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, sop horose monohydrate and laminaribiose, have been recorded in the crysta lline state in the 4000-100 cm-1 spectral region for the IR spectra an d in the 4000-20 cm-1 spectral range for the Raman spectra. These thre e disaccharides exhibit the same monosaccharide composition (i.e. gluc ose residue), but differ in the position and configuration of the glyc osidic linkage (alpha, 1- 1; beta, 1-2 and beta, 1-3 for trehalose, so phorose and laminaribiose, respectively). Most of these spectra have n ot yet been reported, particularly in the low frequency range. They co nstitute the basis of theoretical calculations of normal modes of vibr ation. Normal coordinate analysis has been made in the crystalline sta te using a modified Urey-Bradley-Shimanouchi intramolecular potential energy combined with a specific intermolecular potential energy functi on. The force field parameters are transformed from initial works on b oth anomers of glucose. The vibrational assignments of the observed ha nds are made on the basis of the potential energy distributions. It ap pears that the greatest part of the vibrational modes is very highly c oupled vibrations. The calculated vibrational frequencies agree very w ell with the observed frequencies in the whole spectra, particularly i n the ''fingerprint'' regions and in the low frequency range. The band s observed at 733, 773 and 755 cm-1 for trehalose dihydrate, sophorose monohydrate and laminaribiose, respectively, are calculated at 728, 7 72 and 755 cm-1 and are due to bending modes of heavy atoms involved i n the corresponding glycosidic linkage C1-O1-C(x)'. Moreover, some kno wn characteristic structural regions may be divided into different par ts that have a specific significance. The standard deviation between c alculated 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.