DETERMINATION OF ELECTRONIC EFFECTS IN DISACCHARIDE MODEL COMPOUNDS

ABSTRACT for WATOC'99 (August 1-6, 1999, London):

Determination of Electronic Effects in Disaccharide Model Compounds

A.-M. Kelterer^*,a, G.P. Johnson^b and A.D. French^b
^a Institut für Physikalische und Theoretische Chemie, Technische Universität Graz, Austria
^b Southern Regional Research Center, New Orleans, LA, USA.

The potential energy surfaces of a number of disaccharide models were investigated by MM3(96) and ab initio RHF/6-31G* calculations using the two interlinkage angles phi and psi varying over the whole 360° range. Sucrose is modeled by a THP-O-THF analog (shown below together with the crystal structure of sucrose), the trehaloses and cellobiose by their THP-O-THP analogs.

THP-O-THF analog and crystal structure of sucrose

Minima and saddle points were located by MP2/6-31G* and DFT B3LYP/6-31G* methods. Comparison with the crystallographic data of di-, tri-, and tetra-saccharides show the crystal structures in the low energy regions of the analog maps. In contrast, the MM3(96) force field cannot represent the experimental results of sucrose correctly.

The discrepances in the energetic behaviour of the located minima can be explained by the overlap of anomeric and exo-anomeric effects of the two rings. Basis sets such as cc-pVDZ, aug-cc-pVDZ and 6-31+G were used to estimate the diffuse distribution of the lone pair density of the anomeric oxygens, which hyperconjugate into the anomeric bonds. CHELPG and NPA charges were used to estimate the dipole-dipole interactions of the important minima.

A Fourier analysis is employed to determine the strength of the different electronic effects (dipole-dipole interactions, anomeric effects/hyperconjugation, bond staggering effects) in the disaccharide models.

Informations required by Austrian law (Offenlegung gem. §25 MedienG): Dr. Anne-Marie Kelterer, Graz.