IR spectra and in the 4000-20 cm-1 spectral range for the Raman spectra. These three disaccharides exhibit the same monosaccharide composition (i.e. glucose residue), but differ in the position and configuration of the glycosidic linkage (alpha, 1- 1; beta, 1-2 and beta, 1-3 for trehalose, sophorose and laminaribiose, respectively). Most of these spectra have not yet been reported, particularly in the low frequency range. They constitute the basis of theoretical calculations of normal modes of vibration. Normal coordinate analysis has been made in the crystalline state using a modified Urey-Bradley-Shimanouchi intramolecular potential energy combined with a specific intermolecular potential energy function. The force field parameters are transformed from initial works on both anomers of glucose. The vibrational assignments of the observed hands are made on the basis of the potential energy distributions. It appears that the greatest part of the vibrational modes is very highly coupled vibrations. The calculated vibrational frequencies agree very well with the observed frequencies in the whole spectra, particularly in the ''fingerprint'' regions and in the low frequency range. The bands observed at 733, 773 and 755 cm-1 for trehalose dihydrate, sophorose monohydrate and laminaribiose, respectively, are calculated at 728, 772 and 755 cm-1 and are due to bending modes of heavy atoms involved in the corresponding glycosidic linkage C1-O1-C(x)'. Moreover, some known characteristic structural regions may be divided into different parts that have a specific significance. The standard deviation between calculated and observed frequencies below 1500 cm-1 leads to values of 3.0, 3.7 and 4.2 cm-1 for the three disaccharides, respectively.