Effects of permethylation on disaccharide conformation were studied with relaxed-residue φ,ψ maps for eight disaccharides of glucose and their permethylated derivatives. Many orientations of the methyl groups were generated with the conformation generator in the Chem-X software program and filtered with a rules-based method. For native molecules, clockwise and counterclockwise rings of hydrogen bonds were also used. MM3(96) energies were calculated with a dielectric constant ( ) of 1.5. The native disaccharide maps were also calculated at ) 3.5 and 7.5 to reduce hydrogen-bonding strength. Maps for native and permethylated structures were generally similar. The permethylated structures, which do not make hydrogen bonds, gave maps that were most similar to the native maps that had reduced hydrogen-bonding strength. All φ,ψ values for the global minima of the permethylated models fell within the 1-kcal/mol contours on the ) 7.5 maps. Flexibility values based on a partition function were substantially less for permethylated R,R-trehalose and laminarabiose, compared to their native counterparts at any . On the other hand, strong interresidue hydrogen bonding at ) 1.5 for the global minimum structures of cellobiose and maltose caused those models to be more rigid than their permethylated counterparts. All permethylated models were less flexible than their native counterparts at ) 7.5 and their backbone analogues based on tetrahydropyran calculated at ) 1.5.

Electrospray ionization and collision induced dissociation on a triple quadrupole mass spectrometer were used to determine the effect of spatial crowding of incremented alkyl groups of two anomeric pairs of peralkylated (methyl to pentyl) disaccharides (maltose/cellobiose and isomaltose/gentiobiose). Protonated molecules were generated which underwent extensive fragmentation under low energy conditions. For both the 1 --> 4 and 1 --> 6 alpha and beta isomers, at comparable collision energies the methyl derivative exhibited the least fragmentation followed by ethyl, propyl, butyl, and pentyl. Collision energy is converted to rotational-vibrational modes in competition with bond cleavage, as represented by the slope of product/parent ion (D/P) ratio versus offset energy. Variable rotational freedom at the glycosidic linkage with incremented alkyl groups is hypothesized to be responsible for this effect. Discrimination of anomeric configuration was also assessed for these stereoiosmeric disaccharides. A systematic study showed that an increasing discrimination was attained for the 1 --> 4 isomeric pair as the size of the derivative increased from methyl to pentyl. No anomeric discrimination was attained for the 1 --> 6 isomeric pair. Parent and product ion scans confirmed the consistency of fragmentation pathways among derivatives. Chem-X and MM3 molecular modeling programs were used to obtain minimum energy structures and freedom of motion volumes for the permethylated disaccharides. The modeling results correlated with the fragmentation ratios obtained in the mass spectrometer giving strong indication that the collision induced spectra are dependent on the freedom of rotational motion around the glycosidic bond.

No abstract